Maple Gym Floor After a Roof Leak
A 600 m2 maple gym floor in a community center has a 3 m by 8 m area of cupped boards along the south wall after a roof leak. The water stood for 3 days before extraction. The finish is flaking in the affected area, and several boards show dark staining and a 4 mm gap at the ends. A moisture meter reads 18% on the affected maple and 8% on unaffected boards 10 m away. The gym is scheduled to reopen in 10 days. The building has dehumidifiers and fans, but no floor-drying mats. The subfloor is 25 mm plywood over sleepers on a concrete slab. The maintenance budget allows either sanding and refinishing the whole floor or replacing the affected boards and blending the finish.
1. Decide whether the floor can be sanded and refinished in 10 days or whether replacement is required. Explain the moisture readings and the risks of sanding too soon.
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The 18% reading versus 8% on unaffected boards shows an active moisture gradient, which is why the boards are cupped. Sanding while the top is drier than the core can flatten the surface temporarily, but the boards can re-cup and the sanded surface can become uneven. The 4 mm end gaps and dark staining suggest more than surface finish damage; the staining may be water or iron stain, and some boards may be delaminated. Ten days is very tight.
Before sanding, dry the space and monitor moisture until affected boards are within about 2-4 percentage points of unaffected boards and stable for several days. If the cupping does not flatten, or if sanding would remove too much wear layer, replace the affected boards. Because a patch repair will not match color or sheen, plan a full sand and refinish of the gym. If the schedule cannot move, tell the client the floor cannot be responsibly reopened in 10 days.
2. Outline a drying and verification plan before any sanding, including how to check for concealed moisture in the subfloor.
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Extract standing water, then use dehumidifiers and air movers to create airflow across the floor; do not seal the area. Keep the space at normal occupied temperature and humidity if possible. Record pin-type moisture readings on the maple at multiple depths and locations daily, plus a pinless meter for comparison. Check the plywood subfloor and sleepers through accessible edges or by lifting a small section; a dry top floor over a wet subfloor will re-cup. Look for mold, delamination, and nails backing out.
Set a target: affected wood within 2-4 points of a dry reference and no change over 3-5 days. Verify with a plastic-sheet test on the subfloor if accessible. Only then sand.
3. If replacement is necessary, sequence replacement with sanding and finishing so the floor can reopen safely.
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Remove and replace only the badly damaged boards, using matching maple and allowing the new wood to acclimate to the space. Install so the new boards are flush and tight, then let the patch stabilize. Sand the entire floor as one unit, not just the patch, so the surface is flat and the finish matches. Apply sealer and finish per the gym floor system, with the recommended number of coats for the traffic level, and screen between coats. Keep the area closed until the finish cures.
Ten days may be enough for a small patch plus a full recoat only if drying is already complete. If drying is not complete, replacement does not solve the moisture problem, and the floor will fail again.