Things You'll Need
Covering part or all of the ceiling joists in the attic is a cheap way to make use of wasted space. Installing the subfloor is the easy part. The difficulty comes from the precise measurement and cutting required and the labor of getting the sheets of underlayment in place.
Since ceiling joists are 24 inches on center and wood subfloor material comes in 4-foot by 8-foot sheets, you can potentially use a lot of whole sheets with no need for cutting. The question is, can you get a whole sheet through your attic access door?
Measure the area closest to the attic door to determine if a whole sheet can be used, or what the first cut must be. When laying out for the first sheet, keep in mind the distance between the joists as well as their thickness. Two sheets will meet over a single joist, so each will have only half the width of the joist for nailing. That means 3/4 inch per sheet.
Carry the first whole sheet of flooring into the attic. If whole sheets are too large, start with the longest piece you can fit through the door. Align the edge carefully with the joists so that you are only covering half of each joist at the edge of the sheet.
Nail one corner, leaving the head of the nail slightly above the surface in case you need to move it. Nail the opposite corner lengthwise down the sheet, being careful to check the alignment on the edge of the joist below. Ensure that the sheet is still in the correct position, then nail down all four corners.
Nail down the rest of the sheet, spacing nails about every 12 to 18 inches apart down the joist. Measure the next cut or plan the next whole sheet. Stagger the sheets where possible. When making cuts, remember you are only covering half of each joist. Repeat Steps 3 and 4 until the desired attic space is covered.
Before starting, use a large piece of cardboard to determine the size of flooring material you can get into your attic via the drop-down stairs or trap door.
Use whatever subflooring material is the cheapest in your area. This can be plywood, flakeboard, particleboard or any other wood sheeting that is strong enough to span 24 inches and support weight. Half-inch sheets may be thick enough, but 5/8 or 3/4 inches is recommended.
Keep in mind that a 4-by-8 sheet of flooring will fit the ceiling joists in either direction. It may be easier to lay the sheets cross-ways across the joists instead of length-wise.
Any nails at least 3/4-inch longer than the thickness of the flooring will be sufficient, but spiral shank nails will hold better with less chance of warping.
Regardless of the direction you lay the sheets in, stagger them so the joints between sheets don’t line up. This will add strength and stability.
If you don’t have sufficient insulation in the attic, now would be the time to correct this.
Always wear a dust mask while working in the attic. Fiberglass dust from the insulation is an extreme irritant.
Check under each sheet for pipes and electrical conduits before nailing; remember that sometimes these utilities run through holes drilled in the joists.
Don’t cover up things you may want access to, like ceiling fixtures and cable TV connections.
Make sure there is adequate insulation and ventilation in the attic to maintain a temperature adequate to handle the items you want to store there.
Darryl Brooks is a writer living in Atlanta, Georgia. His experiences include 16 years installing tile, 30 years working in information technology, eight years as a writer, six years as a photographer, 15 years as a competitive runner and 15 years in a travel agency.