Tying in an Addition to an Existing Home
Use these tips and tricks to match new floor heights to old and get everything square.
Synopsis: Building an addition can be tricky, especially when trying to match the height of the new floor to that of the existing structure. Builder John Whritner details his method of using a story pole and a builder’s level to determine the cripple-wall heights and get everything square and level. A detailed drawing and a series of photographs show the step-by-step process of laying out and building the cripple walls, adjusting as needed, and then tying it all together.
For a lot of reasons, building an addition can be trickier than building a new house. On this project, we needed to match the height of the new floor to that of the main floor of the existing structure, and the new foundation jogged and stepped, resulting in two different cripple-wall heights. Any difference in elevation from new floor to old would telegraph, so everything had to be spot on.
Stepped foundations are a bit harder to check for square. Even if they’re good at the bottom, the walls may be out a bit at the top. Setting up stringlines or lasers to check takes time, and in the end only tells you what you have to deal with—they don’t square it up for you.
Unless something is really out of square at the bottom, I don’t bother squaring up the mudsills on the front end; I build the cripple walls so they can be adjusted after they’re up. I also avoid math as much as possible, and rely on a story pole, a builder’s level, and a tape measure to simplify finding the stud heights for the cripple walls. The setup for this is easy and nearly foolproof.
Story pole + builder’s level
Site-made story poles help keep things consistent, whether it’s the height of door and window trim, siding, or framing. In this case, a story pole helps me figure out the cripple-wall heights.
Since the old siding will be stripped, I cut an inspection hole through the side of the house to expose the existing floor and mark its elevation on a scrap of 2x that will stay screwed to the side of the house for reference. For a story pole, I pick a straight 2×4 that runs from the ground to above the finish-floor height. I attach it next to the more permanent block and transfer the floor height to it. Then, I measure down and mark the thickness of the new flooring and subfloor, and the depth of the new 14-in. I-joists, and drive a screw to mark where the new joists will land on the cripple walls to pull a tape from later. Then I turn to the builder’s level.
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