How to Strengthen Floor Joists Before Finishing an Attic
Size, Spacing, and Span Considerations for Structurally Safe Floors
Many people want more room in their homes. How you do this varies, depending on the nature of the existing joist and flooring system. Neglected spaces like basements and attics offer free space for guest bedrooms, kids' playrooms, or home offices. Attics typically do not come with usable flooring, so this must be measured and sometimes corrected before remodeling can begin.
The safest way to know how much weight your attic can support is to get a structural engineer to assess the joists and attic structure. Generally, an attic constructed of 2-by-4s or 2-by-6s is not adequate as a living space. Also, spacing between joists must be at least 16 inches on center (from the center of the plank, not the edge). Anything more than that is not adequate. But you can also use several techniques to strengthen the existing joists and bridge the gaps, eventually allowing for flooring.
What Are Attic Joists?
Attic joists are the wooden planks that span the attic that provides structural integrity to the house. They can be called "ceiling" or "flooring" joists, depending on the load they were designed to carry. Most attic joists are ceiling joists designed only to hold the house, walls, insulation, and supporting materials. Flooring joists were also intended to carry a live load or living space.
Attics That Have No Flooring
Unless expressly built to accommodate a living space in the attic, an attic's joists are only meant for carrying the load of the ceiling below and related elements, collectively known as the dead load. This load may include drywall, ducts, recessed lights, bathroom fans, and attic insulation.
Most homes' attics are built without flooring and are not designed to carry the heavy load of finished space. In rare cases, the joists are intentionally made strong enough for the homeowner to build out the attic later.
Attics With Existing Flooring
Alternatively, your attic may already have some floor, but this doesn't necessarily mean you can use it as living space. The previous homeowner may have covered the joists with plywood to use the area for storage. While this is not the optimal setup, it is acceptable to have ceiling joists covered with plywood or OSB for light attic storage of dead loads.
In attics built purely for storage, you will find 2-by-6 or 2-by-4 ceiling joists. This flooring is intended for dead loads and is insufficient for live loads. Live loads are the weight and movement created by people and living space.
Attic Joists for Live Loads
Attic joists made from 2-by-8s 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) instead of the 40 psf or greater that live-load joists must carry.
The span length is different for every room. So, the span length and width between the spans can be determined only by calculations. It is safe to say that 2-by-6 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 ensure your span calculations are correct is to hire a structural engineer or contractor to run the numbers. A structural engineer is recommended whenever you plan to build out an attic space, whether you need a permit or not. Engineers will work on a per-hour basis, making this an affordable option.
You can get a relative idea of your span options for attic joists by consulting an online span calculator. One good option is from the American Wood Council, which offers a calculator that allows you to enter factors such as wood species, size, grade, and deflection limit to calculate spans for both live and dead loads.
Joists you may have thought adequate for your attic floor may not come close to being strong enough. Using a span calculator, you can estimate that for a 15-foot span; you will need 2 by 10 Douglas fir heart joists spaced every 16 inches. These calculators provide a good baseline or reality check but should never be used as the final authority.
Methods to Strengthen Attic Floor Joists
If the attic joists are inadequate, one way to strengthen the floor for live loads is to "sister" the old joists. Another method is to bridge them or use blocks to distribute weight with less strain on the joists.
Sistering is adding a new joist next to each existing joist. For 2-by-6 joists, you can pair them with an additional 2-by-6 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.
Bridging strengthens attic floor joists by cutting wood pieces or braces and installing them perpendicularly between the existing joists. This serves to stabilize the joists and reduce sagging. Bridging is usually installed in every joist space, and multiple bridge pieces or braces are often required. Bridge pieces can be made of wood, metal braces, or strapping.
Solid wood blocks can also be placed between the joists. This is commonly called blocking. The solid wood blocks are lateral supports between two adjacent floor joists to distribute loads evenly. Lumber with the same width as the joist is used for solid blocking between joists at the mid-span of the joist.
What You'll Need
Equipment / Tools
- Circular saw
- Joist lumber (to match existing joists)
- Blocking lumber
- 10d common nails
How to Reinforce an Attic Floor with Sister Joists
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. These are pieces of wood inserted perpendicularly between joists.
Under no circumstances should any rafter bracing be removed or manufactured trusses be altered without the approval of an engineer. The integrity of the roof will be affected.
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.
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.
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.
Cut and install lumber blocking or bridging between the joists, if required by local code. This consists of lumber the same size as the joists installed between the joists perpendicularly. Once the structure has passed the building inspection, you can run electrical, plumbing, and mechanical lines and insulation, as applicable, then install plywood subflooring.
What is the best flooring for an attic?
If your attic joists won't bear the weight for a living space, but you use the attic for light storage, you can use plywood or oriented strand board (OSB). If using it as a livable space, use tongue-and-groove plywood or OSB subflooring glued and screwed to the joists before installing your desired top flooring. Good attic room choices include resilient flooring like vinyl and laminate, which absorb sound. Carpeting also absorbs sound.
Can you put plywood over attic insulation?
You might be able to put plywood over attic insulation (batting or blown insulation); it depends on whether the joists are adequate to sustain additional weight. If the joists are sufficient, control the height or thickness of the layers so boards can be laid over the top without crushing the filler.
How thick should an attic floor be?
For an attic being used as light storage space, you can install 1/2-inch plywood or oriented strand board (OSB) in 4-foot-by-8-foot panels over existing 16-inch joists. For joists spaced 24 inches, use thicker, 3/4-inch plywood to prevent sagging. Floor decking for the subflooring of a living space should be 3/4-inch tongue-and-groove plywood or OSB.
Understanding Loads and Span Tables. Department of Environmental Conservation, UMass Amherst Building and Construction Technology.
Mastering Roof Inspections: Roof Framing, Part I. National Association of Certified Home Inspectors.