A Decision Maker's Guide to Attic Remodeling

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Fill in the Attic Checklist blanks. For now, assume that all joists and beams are supported from below only on their ends. Once you complete your attic survey, you can measure internal walls on the floor below to see if they offer any structural help.

Structure

Most attic floors were not designed to support the loads common in a normal living space. Standard building codes now specify that the floor of every living space be designed for 10 pounds per square foot (psf) dead load (the weight of the materials themselves) and 30 psf live loud (you,, your furniture, and other belongings). Many older attics were built for only 10 psf dead load and 10 psf live load, half the required total.

Since clear span--the portions of joist and beams that get no support from below—is so important to structural adequacy, you need to locate all load-bearing walls that may break up the spans of your attic's floor. Look for walls that run perpendicular to the joists and that are themselves supported by walls or bean is below. (In a few cases, you may find walls that appcar to be structural but have no support from below.) All load-bearing partitions support the ceiling, so you can consider them to end a joist clear span. Also look for walls or posts under girders; if you find a girder that appears to span more than about 12', look again for supporting partitions.

Consult the span tables to see if your attic's floor is up to specs. These tables are for dimensional lumber (planed to a size smaller than its claimed dimensions) of average strength (eastern spruce). If your attic's joists and beams are full-size, rough-cut lumber or are made of a stronger material, you may have some leeway. (Douglas fir, for example, is about 40% stronger in most specs than eastern spruce.) The stress calculations are also designed with a wide margin for error, so don't panic if you find that your attic isn't even up to the job it's handling now.

If your attic floor isn't strong enough to support chests of drawers, waterbeds, and leaping children, it can probably be brought up to standard. You can strengthen inadequate joists by "scabbing on": attaching new joists to the sides of the old ones. You'll probably have to use deeper boards, which will raise the height of your floor. Because the bending strength of a floor joist is proportional to the square of its depth, a 2 X 6 on edge is actually twice as strong in bending as doubled 2 X 4s.

Weak girders present a harder problem. Because the joists intersect them, you can't just scab additional lumber to the sides. To bolster a girder, you'll need to add posts or structural partitions on the floor below, or suspend it from a built-up truss. Either approach will compromise useful floor area, but with clever layout, it may not be a big problem.

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