Sill Plate is a term used to describe the wooden board used in any one of a number of applications. So we will cover each of the different uses for the term sill plate so that you can find the particular use that fits your needs. Let’s begin our discussion by talking about the type of sill plate used on top of a foundation wall.
Foundation Sill Plate
Here we are going to talk about the type of sill plate that you would use if you were preparing the top of a concrete or block foundation wall for a new floor system. First let’s touch on the basic preparation that you need to have anchor bolts or seismic anchors properly installed in the concrete at the correct interval spacing for your local codes. Make sure these anchors are imbedded in the concrete so that they will land in the center of your sill plate. You must determine if you will place your sill plate flush with the outer edge of the foundation, this will cause your sheeting to hang out beyond the foundation wall. Or will you choose to set your sill plate back the thickness of your sheeting so once in place you can moisture seal the entire enclosure to the foundation with a product like Tyvek straight flash?
The sill plate or mud sill as it is commonly referred to will typically be a pressure treated 2×6 plate anchored in place running the entire length of the wall. It is common to install an approved sill seal or sill gasket between the sill plate and the concrete foundation. DO NOT confuse the use of sill seal as an excuse to use untreated wood for the sill plate. The important thing to remember here is we are considering this connection to be directly to the concrete and there will be a certain amount of moisture that will get to this sill plate. It is our practice to use Borate treated sill plates in all of our projects since this also gives an added level of termite protection.
As a last note on the foundation sill plate if you are trying to increase the ceiling height in the basement of your project then consider adding one or two additional sill plates which can be untreated wood to the top of your first treated sill plate. Each added plate will add an additional 1-½” of height to the ceiling of your project. Just make sure that by lifting the sill height that you do not exceed the standard stud wall height of your basement framed walls.
Wall Sill Plate
The second type of sill plate we will discuss is the bottom plate or sole plate of your primary walls if they are installed directly on a concrete surface. In this case you will use a pressure treated bottom plate and staple the sill seal directly to the bottom of it before setting the wall up into place. Once again it is important to make sure the anchor bolts or seismic anchors are correctly placed to hold down the sill plate and wall. You will also need to make a decision as to if you will place these plates at the edge of the concrete or set them back the thickness of your sheeting to be able to better seal this lower wall transition.
The last area that I want to mention here is the use of a pressure treated bottom or sill plate on any basement wall or any other wall that will come in contact with concrete. It is very important to remember that with any concrete contact it will be possible for small amounts of moisture to wick into the wood in contact with concrete over time. You must use pressure treated wood to protect this contact from rotting.