But sometimes, you need to spend a significant amount of time getting the basic "box" into shape. Attics, which usually do not have flooring, need this type of building-out.
Case 1: Attic Joists But No Flooring
Unless expressly built so, an attic's joists are meant for the ceiling below, not for any flooring in the attic.
Most houses' attics are built without flooring. However, in some cases, the joists are intentionally built substantial enough for the homeowner to later build out the attic. That is a good strategy for cutting costs for homeowners in the short term.
But in most cases, these joists were not intended to support living space.
Case 2: Existing Attic Floor For Dead Load
You may already have flooring in your attic. Does this mean you can build out? Not necessarily. Flooring for dead loads may not be sufficient for live loads.
Flooring does not ensure that you have a solid-enough joist system 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 (non-living space functions) such as storing Christmas decorations, kids' old toys, and the like.
While not the optimal set-up, it is fine to have ceiling joists (i.e., not flooring joists) with plywood on top to use for light storage of dead loads.
In attics built purely for storage purposes, you may find ceiling joists that measure 2" x 6" or even 2" x 4".
Sizing, Spacing, and Span of Attic Flooring Joists For Live Loads
Attic joists that are 2" x 8" 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.
Ceiling joists for dead loads only need to carry 10 pounds per square foot (psf), as opposed to the 40 psf or greater 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.
Sufficient to say, though, that 2" x 6" ceiling joists spaced every 24" on-center--a typical arrangement you will find in attics--will not support live loads for a bedroom, office, or bathroom.
Span Calculation for Attic Floor
The best way to get your span calculations right is to hire a structural engineer or contractor. This will be money well-spent. Some structural engineers will work on a per-hour basis.
However, to obtain a relative idea of your span options for attic joists, consult these span calculators:
- Washington State University: Maximum Span Calculator for Joists and Rafters
- Washington State University: Span Options Calculator for Joists & Rafters
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.
To cover 15 feet of span, you will need:
- 2" x 10" joists
- Douglas Fir, heart
- 16" on-center spacing
Fixing Attic Joists For Live Loads
If the joists are not adequate, you have two options to prepare them for attic flooring.
Sistering is the process of pairing a structural member that is damaged or is inadequate for an intended load with another member.
In the case of 2" x 6" joists, you would pair them up with other 2" x 6" joists by nailing them together.
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.
2. Intersperse Joists
Keep the existing joists as they are, but intersperse them with similarly-sized joists.
For example: 6" ceiling joists that are 24" on-center become 12" on-center when you intersperse them with other 6" joists.