Large crack where ceiling meets wall Cracks develop when attic trusses lose moisture and bow Fix with inexpensive L-brackets Caulk to complete DEAR TIM: I think my house might be falling down. In the past few weeks, a large crack has formed where the ceiling meets the walls in my home.

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When a horizontal gap appears along the top of the wall, the likely culprit is truss uplift. Roof trusses, unlike roof rafters, attach only to specific walls plates. On other walls, the bottom chord of the truss lies on the plate but the roof builder does not attach it. Trusses are specifically engineered to move as a unit if the wood expands due to heat or humidity in an attic. This is an unwelcome, but not uncommon, problem with roof trusses. As the bottom chord expands, it bows upward, lifting the ceiling drywall with it and creating a gap between the wall and the ceiling.

Re-Tape

The least expensive method of repairing the crack involves re-taping the horizontal seam where the wall meets the ceiling. Before applying a new drywall corner bead and drywall joint compound to the crack, scrape torn bits of paper and dried compound off, and sand about 2 inches along the top of the wall and the edge of the ceiling. After filling the crack with new drywall compound and fitting a new corner bead in place, use an inside-corner taping trowel to create a crisp seam. After the compound dries, sand it and paint the wall and ceiling to match the existing colors.

The problem with re-taping is that another crack can appear in the same spot if the truss isn’t done moving. Typically, a truss is more likely to move within the first year after construction. After that, truss movement gradually slows as the wood in the truss ages. To avoid having to re-tape many times, monitor the depth of the crack over a year. When the size of the crack hasn’t increased for at least six months, go ahead and re-tape.

Camouflage the Crack

The second least expensive method for fixing the problem is to camouflage the crack by installing crown molding along the top of the wall. The trick to doing this successfully is to choose a dark molding, such as an oak or walnut stain, and nail it only to the wall. After pushing the molding up to cover the crack, attach it with finish nails, inserting one into each wall stud, but do not nail the molding to the ceiling. If the truss moves again and pulls the ceiling drywall upward, it’s less likely that the resulting crack will be noticed above the dark molding than it would be above light-colored molding.

Float the Trusses

The only permanent way to fix the separation between the top of the wall and the ceiling involves loosening the drywall panels where they connect to the bottom of the truss and then attaching the panels to blocks installed between the trusses on the top wall plate. Called 'truss floating,' this project requires attic access, and it’s a job for an experienced framing contractor. The contractor will loosen the drywall paneling from the bottom truss chord and will push the edges of the paneling downward. He will install blocks on the top wall plate to hold the drywall down. The seam must be re-taped.

If the truss lifts again after that, it won’t pull the ceiling drywall up with it, so no crack will ensue. Many builders float the trusses during the initial installation of drywall.

Hairline Crack Where Ceiling Meets Wall

Attic Ventilation

Wood is a product of nature, and it moves. Green wood, which is new and still slightly damp, moves more than seasoned wood. When subjected to heat and high humidity, almost all wood will move to some extent. The best way to reduce truss uplift is to ventilate the attic. Cool fresh air should enter the attic through soffit vents installed in the roof eaves and exit the attic at the highest point possible, typically through a ridge vent, static vents or a fan-powered vent.

About the Author

Glenda Taylor is a contractor and a full-time writer specializing in construction writing. She also enjoys writing business and finance, food and drink and pet-related articles. Her education includes marketing and a bachelor's degree in journalism from the University of Kansas.

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How to seal cracks in concrete floors, foundations, walls, or other masonry: this article how to seal and repair of cracks in poured concrete slabs, floors, or walls.

We list all of the current methods used to seal control joints or cracks that occur in those building surfaces, giving the properties, general procedure, and pros and cons of each method: control joint inserts, masonry caulks, radon crack sealants, semi rigid epoxy resin crack fillers, special poly urea caulks designed as a joint filler - polyurethane foam injection to seal cracks, grouts including portland cement, latex-modified, epoxy, modified epoxy-supported, and furan grouts or other products used to fill or seal cracks in concrete or other masonry surfaces & structures.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to seal control joint & expansion joint cracks in concrete slabs

Here we discuss how to choose among and apply the alternative methods for repairing or sealing cracks in masonry surfaces such as poured concrete floor slabs, concrete foundation walls, or brick or masonry block foundation walls. We describe use of epoxy sealants, polyurethane foam sealants, and hydraulic cement crack repair sealants.

This article series describes how to recognize and diagnose various types of foundation failure or damage, such as foundation cracks, masonry foundation crack patterns, and moving, leaning, bulging, or bowing building foundation walls.

Types of foundation cracks, crack patterns, differences in the meaning of cracks in different foundation materials, site conditions, building history, and other evidence of building movement and damage are described to assist in recognizing foundation defects and to help the inspector separate cosmetic or low-risk conditions from those likely to be important and potentially costly to repair.

Here we discuss: a list of materials used to fill poured concrete slab control joints. How to seal a cracked masonry foundation wall or floor slab.Typical concrete crack preparation for sealant with an epoxy product.

Use of Polyurea as a control joint filler or crack sealant in concrete slabs (Polyurethane Foam Injection Method). Use of Epoxy-supported Grouts as a control joint or crack filler in concrete slabs or walls.

List of the Types of Tile Grouts, their Bonding Agents and other Properties. Concrete expansion joint sealant products, epoxies, caulks.

Use of Hydraulic Cement for Repairs in Concrete Slabs, Foundations, or Masonry Block/Brick Walls.

When should we not seal a foundation wall or floor slab crack?

Driver updater activation key. Warning: some crack sealing operations may make crack diagnosis or monitoring more difficult

Watch out: First of all do not just seal a crack if the crack size, shape, pattern, location, or other evidence indicate thatsomething important is going on with the foundation. Further evaluation and choice of proper repair method may be in order.

If for example we think that there may be active foundation movement or settlementgoing on and if that condition is going to be monitored for evidence of further movement, just pushing a flexible sealant or caulkinto a crack is leaves some worries unattended:

This photograph of foundation cracks attributed to concrete that included iron sulfide (pyrrhotite), provided courtesy of CCACB - Connecticut Coalition Against Crumbling Basements. The pattern looks like shrinkage but the crack width is larger than normal shrinkage cracking.

Watch out: before sealing concrete cracks like those at page top be sure that you've got a credible diagnosis of their cause. This cracking pattern could be due to iron sulfide pyrrhotite cracking damage from inclusions in the original concrete as mixed and placed.

See FOUNDATION DAMAGE by MATERIAL or INCLUSIONS for details.

• Concete Crack Sealant hides ongoing movement: Using a flexible sealant in a structural crack may make it difficult to see small amounts of future movement since the sealantmay stretch and tolerate small increases in crack width.
  This can increase the difficulty of crack monitoring and diagnosis
• Sealant doesn't fix a structural problem: Using a sealant in a foundation or floor slab crack may fail to address the underlying cause of the problem such as foundationsettlement or frost heaving which need to be addressed
• But some epoxy sealants are in fact also a repair: Some structural epoxies may seal and also 'repair' the damage in a structural sense.
  For example Sika produces epoxy resin & structural engineering systems for concrete crack repair using a structural injection system. http://usa.sika.com/

We may decide to seal a cracked wall or floor slab anyway, to try to reduce water entry in a building, but remember the implicationsof sealing we've just listed.

Types of Sealant Repairs in Concrete & Concrete Structures

Control joint sealant:

control joints, designed to control where cracks appear in concrete placed horizontally or vertically, are themselves are sealed against water, frost, debris and to handle recurrent movement (listed below) using a flexible sealant.

Recurrent movement:

Seal and repair cracks in concrete using injection of either epoxy resin or polyurethane where recurrent movement is expected (in my opinion a flexible sealant is what's called for here) when the movement is considered normal, perhaps caused by traffic vibration or temperature swings, and where the movement is not threatening structural damage.

Stop a water leak:

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Seal and repair cracks in concrete when it is important to quickly stop water leaking through a crack,seal those cracks in concrete using injection of either epoxy resin or polyurethane sealant.

Structural repair:

Seal and repair structural cracks: for some situations (usually where an engineer, expert in masonry repair has made an assessment), actual structural repairs are made using injectable epoxy resin and possibly in some (probably low-load) cases by using injectable polyurethane sealant.

In my opinion (I'm not a P.E.) this application of injected epoxy resin is probably most-appropriate when cracks have appeared due to non-recurrent stresses. When we epoxy cracked structural concrete or a concrete slab that was broken by an initial installation error or by a subsequent event, we expect the repaired area to be as strong or stronger than if no crack were present.

Structural reinforcement:

some sealant manufacturers such as Sika provide seal or wrap-and-seal systems that actually provide additional strength to existing concrete structures such as concrete columns.

Structural reinforcing wrap systems may include use of synethetic fabrics, carbon fibre fabrics, and other wraps. These products might also be used on steel or concrete that has been damaged by weather, frost, corrosion.

Surface repair:

for working surfaces such as industrial work floors or parking decks cracked by spider-webbed shrinkage cracks that are non-structural, special 'healer-sealer' crack sealants may be applied.

Specialty Epoxy Resin and Structural Engineering Systems for Concrete

The situations listed above are those of most-interest to owners of residential and smaller commercial buildings. Below are examples of additional applications of epoxy resins and sealants in construction:

Anchoring epoxy products:

used to inject into holes into which connectors such as bolts or post anchors will be placed.

Bonding agent epoxies: used to promote secure bonding between abutting, often different materials. A special example is the use of segmental bridge adhesives that bond post-tension pre-cast concrete bridge segments to other structural members or to one another.

Epoxy resin adhesives are formulated for bonding concrete to concrete, concrete to other masonry, concrete to steel, and concrete to wood or to other substances.

Overlay expoxy resin coatings:

used as a coating on concrete surfaces for skid protection or for patching damaged surface areas. Some of these epoxy resin products are formulated for use as a high-friction surface or for use to repair spalled or chipped concrete surfaces.

Special acknowledgement: some of this discussion was informed by reviewing products and engineering literature provided by

• Sika Corporation, 201 Polito Avenue, Lyndhurst, NJ 07071 USA, Tel: 1-800-933-SIKA Website: http://usa.sika.com/. Sika produces a wide range of adhesive, bonding, reinforcing and sealing products including automotive adhesives, building envelope adhesives, concrete admixtures, flooring and floor covering adhesives, repair & protection products, rofoing products, sealants, adhesives, waterproofing agents.

How to seal a cracked masonry foundation wall or floor slab

To seal a shrinkage crack or a control joint crackin a poured concrete slab, regardless of whether or not it has occurred at an expansion or control joint,it may be appropriate to seal the surface to resist water entry and radon gas entry.

Control joints are also sealed to provide a smooth and clean concrete surface which does not collect surface moisture or debris. Keeping surface water from entering at a control joint might in some cases also help avoid uneven soil settlement below the slab and tipped or heaved slab sections.

To seal a control joint or expansion joint crack in a concrete slab use a flexible sealant designed for foundation crackrepair or sealing. The radon mitigation industry offers special foundation caulks and sealants for that purpose as well. (If water is coming upthrough a concrete floor or slab, sealing cracks is probably not going to be enough - you need to address the causeof water below the floor.)

List of Types of Materials Used to Fill or Seal Poured Concrete Slab Control Joints

• Pre-fabricated control joint strips of plastic or other material
• Masonry caulks or flexible (typically polyurethane based) radon sealant caulks.
  Pproduct sources are listed at SEAL CRACKS BY POLYURETHANE FOAM INJECTION - live link just below.
• Semi rigid epoxy resin crack fillers (discussed in this article).
  Epoxy products intended for concrete expansion joints.
  These products are described and their sources listed below in our discussion of this method that includes injection ports and (usually) two-part epoxy resins injected into the crack.
• Special poly urea caulks designed as a joint filler - polyurethane foam injection to seal cracks. This method is discussed below.
  Separately we discuss this method in detail along with product sources
  at SEAL CRACKS by POLYURETHANE FOAM INJECTION
• Hydraulic cements for concrete or other masonry floor, foundation, or wall repairs of cracks & leaks

Any sealant that is going to be used to fill a control joint in a slab needs to have the ability to bond to the sidesof the cut or opening of the joint, to remain flexible over temperature and moisture changes, and to withstand bothcompression and expansion as the concrete moves in response to curing and in response to temperature and moisture changes.

Epoxy, for example, is not generally used to fill the expansion joint in newly-poured concrete because the concrete is justtoo wet and has too much movement for the epoxy to bond and perform acceptably. Similarly, a special product would be neededto fill a control joint in concrete in very cold weather.

Watch out: select the right product for your application. Some crack fillers like the spray injection product Good-Bye Cracks elastic crack 'cover' product shown in our photo are not suitable for concrete.

The manufacturer says this product can be used to fill cracks in plaster, drywall, and wood and that it dries to a flexible, paintable finish.

Typical concrete crack preparation for sealant with an epoxy product

Carson Dunlop Associates' sketch shows three common methods used to seal cracks in masonry walls in an effort to stop foundation leaks.

[Click to enlarge any image]

1. Cleaning a concrete crack for patching and sealing:
   The crack or expansion joint must be cleaned of all debris; power washing and vacuuming may work but inspect the crack or joint to be sure it is clean along its entire length.
   Otherwise the sealant may fail to adhere and the crack will leak. Wire-brushing or chipping the crack sides is not normally required when using an epoxy sealant.
   (Traditional concrete or masonry patching (and plaster repairs), on the other hand, was traditionally applied after cracks were chipped and widened into an inverted 'vee' shape to help keep the patch material in the crack.)
2. Mix the epoxy sealant
   according to the manufacturer's directions.
   Some low volume epoxy sealants are supplied in a dispenser which mixes the two parts together in the proper ratio automatically during dispensing from a tube (see your dentist for an example.) Other crack sealants such as prepared caulks, may come pre-mixed in a tube.
3. Install the backer rod
   specified by the sealer manufacturer if you are sealing an expansion joint.
   The backer rod provides a flexible filler that keeps the sealant in the joint during curing and also reduces the total volume of concrete crack sealer epoxy needed.
   We recommend using backer rods also if sealing wide cracks in concrete walls or floors, but first you need to have those cracks evaluated to determine the cause as other important structural repairs could be needed first.
4. Sand
   (clean, dry) may be used as a filler for cracks - an easier approach if you are repairing an irregular crack in a floor. Sand should stop about 3/4' from the top of the floor surface to give adequate volume of sealer epoxy in the crack.
5. Epoxy crack patch cure time:
   Keep traffic off of the sealed crack until the sealant has cured.
   Typically a thin film will form on a crack filler epoxy in 8 hours, the sealant will be hard enough to withstand traffic in 24 hours, and it will fully cure in about a week.

These notes are based on epoxy product application information available from Lone Star Epoxies.

Use of Polyurea as a control joint filler or crack sealant in concrete slabs (Polyurethane Foam Injection Method)

Polyurea joint filler products for concrete control joints have received attention for filling concretecontrol joints and is increasingly used in that application since this material is resistant to moisture, has high adhesionproperties when used with concrete, and will cure in very cold weather.

Polyurea sealant products are also reported to be usefulin sealing control joints in 'green' concrete which has not yet had its full 28 day period of initial curing.

An advantage of polyurea or polyurethane used as a crack sealant is that the flexibility of the material will accommodate slight seasonal or temperature-related movement that might otherwise cause new cracks in an epoxy-repaired structure or reopening of repaired cracks in a mortar or concrete-repaired crack.

Also in our experience, because the injected polyurethane foam expands after it is injected into a crack, you may find it easier to fill the crack through the building floor slab or wall than when using alternative repair methods such as epoxy or masonry repair kits.

Carson Dunlop Associates' sketch (left) illustrates foundation wall crack repair using epoxy or polyurethane injection.

List of materials, product sources, how-to for polyurethane foam crack sealant

• See our separate article POLYURETHANE FOAM INJECTION CRACK REPAIRS for details of the procedure for using polyurethane foam to seal foundation cracks and for a list of sources & brands of polyurea or polyurethane foam sealants, caulks, and crack repair kits. .

Use of Epoxy-supported Grouts as a control joint or crack filler in concrete slabs or walls

Epoxy grouts are available which can be mixed and troweled into a concrete crack or joint. These products are less flexible than the control joint fillers described above. In our opinion, an epoxy grout filler may work fine to provide a well-bonded repair to a stable wall or floor crack in an area where there is no anticipated further movement such as from temperature or moisture variations.

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Watch out: while epoxy, a material stronger than concrete alone, is sometimes used for structural repairs, if your building masonry (concrete, brick, masonry block) is cracking due to ongoing movement, settlement, frost, or other active or recurrent problems, unless those conditions are fixed, sealing a crack with epoxy will not prevent new cracks from forming.
But read the manufacturer's intended application before buying an epoxy reinforced grout for building crack repair. Do not use the product where it was not intended unless a call to the manufacturer provides you with good information that it's acceptable to do so.

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On the one hand we like using a rigid material to fix a supposedly stable crack in a residential building since the fact that the patch is inelastic means it will be easy to see if there is ongoing or new building movement.

On the other hand, a small amount of moment in a foundation wall or floor slab due to changes in temperature will leave a flexible crack filler un-damaged and sealed where a rigid material may fail.

List & Sources of Epoxy Grout Products & Structural Injection Concrete Repair Products

• Elasta Gel 6170 Epoxy® flexible epoxy and expansion joint sealant, two component, no VOC, flexible epoxy resin which is rated for use on parking decks and, according to the manufacturer, Lone Star Epoxies, can also be used to repair concrete spalling on such surfaces.
• Emecole Structural Repair Epoxy products, 50 E. Montrose Dr., Romeoville IL 60446, USA Tel: 800-844-2713, Website: www.emecole.com
  • Emecole 121 Premium - Fast Curing Crack Repair Epoxy
  • Emecole 101 Original Formula - Crack Repair Epoxy
  • Emecole 40 - Concrete Deck Repair Epoxy
  • Emecole cargon fiber reinforcement products: used by professional repair contractors where a building wall suffers large cracks or bowing. (Also see Bulged foundation Repairs.)
  
  Quoting: Emecole Epoxies are recommended for structural cracks. Unlike polyurethane foams, epoxies will not expand. However, they cure to an incredibly hard solid, making them ideal for structural repairs. Our two-component epoxies are strong, durable and reliable. After curing, Emecole Epoxies create a more powerful bond than concrete, resulting in an extremely strong repair.

• Epoxy grouts (ANSI 118.3 1988) for tile applications (watch out for proper tile surface cleaning within the cure time; cleanup is done using water)
• Laticrete™ epoxy grouts
• Mar-Flex Epoxy Injection Concrete Crack Repair Kit, Tel: 800-498-1411, Website: www.mar-flex.com
  Quoting: Low-pressure injection and repair of approximately 8-10 lineal feet. Contains the following; (1 jar 8oz.) Mar-flex Crack Seal (1 jar 8oz.) Port Adhesive (2) Wooden Sticks (12) Surface Ports and Caps (2 Cartridges) Mar-flex Injection Polyurethane/Epoxy Resin 1:1 (2) 3/8' :24 Mixing Nozzles (1) Injection Hose Assembly with plastic shut-off valve (2 pair) Latex Gloves (1) Plastic Trowel (1) Wire Brush (1) Drop Cloth (1) Complete Instructional DVD. The company also offers low-viscosity crack filler that can be injected into both dry and damp cracks and a 'Seal & Peel' crack repair kit.
• Polygem Epoxy pastes, resins, & grouts, POLYGEM, Inc.P.O. Box 609West Chicago, IL 60186, Tel: 888-446-3987, Website: www.polygem.com
  • Polyject #1001 LV for hairline to 1/16” cracks
  • Polyject #1001MV for 1/16” to 1/8” cracks
  • Polyject #1001 HV for 1/8” to 1/4#148; cracks
  • Polyject #1001 EHV for over 1/4#148; cracks

  Quoting: Polygem, Inc. manufactures epoxies, vinyl-esters and silicones for various industries and markets such as: Construction, Waterproofing, Mining, Industrial, Electrical, Zoological, Golf and DIY. Polygem, Inc. has been custom formulating epoxies in excess of twenty-five years and has created over 1,500 unique formulations.

• Ramset™ Epoxy Grout - a pourable epoxy resin, also can be mixed with sand and applied with trowel.
• Shor D 65-80®
• Sika Corporation, 201 Polito Avenue, Lyndhurst, NJ 07071 USA, Tel: 1-800-933-SIKA Website: http://usa.sika.com/. Sika produces a wide range of adhesive, bonding, reinforcing and sealing products including automotive adhesives, building envelope adhesives, concrete admixtures, flooring and floor covering adhesives, repair & protection products, rofoing products, sealants, adhesives, waterproofing agents.
• Other semi-rigid, epoxy products - contact us to add products & information - no fees involved

Use of Peel-and Stick Surface Sealant Kits for Epoxy-Repair Cracks in Concrete Walls

For building surfaces that will remain exposed and where cosmetic appearance is a concern, RadonSeal (and perhaps others) offers a stick-on injection port system that allows the crack material to be injected into a wall or floor crack to leave a smooth surface even with the existing wall or floor surfaces on either side of the crack.

RadonSeal's Easy-Peel Sealer kit uses a surface seal that includes injection ports. After the injected sealant has cured, the surface seal and injection ports are peeled away from the repaired surface.

Quoting: RadonSeal Crack Seal and Port Adhesive 1:1 is a high modulus epoxy gel designed for surface sealing of cracks prior to injection and for attaching surface ports. It can also be used for bonding miscellaneous materials to concrete.As with any epoxy adhesive, surface preparation is critical.

Concrete surfaces should be cleaned by wire brushing or other mechanical means. All loose or unsound material must be removed. Surfaces should be dry and dust free to insure a superior bond. Application onto wet surfaces is not recommended.

• RadonSeal Crack Seal & Port Adhesive, Novion, Inc., 18 L'Hermitage Dr., Shelton CT 06484, Tel: 800-472-0603, Website: www.radonseal.com
• Mar-Flex Epoxy Injection Concrete Crack Repair Kit, Tel: 800-498-1411, Website: www.mar-flex.com
  The company offers low-viscosity creack filler that can be injected into both dry and damp cracks and a 'Seal & Peel' crack repair kit.

How to Choose Between Using an Epoxy Crack Sealant & a Polyurethane Crack Sealant

High Pressure or Low Pressure Slab or Foundation Wall Crack Sealant Injection

Most foundation wall and floor slab cracks can be successfully filled and sealed using low-pressure injection such as that afforded by a caulking gun and cartridge of sealant, or similar device for both epoxy and polyurethane foam sealants.

Professional polyurethane foam injection crack sealing applicators may use a high-pressure injection system using equipment that includes a pump or a device similar to an automotive grease gun and pressures up to 250 psi. Drilling to prepare injection ports along the crack may be required.

Reasons to use an epoxy-based crack repair or sealant method

• Structural repairs are needed. Epoxy, properly mixed and installed, is stronger than the concrete to which it is bonding. But as we warn above, unless you correct the cause of ongoing (such as settlement or earth loading) or cyclical (such as water or frost) structural movement & cracking, new cracks will develop in the structure
• Very fine cracks need to be sealed - so fine that it may be difficult to inject a polyurethane foam, but still possible to inject a low-viscosity epoxy sealant. For wider cracks use a higher viscosity epoxy preparation or consider a polyurethane injection approach.
• May provide a smoother surface for finished surfaces such as a finished basement (but seal & peel systems can also accommodate this need)

Reasons to use a polyurethane foam injection crack sealant

• Structural repairs are not needed but sealing a crack against air or water leaks is required
• If you expect small amounts of recurrent or cyclical building movement that might re-crack or leak where a rigid crack sealant is applied, using a flexible polyurethane sealant may prove more durable
• If the crack is large an expanding polyurethane foam sealant may be easier to install and is also likely to be less expensive per linear foot of crack.
• If the crack extends through the foundation wall and is large, an epoxy sealant may simply leak out of the other side of the wall or floor slab - the foam injection approach may also cause foam to extrude through the structure but is more likely to also successfully fill the crack.
• If the crack is actively leaking an epoxy injection sealant is more likely to fail to bond properly to the masonry surfaces and a polyurethane sealant will perform more reliably. (Polyurethane sealants are moisture activated.)
• Do-it-yourself repairs: homeowners may find this approach easier than the epoxy approach

Use of Hydraulic Cement for Repairs in Concrete Slabs, Foundations, or Masonry Block/Brick Walls

Hydraulic cement used for crack repairs is a waterproof cement product that is sold dry and mixed with water on the job to prepare a slurry that is painted (or troweled) over a crack in a foundation wall or in some cases (DryLok or UGL foundation wall sealants) painted over the wall surface in a paint mixture.

Hydraulic cement is often lower in cost than the epoxy and polyurethane products described here. However, the repair is at risk of re-cracking due to product shrinkage or due to even slight movements in the structure from settlement, earth or frost pressures, or thermal changes.

A principal advantage of hydraulic cement for repairing & sealing cracks in masonry floors or walls is that it can handle and effectively seal a crack even when there is significant active water leakage through the crack. The cement, properly mixed (as stiff as possible) cures and expands to seal the crack in the presence of water.

[As early as 1969 we used this approach to successfully seal holes in a masonry block foundation wall through which groundwater was squirting several feet into the basement. We mixed a plug of hydraulic cement and forced it into and against each hole until it set firmly enough to remain in place - about 3 minutes. - Ed.]

Watch out: any cement product is caustic and can cause skin burns or eye or lung damage if you do not handle it properly and wear the proper protective gear for eyes, lungs, skin.

For an effective repair of a wall or floor crack using hydraulic cement you will need to clean out and widen the crack to approximately 1' wide x 1-1/2' deep, ideally with the crack wider at its innermost and more narrow at the wall or floor surface (a 'V' shape) to give a mechanical bond to the cured concrete patch.

For a wall crack through which water is seeping, seal the crack progressively beginning at its highest point on the building wall. When all of the crack has been sealed except a single pressure relief point, that final leak is sealed with a molded plug of cement.

List of the Types of Tile Grouts, their Bonding Agents and other Properties

• Hydraulic cement
• Portland cement grout (ANSI 108.10 1985)
• Latex-modified portland cement grouts (we've used these with good success indoors in tile floors for better water resistance)
• Epoxy grouts (described above) usually with a water-dissolved mix
• Modified Epoxy grouts (ANSI 118.8 1988.) combining portland cement and epoxy (some sources say the chemical resistance is similar to straight Portland cement but the result is a harder grout - just what chemicals are resisted may vary among products)
• Furan resin grouts use furfuryl alcohol in their products - no water is used. These are highly acid-resistant, also difficult to install.

Questions and answers about sealing floor cracks in Slabs

Question: should we seal floor cracks exposed when we pulled up basement carpeting? Our house had a radon problem

We have had our home mitigated for radon. The latest reading is 2.9. Therefore, we feel we are controlling the radon successfully.We are in the process of removing carpet from a 8' X 12' bathroom in the basement. In preparation for installing an overlay linoleum we found a irregular crack in the concrete floor.

Our question is - should we seal this crack with some time of sealer that could be painted on and would this be effective? If so, what type of product would you recommend? - F.H.

Reply: In general, sealing exposed floor slab cracks is a good idea where there was a radon concern, though if your mitigation system is working it might not be critical.

A competent onsite inspection by an expert might find other cracks, openings, or sources for radon gas leakage that you'd want to address, but usually that would have been done when your radon mitigation system was installed.

Following the installation of a radon mitigation system the installer is expected to make a follow-up radon gas level measurement to be sure that the mitigation is working as expected, and s/he also inspects the home to see if you have exhaust fans or other features that could accidentally interfere with proper operation of the radon mitigation system itself.

If all of that was done, the added radon gas leakage into your home from pulling up carpet to expose a floor crack should not be significant.

Nevertheless it would be good practice to seal any floor slab cracks that you expose.

Here are some things to consider:

If your floor slab cracks are hairline (less than 1/8' across - see our photograph above left) they may be normal concrete shrinkage and they may be too fine to seal with a polyurethane sealant (see our polyurethane sealant link below).

In that case a sealant paint or a combination of mesh tape and sealant used to cover cracks in concrete before installing tile would still work well.

Particularly where ceramic tile is to be installed on a slab we want to protect against telegraphing of slab cracks up through (and breaking) the tile.

See details at TILE ISOLATION MEMBRANES where we discuss Custom Building Products' RedGuard Uncoupling Mat & Uncoupling Crack Isolation Seam Tape and membranes (shown above) and other Concrete Slab Crack Suppression and Isolation Joint Tape products.

and also see FLOOR TILE INSTALL on CONCRETE.

For larger floor slab cracks (such as shown in our photo below) there are some sealants ('caulks') widely used by radon mitigators that work well in floor or wall cracks, typically polyurethanes. The crack is vacuumed or cleaned, and when dry, sealed.

More sophisticated floor slab crack sealing is done if you are going to put down ceramic tile: the repair includes a strengthening mesh glued to the floor over the crack to resist the transmission of the crack upwards through the ceramic tiles. Under carpet you don't need that step.

At CONCRETE SLAB CRACK REPAIR

and SEAL CRACKS by POLYURETHANE FOAM INJECTION at we discuss methods to seal cracks in floors.

At HOW to REMOVE INDOOR RADON we include more details on how to get the radon level down in homes. You'll see that sealing and caulking to stop radon gas from entering at floor or wall cracks or joints is important.

While you're at it, don't forget to check for gaps that may have opened between the floor slab and the foundation wall - seal those too.

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• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
• Avongard foundation crack progress chart for structural crack monitoring
• Basement Moisture Control, U.S. Department of Energy
• Crawl Space Moisture Control, U.S. Department of Energy
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