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What Is a Sill Plate?

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  • Written By: Caitlin Kenney
  • Edited By: Bronwyn Harris
  • Last Modified Date: 01 October 2014
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A sill plate, or mudsill, is a board that is laid horizontally over the foundation of a building. The vertical members of the framing called studs can then be constructed on top off the sill plate, which is anchored to the concrete foundation. This allows a flat, even surface for the construction of the framing and a buffer between the framing and the foundation. As concrete tends to let moisture through, a treated piece of lumber below the framing can help keep the studs from rotting.

Many houses do not have a mudsill, and of those that do, there are several ways of installing it. Commonly, the plate is anchored to the foundation with an anchor bolt, a large bolt with either a hook or expansion wedge at one end and threading for a nut at the other end. The hooked end of the bolts are placed at measured intervals in wet concrete and left while the foundation hardens. Holes are then drilled into the sill plate at the same intervals so that when the board is laid on the foundation, the anchor bolts come through the holes. The nut can then be screwed onto the threaded end of the bolt, usually with a tool called an impact driver.

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Today, many houses also have a sealer made of polyethylene foam that sits between the mudsill and the foundation. This prevents moisture from coming through the concrete or ground and rotting the sill plate. Usually, however, the foundation is built above the finished grade or the top surface of the ground, so that the plate does not come in contact with the soil or lawn. This helps protect the wood from termites and rotting.

Sill plates are often pressure treated to prevent damage from insects and moisture. Pressure treated wood is wood that has gone through a process of being placed in a tank, depressurized, and then repressurized with a preservative chemical. This pushes the preservative, which is resistant to moisture and a deterrent to insects, thoroughly into the wood. The chemical that has been used most prevalently until very recently, chromated copper arsenate (CCA), is very toxic and is today replaced with less toxic alternatives. Buildings that already have CCA treated mudsills or other wood members are considered safe, and removing these boards may release more toxins than leaving them in.

The other purpose of a sill plate is to make a level platform for the framing. Because concrete is uneven, it would cause the studs to reach different heights if they were all cut to the same length and mounted directly onto the foundation. If the concrete was poured well, the sill plate can simply be laid down, but if the concrete is uneven, shims may need to be placed under the plate. A shim is a wedge-shaped piece of wood or another material that lifts the board in place so that it is level. The studs can then be anchored to the mudsill through a variety of methods, depending on the architect’s preference.

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anon302019
Post 3

You can't. The wall is built on top of the plate which is just a piece of lumber laid on top of the foundation piers.

anon291464
Post 2

How would one install a sill plate in a home that has already been built? I have one room in my house that is in need of insulation, but before I can insulate the room I need to install sill plate in the walls. I keep looking online for how to videos, but they are for homes in the process of being built. Any answers?

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