Quick answer
A sill plate, also called a mudsill, is the lowest piece of wood framing in a house: a board laid flat on top of the concrete or masonry foundation and bolted to it.
Floor joists or wall studs sit on it. Because it touches concrete, the code calls for decay-resistant or pressure-treated wood in most cases, and anchor bolts tie it down.
Quick facts
The key facts about Sill Plate, with sources:
- Also called
- Mudsill, mud sill or foundation sill
- Sits on
- The top of a concrete or masonry foundation wall or slab edge 1
- Anchored with
- 1/2-inch bolts, 6 feet or less on center, at least 7 inches into concrete 1
- Bolts per piece
- At least two, one within 12 inches of each end 1
- Wood type
- Naturally durable or preservative-treated wood where it rests on concrete near the ground 2
- Fasteners
- Hot-dip galvanized, stainless, silicon bronze or copper in treated wood 2
- Common add-on
- Foam sill gasket as an air seal and capillary break 4
Key takeaways
- The sill plate is where the wood house meets the concrete foundation, so it is the joint that keeps the house from sliding or lifting.
- The 2021 IRC requires 1/2-inch anchor bolts no more than 6 feet apart, with two per board and one near each end.
- Sill plates touching concrete near the ground must be naturally durable or pressure-treated, because concrete wicks moisture.
- Rot, termite damage and missing bolts at the sill are common finds in older homes and should be fixed before a large remodel.
- Replacing a sill plate means supporting the house while the old board comes out, which is work for a licensed contractor under a permit.
The sill plate in plain English
Every wood-frame house has to stand on something that does not rot. That is the concrete or block foundation.
The sill plate is the flat board that sits on top of it and gives the carpenters a wood surface to nail to.
Everything above, from floor joists to walls to the roof, rests on that board.
Builders call it a mudsill because it sits closest to the ground.
In a house with a crawl space or basement, the sill runs around the top of the foundation wall and the floor framing sits on it.
In a house on a concrete slab, the bottom board of each exterior wall plays a similar role and the code calls it a sole plate 1.
The sill plate also does a quieter job: it ties the house down. Anchor bolts cast into the concrete pass through holes in the sill, and nuts and washers clamp it in place.
That connection is the last link in the chain that carries wind and earthquake forces into the ground, the same chain that a shear wall depends on.
How a sill plate carries the house and resists moisture
A sill does three things at once. Each one depends on how it was installed.
- It spreads the weight. Studs need full bearing on a nominal 2-inch or larger plate at least as wide as the studs (2021 IRC R602.3.4) 3. The sill hands that weight to the foundation.
- It holds the house down. Bolts in the middle third of the board width, with a nut and washer tightened on each, keep the frame from sliding or lifting 1.
- It survives contact with concrete. Concrete can wick ground moisture upward. The code requires decay-resistant wood for sills on a slab in contact with the ground unless an impervious moisture barrier separates them 2.
- It seals the gap. A foam sill gasket under the board fills the uneven concrete surface. The Building America Solution Center says it serves as both an air seal and a capillary break between concrete and wood 4.
- Floor joists or wall studsFraming that rests on the sill
- Rim joistRuns around the edge of the floor framing
- Nut and washerTightened on each anchor bolt; plate washers in high seismic areas
- Sill platePreservative-treated or naturally durable lumber
- Sill gasketClosed-cell foam air seal and capillary break
- Termite shield (where used)Sheet metal under the sill in termite regions
- Foundation wallConcrete or grouted block with anchor bolts at least 7 inches deep
Not every house has every layer. Gaskets and termite shields are common good practice; bolts and treated wood are code items in most cases. Source: [1] [2] [4]
Types of sill plates and plates you may hear confused with them
| Plate | Where it is | What to know |
|---|---|---|
| Sill plate (mudsill) | On top of a foundation wall under floor framing | Anchored under IRC R403.1.6 1 |
| Sole plate on a slab | Bottom of a wall built directly on a concrete slab | Exterior and braced-wall sole plates are anchored like sills 1 |
| Bottom plate | Bottom of any framed wall on a wood floor | Nailed, not bolted; not in contact with concrete |
| Top plate | Top of a framed wall, usually doubled | Carries joists or rafters; not a sill |
| Steel-frame sill track | Bottom of a cold-formed steel wall | Anchored directly or to a wood sill under R505.3.1 or R603.3.1 1 |
| Cripple wall sill | Under short crawl-space walls in older homes | Often the target of seismic bolting retrofits 5 |
Example: counting anchor bolts on a 30-foot foundation wall
This applies the 2021 IRC spacing rules to one wall to show how the numbers work. It is an illustration, not a design. Seismic and wind rules can require more 1.
| Step | Figure | Rule or reasoning |
|---|---|---|
| Wall length | 30 feet, laid as two 16-foot and one shorter sill board | Example layout |
| End bolts per board | One within 12 inches of each end, and not closer than 7 bolt diameters | R403.1.6 1 |
| Seven bolt diameters for a 1/2-inch bolt | 7 × 0.5 = 3.5 inches from the end, minimum | Our arithmetic on the R403.1.6 rule |
| Maximum spacing between bolts | 6 feet on center | R403.1.6 1 |
| 16-foot board | 2 end bolts + 2 middle bolts = 4 bolts | Ends near each end, gaps no more than 6 feet |
| Whole 30-foot wall | About 10 bolts across three boards | Each board counted on its own |
| High seismic site (D0 to D2) | Add plate washers 0.229 × 3 × 3 inches on each bolt | R403.1.6.1 and R602.11.1 13 |
Bolts ≥ 2, and gaps between bolts ≤ 6 ft, with each end bolt between 7 bolt diameters and 12 inches from the board end
Bolt- 1/2-inch diameter, at least 7 inches into concrete
Board- One continuous piece of sill plate
Gap- Center-to-center distance between bolts
Seismic Design Categories D0 to D2 add plate washers and extra rules for interior braced walls.
Source: [1]
The lesson is that bolts are counted per board, not per wall. A short scrap of sill near a corner still needs its own two bolts. Inspectors look for that at the foundation or framing inspection.
Where you see the sill plate: crawl space, inspection report and bid
In the house. Go down to an unfinished basement or crawl space and look where the wood meets the concrete. You will see the sill, the rim joist sitting on it, and the nuts on top of the anchor bolts. In finished basements it hides behind drywall.
In a home inspection report. Inspectors often note "sill plate rot," "no visible anchor bolts" or "wood-to-earth contact." Those notes carry weight when you plan a basement finishing project, because finishing covers the sill for years.
On a bid. A repair bid should say how the house will be supported, how much sill is replaced, what wood treatment is used, and how new anchors are set.
Ask whether the price assumes a set length of sill and how extra rot is billed, the same question you would ask of any allowance.
On the permit. Sill replacement touches the structure, so a permit is usual. The permit checker gives a first read on whether your job needs one.
Take a screwdriver to the crawl space
Push a flat screwdriver into the sill at several spots. Sound wood resists. If the tip sinks in easily, the wood is likely soft from rot, and it is worth a professional look before you cover anything.
Why a sound sill plate matters
What a good sill gives you
What goes wrong when it fails
Limits of treated wood and anchors
Pressure-treated wood resists rot and insects, but it does not make the sill immune to water. Constant wetting from a high water table, poor drainage or a plumbing leak can still damage it over time.
Treatment also changes the hardware: the 2021 IRC requires fasteners, nuts and washers in contact with preservative-treated wood to be hot-dipped galvanized, stainless steel, silicon bronze or copper (R317.3.1) 2. Ordinary steel corrodes.
Anchors have limits too. A bolt only works if the concrete around it is sound and the nut is tight. In old houses, bolts may be missing, rusted through or set in crumbling concrete.
Adding new anchors to an existing foundation usually means drilling and setting retrofit anchors or using steel plates, and the choice depends on the foundation. California's retrofit program describes using anchor bolts or foundation plates for this step 5.
What drives the cost of sill plate repair or replacement
We do not quote prices for structural repair.
What moves the bill is easy to list: how many linear feet of sill are rotted, how hard the area is to reach (a tight crawl space is slower than an open basement), whether the house must be jacked and shored, whether the rim joist and joist ends are also damaged, and whether termite treatment is part of the job.
New anchors add drilling and hardware. A permit and engineering letter may be required where the damage is large or the area is seismic.
The best way to control cost is to find the problem early, before it spreads into joists and studs.
Use the quote comparison tool to line up repair bids, and see the cost guide for how hidden conditions affect a remodel budget.
Lifespan depends on moisture, not age. A dry, treated sill can last as long as the house. Ask any repair contractor what warranty covers the workmanship, separate from any wood treater warranty.
How sill plates are installed in new work and replaced in old work
New installation happens when the foundation is poured. Replacement happens under a standing house and is a structural repair.
- New work: Anchor bolts are set while the concrete is still plastic, at the spacing the code requires 1.
- New work: The crew marks bolt holes, rolls out a sill gasket, cuts around the bolts and caulks around them, as the Solution Center describes 4.
- New work: A termite shield goes under the sill in termite regions where used 4.
- New work: The sill is set over the bolts and the washers and nuts are tightened.
- Replacement: An engineer or experienced contractor plans temporary supports, and a permit is pulled.
- Replacement: The floor or wall above is shored, the damaged sill is cut out, new treated wood goes in, and new anchors are set.
- Both: The inspector checks bolts, washers and wood treatment before the area is covered.
Never remove a sill under a loaded wall without shoring
Cutting out a sill plate without temporary support can drop the floor or wall above it. Sill replacement needs a plan for shoring, a permit and a licensed contractor, with a structural engineer involved when damage is extensive.
Keeping the sill plate dry and sound
Most sill damage starts with water. Simple habits keep it away.
- Keep soil and mulch below the siding. Wood should not touch soil, and splashback wets the sill.
- Send roof water away. Downspouts should discharge well away from the foundation.
- Look once a year. Check the sill from the crawl space or basement for dark stains, soft spots, mud tubes from termites and loose nuts.
- Fix plumbing leaks fast. EPA advises drying water-damaged areas within 24 to 48 hours to prevent mold growth 6.
- Seal the gap. Caulk or foam the sill-to-foundation and sill-to-rim joints in accessible areas, as the Solution Center recommends 4.
Warning signs of a failing sill plate
- Soft or crumbly wood where the frame meets the foundation.
- Sloping floors near exterior walls, or cracks in drywall near the floor line.
- Mud tubes on the foundation wall, a sign of subterranean termites.
- No visible nuts on top of the sill, which may mean no anchor bolts.
- White or dark growth on the wood, a sign of ongoing moisture and possible mold 6.
If you find more than a small soft spot, call a licensed contractor or structural engineer. Treat pest damage with a licensed pest control company before the wood is replaced.
Code rules for sill plates: 2021 IRC R403.1.6, R317 and R602.11
Anchorage. 2021 IRC Section R403.1.6 requires sill plates, and sole plates at exterior walls on monolithic slabs, to be anchored with bolts at least 1/2 inch in diameter, spaced no more than 6 feet on center, extending at least 7 inches into concrete or grouted masonry.
Bolts must be in the middle third of the plate width, with a tightened nut and washer, at least two bolts per plate section, and one bolt no more than 12 inches and no less than seven bolt diameters from each end 1.
High seismic areas. R403.1.6.1 adds plate washers along braced wall lines and tighter rules for interior braced walls in Seismic Design Categories D0, D1 and D2, and for townhouses in C 1. R602.11.1 sizes those plate washers at 0.229 by 3 by 3 inches minimum 3.
Decay protection. R317.1 requires naturally durable or preservative-treated wood (treated under AWPA U1) for sills and sleepers on a concrete slab in direct contact with the ground unless an impervious moisture barrier separates them, and for framing on exterior foundation walls less than 8 inches from exposed ground 2.
Fasteners. R317.3.1 requires hot-dipped galvanized, stainless steel, silicon bronze or copper fasteners, nuts and washers with preservative-treated wood 2.
These are model code sections. Your state or city may use another edition or amend them, and repairs to an existing house may fall under your existing building code. The building department has the final word.
Sill plate vs rim joist vs sole plate vs top plate
| Part | Where | Job | Touches concrete? |
|---|---|---|---|
| Sill plate | On the foundation | Base for floor framing, tie-down point 1 | Yes |
| Rim joist | On the sill, around the floor edge | Closes the ends of the floor joists | No |
| Sole plate on slab | Bottom of walls on a slab | Base for wall studs, anchored like a sill 1 | Yes |
| Bottom plate on wood floor | Bottom of upper-floor walls | Base for wall studs | No |
| Top plate | Top of walls | Carries joists, rafters or trusses | No |
Common sill plate myths
- Myth Pressure-treated sills never rot.
- Reality Treatment resists decay, but constant wetting can still cause damage; keeping water away still matters.
- Myth Any nails or bolts work with treated lumber.
- Reality The IRC requires hot-dipped galvanized, stainless, silicon bronze or copper fasteners with treated wood 2.
- Myth One bolt at each corner is enough.
- Reality Each board needs at least two bolts, near its ends, with gaps of 6 feet or less 1.
- Myth The sill gasket is only insulation.
- Reality It works as an air seal and a capillary break between concrete and wood 4.
- Myth Old houses without bolts are grandfathered, so there is no risk.
- Reality Existing conditions may be legal, but unbolted houses can slide in earthquakes; retrofit programs target exactly this 5.
How termites and earthquakes change sill plate details by region
Two regional variables change sill details. The first is termites. The Building America Solution Center notes termites are most prevalent in the central and southern U.S. and recommends termite shields beneath the sill there, with a map adapted from IRC Figure R301.2(7) 4.
The second is seismic risk. In Seismic Design Categories D0, D1 and D2, the IRC adds large plate washers and stricter anchorage 13, and California runs a retrofit program for pre-1980 raised-foundation homes that bolts sills to foundations 5.
Ask your building department which design category and termite zone it uses.
When the sill plate should change your remodel plan
Check the sill before any project that will hide it or add weight above it: basement finishing, a new bathroom on the first floor, an ADU conversion, or new heavy flooring. Fixing a sill is far easier with the walls and ceilings still open.
If you find rot, fix the water source first, then the wood. If you find no bolts in an earthquake area, ask about a retrofit. Our guide to choosing a remodeling contractor covers what to check before you hire someone for structural repair.
- If The sill is sound and bolted
- Then Air seal it and proceed with the remodel
- If You find small soft spots
- Then Find and stop the water source, then have it assessed
- If Rot runs along several feet or into joists
- Then Plan shored replacement under a permit
- If No anchor bolts in a seismic area
- Then Ask an engineer about a bolting retrofit
Questions about Sill Plate
What is the difference between a sill plate and a sole plate?
A sill plate sits on top of a foundation wall and supports the floor framing. A sole plate is the bottom board of a wall.
When a wall is built directly on a concrete slab, its sole plate touches concrete and is anchored much like a sill, which is why the two terms get mixed up. Upper-floor sole plates are nailed, not bolted.
Does a sill plate have to be pressure-treated?
In most cases, yes. The 2021 IRC requires naturally durable or preservative-treated wood for sills on a slab in contact with the ground unless an impervious moisture barrier separates them, and for framing on exterior foundation walls less than 8 inches from exposed ground. Most builders simply use treated lumber for every sill.
How far apart should anchor bolts be on a sill plate?
Under the 2021 IRC, anchor bolts must be at least 1/2 inch in diameter and spaced no more than 6 feet on center.
Each piece of sill needs at least two bolts, with one within 12 inches of each end.
Bolts must reach at least 7 inches into the concrete and sit in the middle third of the board width.
What size is a sill plate?
A sill plate is usually 2x6 or 2x8 lumber laid flat, but the code sets a minimum rather than one size. Studs must have full bearing on a nominal 2-inch or larger plate at least as wide as the studs. Wider foundations or framing details may call for a wider board.
How do you know if a sill plate is rotten?
Look for dark staining, crumbling or soft wood where framing meets the foundation, and press a screwdriver into it. Sound wood resists; rotten wood lets the tip sink in. Sloping floors near outside walls and termite mud tubes on the foundation are other clues. A contractor can confirm how far the damage runs.
Can a sill plate be replaced without lifting the house?
The house does not usually need to be lifted off its foundation, but the floor or wall above must be supported while the old board is cut out. Contractors use temporary shoring, replace the wood in sections and set new anchors. This is structural work, so it normally needs a permit and a licensed contractor.
What is a sill gasket?
A sill gasket, or sill sealer, is a strip of closed-cell foam laid on the foundation before the sill goes down. It fills gaps in uneven concrete. The Building America Solution Center describes it as both an air seal and a capillary break that keeps moisture in the concrete from reaching the wood.
What fasteners can be used with a treated sill plate?
Use hot-dipped galvanized, stainless steel, silicon bronze or copper fasteners, nuts and washers. The 2021 IRC requires these for preservative-treated wood because modern treatments corrode plain steel. Staples in treated wood must be stainless steel.
Do I need plate washers on my anchor bolts?
Only in some areas. The 2021 IRC requires plate washers at least 0.229 by 3 by 3 inches on anchor bolts along braced wall lines in Seismic Design Categories D0, D1 and D2, and for townhouses in Category C. Elsewhere a standard nut and washer is required. Your building department can tell you your category.
Is sill plate replacement covered by homeowners insurance?
It depends on the cause and your policy. Many policies exclude gradual rot, wear and termite damage, but may cover damage from a sudden covered event such as a burst pipe. Read your policy and ask your insurer before work starts, and keep photos of the damage.
Sources
- ICC, 2021 International Residential Code, Chapter 4 Foundations (R403.1.6, R403.1.6.1), retrieved .
- ICC, 2021 International Residential Code, Chapter 3 Building Planning (R317.1 decay protection; R317.3 fasteners), retrieved .
- ICC, 2021 International Residential Code, Chapter 6 Wall Construction (R602.3.4; R602.11.1 plate washers), retrieved .
- Building America Solution Center (PNNL / U.S. DOE), Air Sealing Sill Plates, retrieved .
- California Residential Mitigation Program, Earthquake Brace + Bolt retrofit, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Anchorage rules were read from 2021 IRC R403.1.6 and R403.1.6.1; decay protection and fastener rules from R317.1 and R317.3.1; plate bearing and plate washer sizes from R602.3.4 and R602.11.1, all on ICC Digital Codes.
Gasket, sealing, termite shield and installation steps come from the DOE Building America Solution Center guide. Retrofit context comes from the California Residential Mitigation Program; mold timing from EPA.
The bolt-count example applies the code rules to a sample wall and is not a design. No prices are given.
Suggest a correction. We fix errors and say what changed.