Strengthening Attic Floor Joists for Safe and Liveable Attic Spaces
Assessing Attic Joists for Live Loads
Before converting an attic into a living space, it’s crucial to assess the strength of the existing floor joists. Attic joists, which are the wooden beams that span the attic, are typically designed to carry only dead loads, such as the weight of the roof, insulation, and ceiling materials. Live loads, on the other hand, refer to the weight and movement created by people and furniture in a habitable space.
To determine if the attic joists can support live loads, consider the following factors:
-
Joist Dimensions: Joists made from 2-by-8s or larger may be suitable for live loads, depending on other factors.
-
Joist Spacing: Joists should be spaced no more than 16 inches on center (from the center of the plank, not the edge) to adequately support live loads.
-
Span Length: The distance between the supports for the joists (span length) also affects their load-bearing capacity. Longer spans require stronger joists.
Calculating Attic Floor Spans
To ensure accurate span calculations, it’s recommended to consult a structural engineer or use online span calculators like the one provided by the American Wood Council. These calculators factor in wood species, size, grade, and deflection limit to determine the appropriate span length for both live and dead loads.
Strengthening Floor Joists
If the existing joists are inadequate, there are several methods to strengthen them for live loads:
Sistering Joists: This involves adding a new joist alongside each existing joist, nailing them together side by side. This effectively doubles the load-bearing capacity of the joists.
Bridging Joists: Bridging involves installing perpendicular wood pieces or braces between the existing joists to stabilize them and reduce sagging. Bridges can be made of wood, metal braces, or strapping.
Blocking Joists: Blocking involves placing solid wood blocks between the joists to distribute weight more evenly. Lateral supports between adjacent floor joists, blocking helps prevent excessive bending and deflection.
Installing Sister Joists: A Step-by-Step Guide
-
Clear Joist Spaces: Remove any insulation, debris, or existing blocking/bridging from the joist spaces.
-
Measure and Cut Sister Joists: Measure the length of the old joists and cut new joist lumber of the same size to serve as sister joists. Check for crowning (slight bowing) and mark the top edge for proper installation.
-
Install Sister Joists: Fit the sister joists alongside the old joists, ensuring full contact and flush top edges. Nail them together with 10d common nails. Also, nail the sisters to the exterior and supporting walls/beams.
Other Considerations for Attic Flooring
Flooring Options: If attic joists are not strong enough for a living space but are suitable for light storage, plywood or oriented strand board (OSB) can be used as flooring. For habitable spaces, use tongue-and-groove plywood or OSB subflooring Glued and screwed to the joists.
Installing Plywood Over Attic Insulation: It may be possible to lay plywood over attic insulation, depending on the insulation type and joist strength. Control the height or thickness of the layers to avoid crushing the insulation.
Attic Floor Thickness: For light storage spaces with 16-inch joists, use 1/2-inch plywood or OSB. For 24-inch joists, use 3/4-inch plywood to prevent sagging. For living spaces, use 3/4-inch tongue-and-groove plywood or OSB as subflooring.
By following these guidelines and using the appropriate strengthening techniques, homeowners can safely and effectively reinforce attic floor joists to create a strong and liveable attic space.
