How To Replace Sill Plates In A Crawl Space

Sean Jarvis
by Sean Jarvis

A sill plate is the bottom piece of a wall where the wall studs are attached. These are a very important component to all houses and if damaged, can result in sagging floors and other problems.

To replace your rotten sill plate under the crawlspace, you need to inspect the damage to find out why it’s happening, and how to fix it. Then, install temporary supports. Remove the damaged sill plate. Replace it and any damaged areas around it. Install the proper moisture and termite protections and then have it inspected.

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What Is A Sill Plate?

A sill plate is the piece of wood that lies directly on top of your existing foundation. This is usually the first piece of wood that is placed during home construction. A sill plate is what anchors your home to the foundation and is a vital part of your home’s structural integrity.

These are usually made with wood lumber that is 2 x 6, or larger, and laid on top of the masonry foundation wall. Running the entire length of the foundation, the first-floor frame is built on top of and then anchored into the sill plates.

Within the floor frame are common joists, which run along the foundation walls, and band (or rim) joists, that cover the ends of the common joists. These band joists are installed flush with the outside edges of the sill plates.

Plywood is then used as a subflooring for the floor joists, to finish the floor frame. On top of this are the wall frames.

Sill Plate Problems

For homes built before the 1990s, it is likely that the sill plate is not protected by pressure-treated wood and therefore susceptible to problems at some point.

Homes that have problems with sill plates are usually having issues due to mold and termite infestations. Once the sill plate starts to rot out, it can quickly spread outward to the joists and vertical structural beams.

The exterior skeletal frame of your home is dependent on the sill plate, to help maintain the proper structure and ensure a correctly maintained home. This is what keeps the joist and studs’ level, as well as the flooring. They are the first point of contact with moisture from the foundation and the first-floor component that will deteriorate.

Issues With Uneven Floors

If you are experiencing issues with uneven floors, it is better that the sill plate be the problem and not the foundation. Repairing a sill place is less expensive, though only a professional will be able to diagnose what the issue is and properly discern where the problem is coming from.

When replacing a sill plate, it is a good idea to use pressure-treated lumber. This will protect the wood from termites and other fungal decays. It will also mean that your new sill plate is waterproof, so the rotting that happened will not be happening again.

When repairing a rotted sill plate, you should also check your face-board. This is the piece of wood at the exterior base of your home and may need replacing as well. It is covered with insulation so you will need to remove that in order to do a thorough inspection.

Installing Sill Plates During Home Construction

When a new home is constructed, the foundation is laid first. This is what holds the weight of the house and provides a flat and level base for the rest of the construction.

There are 3 types of foundations:

  • Slab Foundations: This is one of the most common types of foundations. Made with poured concrete and rock filler.
  • Crawlspace Foundations: These are elevated a few feet off the ground. They allow for easier access to the homes wiring, piping, and ductwork than slab foundation.
  • Basement Foundations: Previous made with cinder blocks, they are known commonly built with poured concrete. They allow for additional square footage, extra living space, and easy access for repairs.

At the top of the foundation wall are heavy-duty steel anchor bolts, called J-bolts. These are set into the wet concrete with the threaded end extending a few inches above. This is what the sill plates will be anchored to with large washers and nuts.

In older homes, the sill plates would rest directly on top of the concrete foundation. Nowadays there are added protections between the foundation and sill plate, to protect against moisture and rot.

The sill plate must be installed flush against the foundation. If there are any gaps it will allow space for water to leak through into the foundation, reducing the structural integrity of your home.

If the home is built in a location prone to termites, there is often a metal barrier installed to keep them from climbing the foundation wall.

Replacing a Damaged Sill Plate

In old houses, a damaged sill plate is a common issue. This is mostly because the sill plates in old homes were not made with pressure-treated wood. As time goes on, moisture will collect and wreak havoc to the wood. If you live in a termite prone area, these creatures will also eat away at the wood.

Because there are so many individual factors that go into replacing sill plates, it is hard to find a one-size-fits-all approach to replacing them. It is also not usually recommended for the average homeowner to try and replace sill plates themselves.

The general guidelines for replacing sill plates is:

  • You must remove the weight from the sill plate in order to reach it
  • Remove the sill plate and damage to any surrounding area
  • Redesign the sill plate so that it meets current building codes
  • Install the sill plate
  • Inspect the installed sill plate to make sure that everything was done correctly
  • Insulating the sill plate to prolong its lifespan and protect against moisture and termites

Signs of a Rotten Sill Plate

Often times, you will not notice that anything is wrong with the sill plate until you are working on something else and come into contact with it.

There are a few occasions where the exterior siding is buckling or the floor has a noticeable dip, which may prompt you to do a little investigating and notice that something is wrong.

Repairing a Sill Plate Yourself or Hiring Out

Deciding on whether or not to tackle this job yourself depends on a number of factors. How much of the sill plate is rotten? How much of a load is it supporting? There are many questions that need answers before moving ahead with this project.

For most DIY’er’s, this is a massive project to tackle. Unless you are familiar with framing a home, it would be best to hire this one out. Prices can range from anywhere to $1500, all the way upwards to $15,000, depending on the damage, the size of the home, etc..

It is good to start by hiring a structural engineer and a contractor to come and take a look and give their professional opinion. They will be able to guide you in the right direction, tell you what the actual damage is, how much it will cost to repair and how long it will take.

Replacing a rotten sill plate is an important measure in making sure your home is safe and stays structurally secure for years to come. If you replace the sill plate with the right kind of treated lumber, this will not be an issue you will have to deal with for another 100 years.

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Related Questions

What is the Difference Between a Sill Plate and a Sole Plate?

A sill plate is a piece of wood that is on top of the foundation. This is one of the most important pieces of housings construction. A soleplate is a piece of wood on a concrete floor used in a basement partition wall.

What Goes Between a Sill Plate and the Foundation?

To add additional moisture protection, a gasket or strip of closed-cell foam can be installed. You can caulk between the top of the sill plate and exterior sheathing, as well as sill plate joints and seams. It is also now common to add a sheet of metal which stop termites from being able to climb up the wall.

Is a Sill Plate Structural?

The sill plate is one of the most important elements of a houses structural framing. This is what anchors the house to the foundation.

Sean Jarvis
Sean Jarvis

Sean Jarvis is an interior decorator, writer, and expert handyman. Well versed in everything home improvement, he is a savant at manipulating words and spaces and upgrading everything around him. Sean specializes in writing concise guides about appliance repair and installation, home and lifestyle, and other residential projects.

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