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How to Waterproof a Block Foundation From the Exterior

Waterproofing a block foundation wall is critical to keeping the underside of a home as dry as possible and preventing the growth of mold and mildew. Homes built to current building codes should have waterproofed foundations but sometimes the waterproofing is poorly done, or not done at all. Today we will discuss how to solve this water management problem and how to prevent it from recurring in the future.

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Why Do I Need to Waterproof My Block Foundation Wall?

Modern building codes require the expulsion of moisture laden air from crawl spaces via foundation vents. However, most vents are static (meaning they don’t have a motor) and struggle to keep up with additional moisture in less than ideal conditions. One of these conditions can be a block foundation wall that is allowing water to pass through into the crawl space.

Block foundation walls are designed to be porous so they will absorb the moisture from the mortar. However, the porosity also allows the block to absorb groundwater, which can be transferred to the framing of the home. To prevent this absorption block foundations should be waterproofed, a vapor barrier installed, and in some cases a french drain may be needed as well. Together, these actions should prevent moisture from entering the crawl space or basement block wall from exterior moisture.

Can I Waterproof My Foundation Wall Myself?

Most homeowners will have the skills to waterproof a foundation, but the project can be a lot of work. The project will require digging, but hiring a trencher and operator will ease the workload. Also consider drafting help for this part of the project as some steps may require an extra pair of hands. Waterproofing is a dirty job, so you’ll want to dress appropriately to avoid getting the waterproofer on your skin.

Block foundations differ from poured concrete foundations in that they have mortar joints, which are very water permeable. Most DIY applications will begin by covering all of the mortar joints with waterproofer first, which adds extra thickness to the joint. The rest of the foundation is then waterproofed the same way, covering the mortar joints again. After the material has dried for 24 hours, a second coat is applied (the third for the mortar joints) and allowed to completely cure for 3-5 days before replacing the dirt.

Will I Need Expensive Tools to Waterproof My Block Foundation Wall?

Not necessarily. Having power tools like a trencher will certainly make the job easier, but it can be done with just a shovel and paint brush. Most of the work (approximately 70-80 percent) will involve exposing and replacing the dirt, so some form of earth moving tool is recommended. Extra help is always welcome on these projects, so consider drafting a friend or neighbor for additional digging help if a powered tool is unavailable.

How Do I Waterproof My Block Foundation From the Outside?

Step 1 Remove the Soil

The first step to waterproof a block foundation is to gain access to the foundation wall. Often shrubbery, paving stones, and landscaping must be relocated to make room to work. Exposing the foundation can be done by hand, but is usually accomplished with a small trenching machine designed to bury cables and pipes. The soil should be removed approximately 12”-16” away from the block foundation wall to a depth that reaches the concrete footing of the home.

Step 2 Prepare the Wall For Waterproofing Material

Foundation waterproofing is usually available in liquid form and sold by the gallon. The waterproofer can be made from any number of materials, but the most common are tar, rubber and elastomeric paint. Each method requires a dry, relatively clean masonry surface to adhere to the foundation wall. Professionals will usually pressure wash or rinse the foundation after  excavation of the topsoil to remove any mud or rocks that might prevent adhesion.

Another common method uses a thin rubber membrane or polymer modified asphalt sheet to deflect water from the foundation. These products work much like flashing does on a roof, by deflecting groundwater away from the foundation wall. Used in conjunction with liquid waterproofing, waterproofing membranes extend down the wall and overlap the footings to prevent water from reaching the block foundation wall.

Step 3 Spray, Roll, or Brush On the Waterproofer

Foundation waterproofers can usually be either sprayed or brushed onto the wall. Professionals tend to spray the waterproofer because it is fast, but most homeowners will use a coarse brush. Coarse (often disposable) brushes are commonly used to apply waterproofing because the material can be thick and difficult to spread around with a small brush. Most waterproofing products require at least 24 hours of drying time, so it’s a good idea to plan around the weather forecast.

Sprayed waterproofers are generally oil based sealers or elastomeric paint, and applied in several thin layers. Oil based waterproofers are caustic to the skin, so pros will wear protective clothing and a respirator while applying it. Elastomeric paint waterproofer is the most common version used by homeowners, as it is simple, often water soluble, and relatively safe to use. Elastomeric paint is made from elastic polymers (hence the name) and typically includes a mildewcide as well.

Fortunately, elastomeric paint does not require more than two coats for effective waterproofing. In contrast to sprayed waterproofers, elastomeric paint is very thick and forms a dense but flexible coating after it cures. Most manufacturers of elastomeric paint recommend two coats and an estimated 3-5 days for complete curing. Elastomeric paint tends to emit fewer fumes (also known as VOCs) as well, making it a popular choice for indoor basement waterproofing.

Keep Your Home Healthy

Block foundation waterproofing is a relatively simple project, but it can be labor intensive. Using the appropriate tools makes the job easier, but doing it correctly makes it last. Being thorough and covering every inch of the foundation wall will ensure you only perform the project once, so diligence and attention to detail is important. Waterproofing a block foundation wall requires time and elbow grease, but the resulting dry foundation is worth the trouble.

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