A sill is a board anchored to a concrete foundation, creating a level surface for framing construction and preventing moisture damage. Sills are often pressure treated and protected with polyethylene foam. Anchor bolts are used to install sills, and shims may be needed for uneven concrete. CCA-treated sills are safe but should not be removed.
Also referred to as a sill, a sill is a board that is horizontally anchored to the concrete foundation of a building. Vertical framing elements, known as struts, can then be built on top of the sill.
Why use a windowsill?
The primary use of a window sill is to create a level, even surface for framing construction. Since the concrete is uneven, the posts would reach different heights if they were mounted directly on the foundation.
The sill also acts as a buffer between the frame and the foundation. Because concrete tends to let moisture through, a piece of treated lumber between the foundation and the framing can help prevent the studs from rotting.
If the concrete has been poured well, the sill plate can simply be laid, but if the concrete is uneven, shims may be needed under the plate. A shim is a wedge-shaped piece of wood or other material that lifts the sill to level it. The studs can then be anchored to the windowsill.
Installing a window sill
The most common method of installing a window sill:
The plate is anchored to the foundation with anchor bolts, which are large bolts with a hook or expansion wedge at one end and thread for a nut at the other end.
The hooked ends of the bolts are placed at measured intervals into the wet concrete and left while the foundation hardens.
Holes are then drilled in the sill plate at the same intervals so that when the board is laid on the foundation, the anchor bolts will pass through the holes.
The nuts are then screwed into the threaded ends of the bolts, usually with a tool called an impact driver.
Sill protection plates
Many homes today also have a polyethylene foam sill sealer that sits between the sill and the foundation. This prevents moisture from passing through the concrete or soil and rotting the windowsill.
Window sills are often pressure treated to prevent insect and moisture damage. Pressure treated wood is wood that has been placed in a tank, depressurized, and then repressurized with a chemical preservative. This pushes the preservative, which is moisture resistant and an insect deterrent, all the way into the wood.
Until recently, chromated copper arsenate (CCA) was the most commonly used chemical preservative. However, CCA is very toxic and has been largely replaced with less toxic alternatives. However, older buildings that already have CCA-treated sills are considered safe for habitation. In fact, removing these tabs is likely to release far more toxins than leaving them in place.
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