Installing new sill plate in a garage. Garage Remodel:

Replacing Rotted Sill Plates

In This Article:

A small garage is lifted up with a hydraulic jack and the rotten bottom board is replaced with treated lumber.

Related Articles:
Skill Level: 3-4 (Intermediate to Advanced) Time Taken: About 8 Hours

By , Editor



If there is one fundamental flaw with older wood-framed buildings, it's the fact that ordinary lumber was used adjacent to masonry. Concrete, brick, stone and mortar are porous and absorbent materials that are capable of wicking moisture upward from the ground. And since the ground in many areas is usually damp, this means that any wood placed next to masonry could be continually damp. A common problem with older buildings is decay (rot) that attacks the lower-most wood components.

The lowest structural component on many wood-frame buildings is the sill plate. There are regional variations in terminology, so in some places this piece may be known by another name, such as the mudsill. The term sill plate normally means a piece of wood that is attached to the top of a foundation wall. Floor joists would normally rest on the sill plate, then floor sheathing on top of the joists.

This garage was built on a concrete slab, a fairly common method of construction. 

It's also common to find the walls resting directly on a short foundation wall, with the floor slab perhaps a few inches below the top of the foundation.

In either case, it's typical to find the bottom plate of the stud wall (also called the sole plate) directly above concrete. In these cases the bottom plate could be considered to be a "sill plate", though it may not be perfect use of the term.


Some sections of the bottom plate were visibly crumbling (red arrows), while other areas appeared to be intact because the wood was hard on the upper surface.


But when I struck these intact-looking sections with the claw of a hammer, they broke up easily. Not a good sign.


Another Symptom:

Somebody had poured a topping slab (an additional layer of concrete) over the original slab, perhaps because the first slab was badly cracked or pitted. The smooth edge of the slab had been formed by the bottom plate of the wall.

There was this suspicious tapered gap between the wall and the concrete topping slab. (Red arrows)

It appeared that the garage wall had moved outward over time. The space between the red arrows was about 1½ inches. Movement like this is consistent with a rotted bottom plate (a common problem) or broken anchor bolts (rare).


There was evidence of this topping slab being done in two pours.

Arrow 1 points to the clean edge from the older-looking concrete, and arrow 2 points to the rough edge on the newer concrete, at the rear of the garage. Perhaps the previous owner (who, ironically, is a local contractor) had broken up the rear section of an earlier topping slab and poured more concrete to replace it.

Whatever the story, someone did a sloppy job. There was excess concrete on top of the bottom plate, which only helps to hold moisture in.

Arrow 3 points to the edge of the "really bad rot".


But Where To Start?

The first step before lifting a small structure is to remove or disconnect any anchor bolts. I couldn't find any in the bottom plate. Many older structures don't have anchor bolts.


Installing rafter ties to fasten wall studs to top plate. The next thing I did was install at least one steel connector between the top plates and each stud. 

I'm going to be lifting the top plate and without some serious connectors, the studs will stay put while the top plates lift away... not good.


I used a diamond blade in a circular saw to trim away the excess concrete from the rear (newer) topping slab. The material would interfere with placing the new bottom plate back into the proper location.

I just snapped a chalk line from the inside corner to the smooth edge of the front topping slab, to create one long straight line.


The red arrows indicate the wedge of concrete that I removed. 

Note how the concrete dust covers everything. It's a good idea to remove things from the area to prevent dust damage. This dust can be rinsed off but should not be wiped off, because it is so abrasive. I've ruined safety goggles by simply wiping concrete dust from them.


Getting A Lift:

I used a 6-ton hydraulic bottle jack and a heavy-duty steel lally column to do all the lifting of this garage, and it was more than adequate. I also brought along a 12-ton jack, but it was overkill.

I always place the jack on a sturdy base of wood blocks.

On top of the jack was a piece of ¼" steel plate. This is heavier than the steel plates that come with the lally columns.

Hydraulic jack and lally column used to lift garage wall.

Note: If there was bare dirt instead of a concrete slab in this garage, I would have needed to make a large base of several blocks of wood or solid concrete blocks. Soil will be compacted when a heavy load is placed on it, and the weight must be spread over a large area, perhaps as big as 2 feet by 2 feet. The size of the load bearing area depends on:

  1. The type of soil, as some soils can only withstand about 1,000 pounds per square foot, 
  2. The weight of the structure, and 
  3. The number of columns being used to temporarily support the structure.


The Top:

I use a piece of duct tape to keep the top steel plate (one that comes with the lally column) from falling on my head.

I positioned the top end of the lally column under the top plate, just beside a stud.

Then I pumped the bottle jack to raise the structure a fraction of an inch.

Listening to the structure is very important here. When a building is raised you will hear lots of snaps, crackles, and pops. These are roughly the same sounds that you will hear when a building creaks and groans during a strong windstorm, but the sounds are a little louder and much more frequent. Very loud popping or snapping sounds should be cause for concern.

After raising part of the structure by about 3/4 inch, I place a light-duty lally column next to the jack.  Jack to raise wall beside column used to support wall.


This light-duty column (the nearer column) supported the underside of the top plate, but on the opposite side of the stud.


I used a wrench to tighten the adjuster screw. Caution is required here because the end of the screw tends to "walk around" and drift away from the center of the steel plate.

A steel plate must be used, or else the screw could dig into the concrete.


After a few minutes I had raised the corner of the garage and placed two lally columns under the top plate.

I could push on the lower part of the wall and it would move in and out about an inch. This told me that the bottom plate was free of the concrete foundation.

The sequence of lifting could be important. I usually start by lifting near a corner. Sometimes I've lifted a small structure in the middle of a wall and the entire wall was picked up, to my surprise.


After about an hour of work I had lifted the entire side wall of the garage. The red arrows point to the temporary support columns, which are steel lally columns or 4x4 posts. Row of lally columns used to raise wall so bottom plate can be replaced.


It's hard to see here, but the bottom plate has been lifted off the concrete by at least half an inch.

Originally the gap between the concrete and the bottom plate was filled with dead grass and debris. Now you can see a gap between that debris and the wood. 


The lowest piece of siding also had some rot damage, and I needed to remove it. Even without that damage, I would have needed to remove some siding so I could get a new piece of wood into place.

I used a reciprocating saw to cut the nails that held the siding to the studs.


Removing siding to gain access to rotted bottom plate. Once all the nails were cut I went outside and pried the siding loose.


At the end of the black pry bar you can see more rot damage on the bottom plate. This rot was not noticeable from inside the garage.


The view from inside the garage.

Since the extra-thick concrete slab was in the way, the only way I could remove the old wood was by pushing it towards the outside.


I snipped the nails that held the bottom plate to the studs. Many of these were badly rusted. Cutting nails between bottom plate and wall studs.


I pushed the old bottom plate outside. It didn't put up a fight.


Then I discovered some anchor bolts. Since the wood had rotted so badly, these never got in the way


Towards the back of the garage the bottom plate seemed to be much sturdier. I pried on it and it didn't crumble.


My initial plan was to only replace the rotted sections of the bottom plate, so I proceeded to cut the plate where it seemed intact.


This was another anchor bolt. It was hidden beneath a stud.

After cutting the bottom plate, I discovered that there was rot on the underside and center of the board. So the I decided to replace the bottom plate on the entire wall.

I had to remove the old anchor bolts before I could install any new wood. It would be almost impossible to reuse these anchor bolts.

A couple of the bolts were simply cut off with a metal-cutting blade in my reciprocating saw.


This anchor bolt became loose, so I dug it out of the concrete. It appeared to be some sort of discarded piece of hardware.


I swept the concrete clean.


New piece of treated 2x4 used to replace rotted sill or bottom plate. I slid a 12-foot long pressure treated 2x6 into position. Since I had removed that lowest piece of siding, it was easy to get this board in place.

If there hadn't been a topping slab in my way, I might have been able to slide this new board in place from the inside.

I also slid an 8-foot 2x6 board into place to complete the 20-foot bottom plate.


Elevator... Going Down

Once the new wood was in position, I lowered the structure back down. This involves:

  1. Placing the bottle jack next to a temporary support. 
  2. Raising the building a bit more.
  3. Removing the temporary support.
  4. Slowly lowering the bottle jack.

I had to do this 5 times, once for each of the temporary supports.

While lowering the building, I used a hammer to tap the bottom of the studs inward or outward, whatever it took to make them line up with the bottom plate.


I used some of these angle brackets to help hold the studs to the new bottom plate.


Metal angle bracket used to connect new bottom plate to old wall stud. The ends of the bracket can be bent over.

I used Simpson Strong-Drive Screws in some places here, mostly to prevent splitting the wood. Nails are better because they have greater shear strength.


Once the bottom plate was securely attached to the studs, I drilled some holes for anchor bolts.

I used my fancy and expensive hammer-drill to drill through the wood and into the concrete.

Drilling hole in bottom plate and concrete floor to install anchor.


Inserting lead anchor into hole in concrete floor. I placed 3/8" lag shields into the holes. These are made from lead, which is soft and conforms to the threads of the lag screw.


I used a punch to drive the lag shield all the way down into the hole.


Lag screw used to fasten bottom plate of wall to concrete slab. I threaded a 3/8" lag screw into the hole and tightened it with a socket wrench. It's easy to over tighten this type of fastener and strip the soft lead. I think next time I'll use big Tapcon screws or something.

I installed 3 anchors along the 20-foot wall.


I fastened some treated 2x4 blocking between the studs, resting on the bottom plate.

The main reason I did this was to keep a certain chipmunk out of the garage. I knew it would take a couple of weeks before I could get around to installing replacement siding (because I had to custom-mill the siding).


A closer view of the new bottom plate, or sill plate. The red arrow points to one of several deck screws that I used to fasten each piece of blocking to the bottom plate.

I also used metal angle brackets to hold things in place. This prevents the stud or block from shifting while nails or screws are driven at an angle.

Most sill replacement projects would not require these extra blocks of wood, though they do improve the structure somewhat.


A Bit About Rot:

Ordinary wood will rot if:

  • The moisture content is 20 per cent or higher.
  • The temperature is above 35 to 40 degrees Fahrenheit.
  • There is adequate oxygen surrounding the wood.

The decay process is caused by fungi that are everywhere. The fungi use the cellulose in wood as a food source. Keeping the fungi spores away from wood is impossible. Keeping oxygen away from wood is impractical. The key to prevention of decay is to keep the wood dry, or at least provide a quick drying time for wood that gets damp or wet. 

It's my understanding that fungi spores go dormant if conditions are not right. Under favorable conditions the fungi cells will consume the wood. Since these are single-cell organisms, they reproduce by dividing in two. But this process takes time.

If wood is wet only for brief periods (a few hours at a time), the fungi do not have adequate time to undergo their cell-division process, so the fungal colony doesn't grow. If the fungi are exposed to sunlight, the ultraviolet light will kill them. Lots of old houses and barns have wood siding with almost no paint. The wood gets wet during rainstorms but dries out quickly, so it never rots (but there are other problems, such as warping and splitting).

If wood structures can be designed and built to provide adequate drainage for stray water (such as the rain that gets driven behind the siding by strong winds) and adequate airflow around moisture-prone areas, then the structure may survive indefinitely. It's been my experience that most carpenters and builders have little concern for such details. In many cases it seems that only building codes and building inspectors have any real power to enforce good building practices.

Masonry tends to act like a sponge, remaining damp for long periods of time. Masonry can also wick the moisture upwards from the soil, so even if concrete appears dry on the surface, it may be damp just below the wood wall structure. Today, building codes state that any wood placed next to concrete must be treated to prevent decay. This typically means using CCA-treated lumber (the greenish-tinted lumber sold for decks). 




Tools Used:

  • Basic Carpentry Tools
  • Reciprocating Saw
  • Cordless Drill-Driver
  • 6 Ton Hydraulic Bottle Jack
  • Lally Columns


Materials Used:

  • Pressure-Treated Lumber, 2x6'
  • Galvanized 16d Ardox Nails
  • 3" Deck Mate® Screws
  • Lag Screws and Lag Shields


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Written February 4, 2003