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Sill plate: what it means on an inspection

Rafter Tie editorial teamUpdated 4 min read

A sill plate is the piece of lumber laid flat on top of a foundation wall that the floor framing sits on and that is bolted down to the foundation. It is the first wood in the house. Because it touches concrete or masonry near the ground, it also has decay protection rules.

A cross-section of the top of a foundation wall with a sill plate laid flat on the concrete, an anchor bolt with washer and nut through the plate, and floor framing bearing on the plate beside it
The sill plate is the first wood on the foundation, tied down by anchor bolts.
Use this diagram

How to check a sill plate

  1. Find the plate. On a house with a crawl space or basement, the sill plate lies on the foundation wall, under the band joist and the floor joists. On a slab, the bottom plate of the wall takes its place and the anchorage rules still apply.
  2. Check the anchorage. The IRC sets a minimum bolt diameter, a maximum spacing along the plate and a minimum embedment into the foundation, and each piece of plate needs at least two bolts with one near each end; check the current edition for the values. Straps or other approved anchors may replace bolts. Wind and seismic regions add requirements, so confirm in your edition.
  3. Look at the decay protection. Wood that rests on concrete or masonry close to the ground generally has to be preservative-treated or a naturally durable species. The clearance rules are in the decay protection section of Chapter 3; check them for your case.
  4. Check the bearing. The plate should sit flat and full on the top of the foundation wall, with the floor framing bearing on it.
  5. Look for sealing. A sill sealer or gasket between plate and foundation is an air sealing detail in many areas, not a structural one.

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Common mix-ups

  • Sill plate versus bottom plate. Bottom plate, also called sole plate, is the lowest horizontal piece of a stud wall. A sill plate is the one resting on the foundation. In a house with a first floor over a crawl space, you can have both.
  • Sill plate versus window sill. A window sill is the bottom of a window opening. Different part, same word.
  • Anchor bolt versus hold-down. An anchor bolt ties the plate to the foundation along the wall. A hold-down is a heavier connector at the end of a braced wall panel that resists overturning. Look at braced wall panel for the wall it supports.
  • Treated lumber is not a license to skip anchorage. Decay resistance and anchorage are two separate requirements.

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Sill plates on the B1

Sill plates sit between footings and foundations at 16% of the B1 and floor construction at 14%. The questions are often about anchorage spacing, how close a bolt must be to a plate end, or which wood needs treatment. Practice looking up the foundation anchorage section in Chapter 4 and the decay protection section in Chapter 3. Section numbers vary between editions, so confirm them in your own book and tab them.

Practice it

Use the IRC tabs guide to mark the anchorage and decay protection sections in your book. Then read band joist, which sits on the plate, and footing versus foundation wall for what is below it. The IRC terms glossary links the rest.

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