Everyone wants more room in their homes. Neglected spaces like basements and attics offer seemingly free space to use for guest bedrooms, kids' bedrooms, or home offices. Often, though, you need to get the basics into shape before you can tackle the real remodeling work. Attics typically do not come with usable flooring. So, flooring your attic is the first step to remodeling an attic.
Attics That Have No Flooring
Unless expressly built so, an attic's joists are meant for carrying the load of the ceiling below and attendant items, not for any flooring in the attic. This ceiling load may include items like drywall, ducts, recessed lights, bathroom fans, and attic insulation. Most houses' attics are built without flooring. However, in some cases, the joists are intentionally built strong enough for the homeowner to later build out the attic.
Attics With Dead Load Flooring
Alternatively, your attic may already be floored. Does this mean you can build rooms? Not necessarily. Flooring for dead loads may not be sufficient for live loads.
The presence of existing flooring does not ensure that you have a joist system that is substantial enough to support a floor for living space. The previous homeowner may have simply covered those joists with plywood to use that space for a dead load (literally, non-living space functions) such as storing Christmas decorations, kids' old toys, and keepsakes. While this is not the optimal set-up, it is acceptable to have ceiling joists with plywood or OSB on top to use for light attic storage of dead loads. In attics built purely for storage purposes, you may find two-by-six ceiling joists or, in some cases, even two-by-four joists.
Attic Flooring Spans for Live Loads
Attic joists that are made from two-by-eights may be acceptable for building your attic floor. Because every room is variable, there are no absolutes. Joist dimensions are only part of it, though. You need to ensure that the joist spacing is adequate.
In many cases, ceiling joists for dead loads only need to carry 10 pounds per square foot (psf), as opposed to the 40 or greater psf that live-load joists must carry.
Span length is different for every room. So, span length and width between the spans can be determined only by calculations. It is safe to say, though, that two-by-six ceiling joists spaced every 24 inches on-center (a typical arrangement found in attics) will not support live loads for a bedroom, office, or bathroom.
Span Calculations for Attic Floors
The best way to get your span calculations correct is to hire a structural engineer or contractor to run the numbers for you. In many cases, engineers will work on a per-hour basis, making this an affordable option.
However, you can obtain a relative idea of your span options for attic joists by consulting any number of online span calculators. One good reference is Washington State University's Maximum Span Calculator for Joists and Rafters, which allows you to enter factors such as wood species, size, grade, and deflection limit in order to calculate spans for both live and dead loads.
For example, with such a calculator, you will find that for a 15 foot span, you need two-by-ten Douglas Fir heart joists spaced every 16 inches. While not the ultimate voice in spans, these calculators do provide a reality check. Joists you may have thought adequate for your attic floor may not come close.
Strengthening Attic Joists for Live Loads by Sistering
If the attic joists are not adequate, you have two options to prepare them for attic flooring: sistering or interspersing the joists with additional joists. Sistering is the process of pairing a structural member that is damaged or is inadequate for an intended load with another structural member. In the case of two-by-six joists, you would pair them up with other two-by-six joists by nailing them together, side by side. The best-case scenario is to run the sisters the entire length of the existing joists so that you have two additional resting points. Your attic may have electrical wires running through the joists that make sistering more difficult.
Strengthening Attic Joists for Live Loads by Adding Joists
The other method of creating a stronger attic floor base involves keeping the existing joists as they are but interspersing them with joists of the same size. For example, two-by-six ceiling joists that are 24 inches on-center will become 12 inches on-center when you intersperse them with other two-by-six joists.