An Addition Foundation
Grafting the right foundation onto a house that's neither square nor level.
Synopsis: Learn techniques for adding a concrete foundation to an old house that may be irregular. The author covers foundation layout, bracing, and how to set up the forms.
I recently poured a foundation for a fussy builder. After the forms were stripped, he checked the foundation. The length and width were okay. But the foundation was 1 1/4 in. out of square, and it was 1 in. out of level from one end to the other. The builder paused for a moment and proclaimed the foundation perfect. The foundation was for an addition to an old house.
A new-house foundation and an addition foundation are two different things. With a new house, I start with an empty hole in the ground and follow the plans: straight, square, plumb and level. But with an addition foundation, the existing house and its foundation play a big part in molding my strategy.
Sizing up the existing foundation
When I’m planning an addition foundation, the first thing I check is the existing foundation. Here in New England, we encounter three basic types of foundations: poured concrete, stone and what I call a modified-pier foundation. Poured concrete is the easiest to add to. The walls are usually straight and smooth, and there is a good chance that the foundation is somewhat square and level.
Stone foundations, standard fare in this area before 1900, are the most challenging to add to. It’s a lot of extra work to match concrete forms to the irregular surface of a stone foundation. Stone foundations usually move a great deal over their long lives, so you can bet that they’re well out of square and level.
With the project shown in this article, the house was set on concrete piers poured into holes dug in the ground. Over the years, the spaces between the piers had been filled in with block that was not on footings. As is typical, this foundation as well as the house on it was neither square nor level.
The existing frame guides the addition foundation
Once I’ve looked at the existing foundation and determined where the addition foundation ties into it, I’m ready to begin layout. I locate the addition foundation according to the frame of the existing house to allow the addition framing to match seamlessly with the house framing.
The short explanation is that I run two sets of strings: one above grade off the mudsills of the house and the other on the floor of the excavation. The type of addition and its attachment to the house determine the degree of difficulty of this process.
If the addition is to be along one wall and away from its ends, such as for a simple bump-out, I set up a string parallel to the existing wall for the addition’s outside wall and then square back to the house for the sidewalls.
For the addition in this article, the foundation began at one corner of the house, with the wall of the addition continuing in the same plane as the house wall. If that plane wasn’t maintained, then the transition between the old house and the new would have become apparent.
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