Box Sill

A box sill is a type of sill which is used in framing. The box sill is nailed directly onto the floor joists. Onto the block sill is nailed the vertical header joist and outside and horizontal joists are nailed to the header joist and outside joist, comprising the frame on which the sub-flooring is attached.

Box Sill for Platform Construction

The box sill is used most often in deck and platform construction. When needed, as in the interior of a house, insulation and metal termite shields are sometimes put underneath the box sill.

If using a termite shield under a box sill, the termite shield should be 26-gauge aluminum, copper or galvanized sheet metal. The outer edges of the termite shield should be bent down slightly

Box sills are generally single, but double sills are sometimes used. In laying out box sills, it is good to note that the corners should be halved together. Box sill corners can also be butted or mitered together.