The sill plate, also known as sole plate, is the first member of a house’s wood frame to be installed in platform framing. The sill plate consists of 2 by 4 or 2 by 8 pieces of lumber attached over the concrete slab perimeter or foundation wall via holes drilled to accommodate the J anchor bolts which are fixed in the foundation.
Building codes mandate that sill plates be treated with a wood rot resistant preservative. This is because the sill plate comes in direct contact with concrete, which holds moisture. The preservative, which is injected into the wood through heat and pressure at the factory or lumber mill, gives the lumber a dark greenish or brown tint.
The preservative treated wood functions as a barrier to halt migration of moisture from the ground and foundation into the wood framing’s untreated lumber.
The sill plate ties the framing of the building into the foundation. It also serves as a way for the builder to adjust a wall or floor that is out of level due to an uneven foundation, through shimming.
The sill plate is shimmed by inserting tapered, thin strips of metal or wood underneath the sill plate prior to tightening the anchor bolts. This will prevent problems from cropping up in a house that has a slope or slight high or low spots in the sill, including sloping floors, doors that will not close properly or stay open, windows that do not sit properly in their frames, and so on.
In certain situations, building codes allow the builder to eliminate use of a sill plate. If a framing component can be eliminated without altering the overall structural integrity of the building, then such value-engineering methods can save money and resources. Value engineering is a design methodology which analyzes the impact of cost and structural value of a material or component to the overall structure.
There is disagreement among code officials, framing contractors, inspectors and builders over leaving the sill plate out of a traditional platform framing house. Many builders say the cost savings are not significant, and some inspectors will take extra time making sure such a house meets minimum structural standards.