Replace a Rotten Sill Plate
By Mark Clement
Removing and Replacing a Rotten Sill Plate.
A rotten sill plate is a common occurrence in old houses—and a big problem. The sill is the piece of wood closest to the ground, either on a foundation or piers, and is usually a “timber-sized” board: 3×6, 3×8, 4×6, 4×8, and so on. The studs often rest directly on the sill and are toe-nailed in with no bottom plate as you’d find in a modern house. Often installed too close to the ground or otherwise exposed to water or insect infestation, sills can—and do—literally rot out from under the building.
The good news is that many of them can be replaced using common tools, common materials, and common sense. The bad news is that every sill replacement is loaded with individual factors so this article is more about explaining our approach rather than specific steps that’ll cover every job.
That said, one constant is gravity so we know we’ll encounter the same general things on most projects.
- Remove weight from the sill.
- Remove the sill (and any other damage).
- Re-design a sill and/or stud system that meets current code and fits the needs of the space.
Signs the Sill Is Rotten
Unless you can see it from your basement or crawlspace, a rotten sill is a hidden problem you might not notice until you’re doing something else. In this case, we were demo-ing the existing plaster while upgrading a mudroom and saw the problem.
On the other hand, there may already be telltale signs: the exterior siding is buckling or cracking, the floor inside is spongy to walk on or the room has a noticeable dip. Once you see that the sill is better suited for mulching your petunias than supporting a roof, you might ask, “What’s holding the building up?”
The answer is: the membrane created by sheathing and siding—and “habit,” as some carpenters say. It won’t last forever, but it’s what’s doing the job now.
Carry the Load
Sub it Out? Technique aside, the first step to repairing a rotten sill is to determine if this is the right job for you. If, for example, the sill sits directly under two stories of house, you’re talking about managing a lot of weight. Time, preparation and installation process—not to mention permits and inspections—are critical.
If phrases like “continuous load path,” “point-load” and “header” aren’t a central part of your lexicon, we’d recommend subbing this one out. And if you do, this article contains lots of information you’ll need to make sure your contractor knows what he’s doing.
On the other hand, while the steps are the same, there is a lot less at stake if you’re talking about a small, single-story structure like the one we repaired here. We still needed to manage the weight of the roof and walls—and used similar principles as if we were managing the weight of an entire building—but there wasn’t an entire building held up on temporary framing over our heads.
Continuous Load Path. The thing about removing a sill plate is that you have to support all the weight above it while you remove and replace it. That means you must design a temporary framing system that carries the entire load rather than portions of it.
For example, it may seem expeditious to jam a 6×6 under a single rafter or joist. However, that’s only carrying a “point load,” which is the member above it and little else. In other words, you have to send the weight of all the overhead framing into the ground, thus suspending the wall.
Generally, the most effective way we’ve found to do this—especially the heavier the weight—is to build a temporary stud wall. The way we do it is to screw a top plate to the bottom of the rafters. Next, fasten a stud plumb under each rafter. But you can’t cut studs willy-nilly. They have to fit extra tight. If you have to pound a stud into place with a 2-pound sledge (we’re assuming a concrete basement floor here, rather than a finished interior floor, dirt floor or grass), that’s the kind of tight we’re talking about.
Alternatively, you can build a similar structure outside, which is what we did. We screwed a top plate across the length of the eave, then dropped double 2×6 studs every 24 inches to the ground below (a concrete apron). We tied the studs together with diagonal bracing. Were there not a concrete apron, we would have laid 2×12 on the ground as a pad.
The load from the top plate is transferred across the plate and down the studs into the ground. The other reason for fitting the studs and plates super-snug is that you want to keep the framing above from moving as the existing framing is removed. Tension on the studs (and the existing membrane mentioned earlier) helps prevent any movement at all.
Removal—Studs and Sill
In addition to our sill plate being trashed, water had wicked up the studs and sheathing, rotting the bottoms. This meant we also had to extract the studs from the sheathing and re-structure the wall.
To get the studs out we cut them at least 6 in. above the visible rot. And while a recip saw would work for making the entire cut, they are often too aggressive and can dislodge framing from perfectly solid connections. Instead, we made a starter cut with the worm-drive saw (which turns out to be cleaner, faster and easier), and then finished the cut with the recip.