Water from outside
Soil or mulch piled against siding, grading that slopes toward the house, overflowing gutters, and leaking hose bibs wet the top of the wall.
Crawl space framing guide
Sill Plate: What It Is, Signs of Damage, and Repair
Where the sill plate sits, how it differs from the rim joist and bottom plate, what rot, termite, and crushing damage look like from the crawl space, and what repair usually involves and costs.
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See it
Where the sill plate sits
A section through the edge of the house, from the foundation wall up to the floor you walk on.
Under the model residential code, anchor bolts are typically at least ½ inch in diameter, spaced no more than 6 feet apart, and placed within 12 inches of the end of each sill piece, with the treated sill set on a gasket that seals the joint and keeps wood off bare masonry. Local amendments vary, so treat those as the common baseline rather than your house's requirement.
Terms
Sill plate vs. rim joist vs. band board vs. bottom plate
Four boards that get confused in inspection reports and quotes. Knowing which one is damaged changes the repair.
| Part | Where it is | What it does |
|---|---|---|
| Sill plate (mudsill) | Flat on top of the foundation wall, bolted down | Anchors the house to the foundation and gives the floor framing a wood base to bear on |
| Sill gasket (sill sealer) | Thin foam strip between the wall and the sill | Seals air leaks and separates wood from masonry; not structural |
| Rim joist | Stands on edge on the outer edge of the sill | Caps the ends of the floor joists and carries the wall above |
| Band joist / band board | Same position as the rim joist | Regional name for the rim joist — contractors use the terms interchangeably |
| Bottom plate (sole plate) | Under the studs of a wall, on top of the subfloor | Base of a framed wall — one floor up from the sill plate, not part of the foundation |
Rot rarely stops at one board. Water that reaches the sill usually reaches the rim joist sitting on it too, and sometimes the ends of the floor joists — which is why good quotes price the perimeter as a system.
Why it fails
Why sill plates get damaged
The sill sits at the one spot where outside water, foundation moisture, and crawl space air all meet.
Moisture from below
Older sills were sometimes untreated lumber set straight on block with no gasket; masonry wicks ground moisture into the wood.
Insects
Termites and carpenter ants target damp wood, and the sill is the first wood they reach climbing the foundation.
Diagnose
Sill plate damage: what you see, likely cause, who to call
Probe the wood with a screwdriver from inside the crawl space: sound wood resists; rotted wood lets the tip sink in. These patterns commonly point to the causes below — an inspection confirms them.
| What you see | Likely cause | Who to call |
|---|---|---|
| Soft, dark, or crumbling wood near a downspout, hose bib, or low grade outside | Rot from water reaching the top of the foundation | Crawl space or framing contractor; fix grading or gutters too |
| Mud tubes on the foundation wall, or wood that sounds hollow and breaks in layers | Subterranean termites | Licensed pest (WDO) professional first, then framing repair |
| Fine sawdust-like frass, clean galleries, or ants near damp wood | Carpenter ants or wood-boring beetles, usually following moisture | Pest professional plus moisture correction |
| Sill compressed or split under a post, beam end, or heavy wall | Crushing from concentrated load or old rot under load | Framing contractor; engineer if the load path is unclear |
| Daylight or drafts between sill and wall; gaps along the top of the foundation | Missing or failed gasket, uneven wall, or sill that has shifted | Air sealing, or anchorage review if the sill has moved |
| Dark staining but hard wood that resists a screwdriver | Old or surface moisture; sill may still be sound | Monitor, find the water source, recheck after wet weather |
| Sagging floor or sticking doors along one exterior wall | Sill or rim damage, or foundation movement below | Inspection first; foundation contractor or engineer if the wall itself has moved |
If termites are possible, have a licensed pest professional inspect before any wood is replaced. New lumber set on an active colony is the repair most likely to fail.
Repair
Sill plate repair and replacement options
Scope depends on how much of the sill width and length is lost, and whether the rim and joist ends are involved. Costs below are planning estimates from published cost guides, not quotes.
| Approach | Usually fits when | What it involves | Planning estimate |
|---|---|---|---|
| Treat and monitor | Surface staining, wood still hard when probed | Moisture correction and borate treatment; no framing replaced | Cost of the moisture fix |
| Sister or partial repair | Short damaged section; most of the sill width still sound | New treated lumber fastened alongside, or a small section cut out and replaced with local shoring | Rim/band board sistering is often quoted around $40–$50 per foot |
| Section replacement with jacking | Rot or insect damage along a wall run | Floor shored on temporary jacks, damaged sill cut out, new treated sill and gasket set, anchors restored, load lowered back | Commonly $100–$200 per linear foot |
| Whole-perimeter or long-run replacement | Damage along most of the house | Staged jacking around the house; rim joist, joist ends, and siding often opened | Roughly $10,000–$40,000 |
At $100–$200 per linear foot, a 10-foot run works out to about $1,000–$2,000 and a 40-foot wall to about $4,000–$8,000 before rim joist, siding, pest treatment, or permit costs. Most of the price is labor and temporary support, not lumber. Whatever the scope, the moisture source — grading, gutters, a leak, or high crawl space humidity — should be corrected in the same plan.
Where our lane ends
When it is an engineer or permit question
Many sill repairs are routine carpentry. Some are not.
Damage runs along most of a wall or around corners.
The wall above has dropped, or doors and windows on that wall stick.
The foundation wall itself is cracked, bowed, or has moved.
Anchor bolts are missing, sheared, or pulled out.
The repair is part of a home sale or insurance claim and needs documentation.
Your jurisdiction requires a permit for structural framing replacement.
In those cases a structural engineer's letter or a building permit is common. Foundation movement belongs with a foundation specialist before the wood is replaced.
Quote review
Sill plate repair quote checklist
A clear written scope makes quotes comparable.
How many linear feet of sill are being replaced, and where?
Is the rim joist or are joist ends included, or priced if found?
How will the floor be temporarily supported, and will anything be lifted?
Is new sill gasket and pressure-treated lumber rated for ground/foundation contact specified?
Are anchor bolts or connectors being restored or added?
Is pest treatment done first, and by whom?
What moisture correction is included or recommended?
Are siding, interior finishes, permit, and engineer fees included?
Related pages
Related framing and moisture guides
The boards around the sill, and the moisture work that protects them.
Rim joist
The board standing on the sill — what it is, how to insulate it, and rot repair.
Learn moreFloor joist repair
When the damage has spread into the joist ends that bear on the sill.
Learn moreWood rot repair
Moisture-related damage elsewhere in the crawl space framing.
Learn moreCrawl space wood rot
What wet and dry rot look like and where decay usually starts.
Learn moreTermite inspection gap
Keeping the top of the foundation visible when you line or seal the crawl space.
Learn moreCrawl space repair cost
Planning ranges for the moisture and structural work that often travels with sill repair.
Learn moreFAQs
Sill plate FAQs
Short answers to the questions homeowners ask most.
What is a sill plate?
A sill plate, also called a mudsill, is the horizontal board bolted flat on top of the foundation wall. It is the lowest wood in the house: the rim joist and floor joists sit on it, and anchor bolts tie it to the foundation. Because it touches concrete or masonry, codes require it to be pressure-treated or naturally decay-resistant.
Can you replace a sill plate?
Yes. Contractors shore the floor on temporary jacks next to the damaged run, cut out the old sill, set a new pressure-treated sill and gasket, restore the anchor bolts, and lower the load back on. Short sections can be done without lifting much of the house. The work should be done by someone experienced with temporary support, and moisture or termites should be dealt with first.
How much does sill plate replacement cost?
As planning estimates, not quotes: published cost guides commonly put sill plate replacement around $100–$200 per linear foot, so a 10-foot section might run about $1,000–$2,000 before related repairs. Long runs or whole-perimeter replacement are often quoted at $10,000–$40,000. Rim joist damage, jacking, siding, pest treatment, and permits move the number most.
What is the difference between a sill plate and a bottom plate?
The sill plate sits on the foundation and anchors the floor system to it. A bottom plate (or sole plate) sits under the studs of a framed wall, on top of the subfloor. They look similar, but the sill plate is below the floor and the bottom plate is above it.
Does homeowners insurance cover a rotted sill plate?
Usually not. Most homeowners policies exclude rot, termite damage, and damage from long-term leaks or wear. Some policies may cover sill damage caused by a sudden, covered event such as a burst pipe. Coverage depends entirely on your policy, so read it or ask your insurer before assuming either way.
Does a sill plate have to be pressure-treated?
Under the model residential code that North Carolina's code is based on, wood sills resting on concrete or masonry foundations must be preservative-treated or a naturally durable species, unless separated by an approved moisture barrier. Replacement sills should be treated lumber rated for that use, set on a sill gasket.
Sources
Research notes and review date
Code references and published cost guides used for this page.
- ICC — IRC R317 protection of wood against decay for treated or decay-resistant sills on concrete and masonry.
- ICC — IRC R403.1.6 foundation anchorage for anchor bolt size, spacing, and end distance.
- ArchitectureCourses sill plate replacement cost for the $100–$200 per linear foot and $10,000–$40,000 planning ranges.
- Acculevel sill plate and sistering costs for sill repair per foot and rim/band board sistering ranges.
- Angi sill plate replacement cost guide for national cost context.
Code requirements vary by jurisdiction; North Carolina's residential code is based on the IRC with state amendments. Costs are planning estimates, not quotes.
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