Cross-bridging on joists may stop floor bounce

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This article was published 04/01/2004 (7918 days ago), so information in it may no longer be current.

QUESTION: We have a bungalow that has a 30-foot width with a beam down the middle. The floor joists are 7 1/2-inch fir. I suspect that if a larger floor joist had been used, this would have eliminated the bounce that we experience when we walk across the floor. Is there any simple strapping, etc., that could be added to the joist to eliminate this bounce? We have been considering putting hardwood flooring down in the dining area and hallways and I have concern that this bounce will interfere with the solid deck that you would want for laying hardwood down. Is this a real concern or would the hardwood itself provide some of this strength that would be needed in the floor? Thanks for your reply to this matter.

— Brian Bodner, e-mail

ANSWER: If I am reading your question correctly, your home with a single main beam has a 30-foot width, and that should leave a span of a little more than 14 feet for the 2×8 joists in your floor structure. This may be somewhat over-spanned, depending on the space between the floor joists and if there are no other supporting members below. It will likely be the cause of the “bounce” in the floor that you mention. Alterations or reinforcement to help minimize this movement will depend considerably on the finish on the underside of the floor joists and the arrangements of the area in the basement.

Addition of reinforcement or framing to strengthen the flooring system will be much less difficult if your basement is unfinished and has minimal obstructions. This is not usually the situation in many older homes. Access to the underside of the floor structure will be critical, and the first step may be to remove any basement ceilings or drywall attached to the underside of the floor joists. Once this area is open and visible, it can be assessed and the method of reinforcement, if any is possible, can be decided upon.

Once the underside of the floor is visible, the first item to look for is proper cross-bridging between the original floor joists. This bridging is normally comprises two small boards nailed diagonally between two joists, forming an X between them. This bridging serves a couple of purposes, but will primarily function to strengthen the floor structure once the home is beyond a few years old. Initially, this cross-blocking will help prevent twisting of joists as they dry out after initial installation, but this is not their main purpose. These braces help to add rigidity by providing continuity between all the joists in a floor, utilizing the entire joist depth rather than just the tops, which are attached together by the floor sheathing.

Cross-bridging gives the floor structure considerably more strength and will help reduce deflection or the bounce you feel. This will reach its maximum effectiveness only if the bridging is continuous throughout the entire length of the floor. This is not normally the case. This small blocking is often removed and discarded when heating ducts, plumbing pipes and other systems are installed. Basement and rec-room renovators often remove these 2×2 boards to accommodate new items installed. This bridging can often be easily reinstalled if missing, or replaced with other blocking near the bottom of the joists. Ensuring this blocking is continuous will be the simplest and least costly way of adding strength to the floor structure.

Adding additional or larger floor joists, doubling up existing joists or reinforcing cracked or damaged existing ones with plywood are all options to consider, but may be difficult to accomplish. Wires, pipes and heating ducts are often run between floor joists, making the addition of more support structures complicated. It is impossible to suggest proper remedial action without seeing the existing basement.

Installation of new hardwood flooring will help to add some strength to the entire floor structure, but the extent of this may be limited. Any component that adds depth to the floor will increase its support capabilities somewhat. How the flooring is installed and the pre-installation preparation may determine how much this helps. The key to this will be resecuring of the old subfloor.

The plywood or plank-wood subfloor should be checked, after removal of the old floor covering, for proper securing to the floor joists. If there are loose nails, damaged boards or major squeaks, they should be repaired or removed as a first step. Pre-drilling and screwing down this older subfloor will help strengthen this area and prevent concerns later on. If the old subfloor is in poor shape or minimal in thickness, another layer of plywood may be installed over top. This should be done with subfloor adhesive and screws, which will give the new hardwood a more solid and proper base for installation. If the new hardwood is nailed or secured with special flooring fasteners rather than glued down, this will also add some stability to the old floor system.

It is commendable that you are thinking ahead and trying to improve a problematic situation in your home while doing renovations. This is the ideal time to try and fix old issues and will help prevent dissatisfaction with the new flooring. Extensive reinforcement of the floor structure, enough to eliminate the bounce, may not be possible, but attention to these other details may provide a noticeable improvement.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and vice-president of the Canadian Association of Home & Property Inspectors (www.cahi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P.O. Box 69021, #110-2025 Corydon Ave., Winnipeg, Man. R3P 2G9. Ari can be reached at (204) 291-5358.

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