How to Strengthen Floor Joists Before Finishing an Attic

Attic Flooring With OSB Wood

BanksPhotos/Getty Images

  • Working Time: 16 hrs
  • Total Time: 3 days
  • Yield: 16 x 30 attic space
  • Skill Level: Advanced
  • Estimated Cost: $300 to $500

Many people want 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 this must be corrected before remodeling can begin. How you do this varies, depending on the nature of the existing joist/flooring system.

Attics That Have No Flooring

Unless expressly built so, an attic's joists are meant for carrying the load of the ceiling below and related elements, collectively known as the dead load. The ceiling load may include items like drywall, ducts, recessed lights, bathroom fans, and attic insulation. Most houses' attics are built without flooring and are not designed to carry the heavy load of finished space. However, in some cases, the joists are intentionally built strong enough for the homeowner to later build out the attic.

Attics With Existing Flooring

Alternatively, your attic may already have some sort of floor, but this doesn't necessarily mean you can build rooms. The previous homeowner may have simply covered the joists with plywood to use the space for storage. While this is not the optimal setup, it is acceptable to have ceiling joists covered with plywood or OSB to use for light attic storage of dead loads. In attics built purely for storage purposes, you may find 2x6 ceiling joists, or in some cases, even 2x4 joists. Flooring for dead loads may not be sufficient for live loads—the loads created by people and the features of living space.

Attic Joists for Live Loads

Attic joists that are made from 2x8s may be acceptable for building your attic floor, but because every room is variable, there are no absolutes. Joist dimensions are only part of the equation. You also need to ensure that the joist spacing is adequate. In many cases, ceiling joists for dead loads are designed 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 2x6 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 2x10 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 to being strong enough.

Strengthening Attic Joists for Live Loads by Sistering

If the attic joists are not adequate, one way to strengthen the floor for live loads is to sister the old joists. Sistering is the process of adding a new joist next to each existing joist. In the case of 2x6 joists, you can pair them up with additional 2x6 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.

What You'll Need

Equipment / Tools

  • Circular saw
  • Hammer


  • Joist lumber (to match existing joists)
  • Blocking lumber
  • 10d common nails


How to Reinforce an Attic Floor with Sister Joists

  1. Clear the Joist Spaces

    Pull out all insulation and any debris from all of the joist spaces so you can see the full length of each joist, including where it meets the exterior walls. Also remove any blocking or bridging between the joists, as applicable.

    Under no circumstances should any rafter bracing be removed or manufactured trusses be altered without approval of an engineer. The integrity of the roof will be affected.

  2. Measure the Old Joists

    Measure the length of the old joists and note how much they overhang any supporting walls or beams and the exterior walls. The top corners at the outside ends of the joists may be cut at an angle to fit under the roof decking.

  3. Cut the Sister Joists

    Cut new joist lumber for each of the sister joists, using a circular saw. Check each joist for crowning (slight bowing along the length of the board) and mark the top edge to ensure that the joist is installed with the crown facing up.

  4. Install the Sister Joists

    Fit each new joist in place next to an old joist so their faces make full contact and their top edges are flush. Nail the sister to its mating joist with 10d common nails. Also, nail each sister to the top of the exterior wall and any supporting walls or beams.

    A pneumatic nail gun would be recommended by professionals. Hammering will likely loosen the drywall nails in the ceiling below causing them to "pop" out the finish compound.

  5. Install Blocking

    Cut and install lumber blocking or bridging between the joists, if required by local code. Once the structure has passed the building inspection, you can run electrical, plumbing, and mechanical lines and insulation, as applicable, then install plywood subflooring.