How Do You Keep Water From Going Under A Sill Plate?

The sill plate is the bottom horizontal piece of wood that supports a home’s wall. It is made from pressure-treated lumber or metal, and it usually sits on top of a concrete foundation.

To keep water from going under the sill plate, you should ensure that there are no cracks in it or around it. It would help if you also sealed any gaps between the sill plate and the foundation with waterproof caulking.

How Does A Sill Plate Work?

In wall construction, a sill plate (also known as a sole plate or simply a “sill”) is the bottom part of the wall structure that attaches to the wall studs.

As a very important component of every house, they are usually anchored to the foundation.

Sagging floors can occur if the sills become damaged or rotten. In addition, both air and water can leak when two different materials come into contact.

There is a gap between the concrete foundation and the wood sill plate because of the different rates at which they contract and expand in response to heat and cold.

In the construction of a home, a filler is used to fill the gap between the sill plate and foundation.

Water can seep into a basement through cracks or chips over time. However, the sealant can be repaired even by a DIY novice, so water cannot enter under the sill plate.

Inspect And Repair Sill Plates

It occurs when the ground outside is higher than the wall or slab on top of your foundation, causing water to leak into your basement or slab.

Exterior siding buried below dirt or concrete can cause water to seep into the 1st-floor framing or slab if the siding is buried below the surface.

The ground level of your home is usually raised just enough for water to enter through the ground when a contractor installs pavers or new concrete over old concrete.

An exposed foundation should be at least three inches above the ground, and the siding shouldn’t be buried below the ground. 

In addition to inviting bugs (especially termites), buried siding is also an invitation to pests. It can be a direct path for insects to travel up the soil when it is up against your house’s framing and siding.

How To Block Water From Going Under A Sill Plate?

This is a guide for homeowners and DIYers who are looking for ways to prevent water from going under a sill plate.

The most common way to seal off water from going under the sill plate is by using caulk. This will not only block the water but also provide a nice seal that will last for years.

However, if you are not comfortable using caulk or want to make sure that the caulking job lasts and does not need any repairs, there are other things you can do. One of these things is to install flashing around the perimeter of the sill plate.

The flashing will go on top of the siding, and then it will be nailed in place with galvanized nails. Once this has been done, it should be sealed off with tar paper or asphalt.

The following section will provide you with a DIY guide on how to block water from going under a sill plate.

Step 1

Ensure you read the sealant’s instructions carefully so that you know the safety requirements and how to use it properly. For sealing an area that requires more than one can of expanding foam sealant, you may need to use more than one can. 

Step 2

It is possible to reseal the perimeter as a whole, which would require several cans, or just the area that appears to be leaking, which would require one can.

As you seal the sill plate, choose a day when the weather is dry to prevent water from entering.

Step 3

As the foam sealant is being applied, open the basement windows and doors to allow proper ventilation. Protect your skin, lungs, and eyes while applying sealant with work gloves, masks, and goggles.

Step 4

Foam sealant cans should be lidded. Remove the lid. To remove the plastic covering from the applicator, remove it from the can by untapping it or wrapping it in plastic. A foam sealant’s nozzle has a mark where the applicator should be inserted.

Step 5

If you are sealing the entire perimeter of the basement, start at one corner or 4 to 6 inches from the leak. Press firmly on the release button after aiming the applicator tip into the space between the cement foundation and the sill plate.

Step 6

Using foam sealant, fill the gap, ensuring it touches both the top and bottom of the space before moving the applicator slowly along the seam that needs resealing. When the sealant dries quickly, it expands, creating an air- and watertight seal.

Step 7

Apply the applicator all the way down the sill plate and fill in either the entire perimeter of the basement under the sill plate or 4 to 6 inches on the other side of the leaking area.

Step 8

The next step is to seal the rim joists, where the beams that support the floor intersect with the walls of the foundation.

On the interior and exterior sides of the basement, spray foam is around the bottom and top of these joists. In addition, the space surrounding them on either end needs to be sealed airtight and watertight. 

Step 9

As a precaution, if you are not spraying the perimeter, you should seal the rim joists where the water leaks.

Step 10

Make sure the foam has completely dried and that the basement is completely ventilated before you close the basement doors and windows and they close all the way. Make sure sealant cans are disposed of in accordance with the manufacturer’s instructions.

The Bottom Line

Using a sill sealer (also called a sill gasket) before laying down the sill plate is the best way to air seal the sill plate. There are several widths of sill sealer, from one inch up to ten inches.

The foundation perimeter is rolled out with the sheet over the concrete. In addition to conforming to the concrete surface, the flexible sill sealer product makes it possible to seal any irregularities in the surface.

If water migrates up through the concrete, you should select a closed-cell foam that provides both an air seal and a capillary break to prevent moisture from reaching the wood of the sill plate. In addition to preventing rot, the product also prevents insect and rodent infestations. 

The sill plate is sometimes sealed to the foundation wall with two large beads of caulk, but sill seal products which cover the whole sill plate area are preferable, as they offer waterproofing capabilities as well as uniformity.

In this case, the lip of the sill plate is sealed with caulk, while that of the rim joist above is sealed with paint. 

Sill plate joints and seams with rim joists should be caulked before air sealing and insulating the entire area between the sill plate and rim joist. Caulk can also be applied to the hole in the sill plate where the anchor bolt protrudes.