Foundation on a Flatbed
The new foundation at the Carlisle Project won't be arriving in a cement mixer. It will show up on a truck, be installed in less than a day, and stay weather-tight for decades.
Foundation walls are surprisingly complicated things — they have to hold up the weight of the house, resist the pressure from the ground surrounding them, and keep the basement dry and warm. That's why building codes are so strict about them, and that's why they require so much painstaking planning. Even so, not every foundation is a decade in the making.
Update: Installation Complete
The foundation wall panels were installed on July 1. If you missed it, check out the time-lapse photos on our WebCams page.
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It was in 1994, at the This Old House project house in Acton, that we first used a precast foundation system — concrete walls that were built to spec in a factory and then delivered to the construction site ready to install. Tom Silva was impressed with the product, and we've been saying for ten years that we'd look forward to using it again. As fate would have it, at a trade show this winter Tom ran into Mel Zimmerman, founder and CEO of Superior Walls, which provided the foundation panels we used in Acton. This year, his January meeting with Tom led Zimmerman to a much bigger challenge: in June, his precast walls will become the foundation of the 25th anniversary project in Carlisle.
It's a tall order — literally. In Acton we used precast walls for a crawl space, but in Carlisle we'll be using 9- and 10-foot-tall panels for a full basement. It's a long order, too, since this is no simple addition. We'll be installing a new foundation not only under the master bedroom extension behind the main house, but also under the entire 65-foot-wide central ell we're building to replace the structure we demolished there last month — for a total of 250 linear feet. The new foundation is a complex shape, with bump-outs and bays, and it has to be tied into the existing foundation under the original house on one side and into the foundation under a jacked-up barn on the other.
The size and shape of the basement is not what makes this interesting, of course; it's that foundation walls are generally built or poured on site, with all the forming and curing time that normally entails. Doing it right is a painstaking and time-consuming process, which can't begin until excavation is complete and during which all other construction has to wait. Our Carlisle foundation, however, will be manufactured to our specifications in a factory while we excavate and delivered via flatbed truck the day we need it. The panels will arrive already insulated, fortified with rebar, and ready to be sealed together into solid walls. The day after they're installed, we can start to build.
Tom is impressed with the speed, of course, but he wouldn't be using this system if speed were all it had in its favor. "It's very strong, and it's sealed," he says. "It's also dead-on after it's set: level, plumb, and square. I found that out when we did Acton." That project also demonstrated just how fast fast can be. "In Acton," says Tom, "we were able to pour the concrete floor for the basement the day after the panels went in. The next day we started the frame. Now that's fast."
All that speed of installation requires some lengthy advance planning. The Carlisle foundation plans have to undergo scrutiny from the local building department, which requires extensive documentation. Architect Jeremiah Eck has already drawn up a detailed spec that provides measurements and positions for the foundation walls, indicates where ground level is, and provides information on point loads (how much weight the foundation walls will be expected to carry, and where). From that spec, Superior Walls created its first schematics of the foundation.
Once the building department signs off on the spec, a number of teams will get to work. Superior Walls will finalize the schematics and start the manufacturing process on the panels. Tom's team will finish the excavation and install 4-inch perimeter drains. A company called Rock-It Stone Spreaders will truck in nearly 60 tons of 1/2-inch stone and fire it down into the excavated hole, where Roger Cook's landscape team will level it. When the Superior Walls flatbed arrives on installation day, a crane will lift the panels off the truck and set them in place on the stone bed, where workers will bolt them together and apply a triple bead of special concrete sealant to each vertical joint. Presto! the foundation walls are done.
Once the walls are in, construction will start moving at a rapid pace. We'll put down a vapor barrier over the bed of crushed stone, followed by 2 inches of sand. Finally, we'll pour a 4-inch-thick concrete slab for the basement floor. At the top of the foundation panels, we'll put the first-story floor into place. With the foundation panels held in place by the basement slab at bottom and the floor frame at top, we'll backfill the outside. "Then," adds Tom in a classic understatement, "we build the building."
Unlike a new construction project, the Carlisle foundation walls will need to tie in to existing stone foundation walls. To make the connections, some parts of the older foundations walls will have to be taken out. "We're going to remove as much as we need to, but not more," says Tom. "Some parts of the old stone will be tied in to new concrete block, then the Superior Walls system will tie into the new block foundation with bolts — a piece of cake."
A 9-foot-tall basement, of course, cries out to be finished space, and the wall panels anticipate that. The concrete studs are covered with wood nailers that will make drywalling the basement a snap. Predrilled holes in the studs will allow us, or the future homeowners, to run wiring through the walls with ease. We're still in the dreaming stage about what we'll put in the basement — wine cellar, anyone? — but we already know how straightforward the finishing work will be.
So the panels are fast and easy to install and convenient to finish — but that's not what concerns homeowners most about their basement walls. The feather in the cap of these precast walls is that they're designed to stay warm and dry. Built in a factory-controlled setting, they're not subject to some of the errors that can plague traditional foundations, like pouring concrete in too-cold temperatures, or not allowing it enough time to cure. Superior Walls has been around since 1981, which means 23 years of watching the product to see how it holds up. So far so good — 50,000 foundations, the company reports, and all still dry. That sure works for us, and for the family who will be calling this house home in another 23 years.
Where to Find It:Superior Walls
Rock It Stone Spreaders