How to repair a bouncy floor

Inspired by the Skill Builder channel. This guide is based on the approach shown in the Bouncing Floor Joist Mystery Solved video from the UK's Skill Builder channel. Roger Bisby has a knack for explaining why something is failing before reaching for a fix — a refreshing change from guides that jump straight to the answer without understanding the problem.

1. Diagnose the cause

Walk the floor slowly and note where the bounce is worst. A springy zone in the middle of a large span usually means the joists are undersized or lack intermediate support. Movement at the edges points to loose subfloor fixings or a gap at the wall plate.

If you can access the underside from below — through a crawl space or a partial basement — look for joists that are notched too deeply, split, or simply too small for the span. Also check for missing or badly fitted herringbone strutting, which is the timber bracing between joists that stops them twisting sideways under load.

2. Check for rot or structural damage

Probe each accessible joist with a sharp screwdriver at the ends, near the wall plates, and anywhere that looks discoloured. If the screwdriver sinks in without resistance, the timber is rotten and needs replacement rather than stiffening.

Rot at the wall plate end is common in older houses where ventilation under the floor has been blocked over the years. Before you stiffen anything, make sure the ventilation is adequate — fixing a bouncy floor on a damp structure just delays the next problem.

3. Add solid or herringbone strutting

For a bouncy mid-span floor where the joists are sound but unbraced, adding herringbone strutting is often all that is needed. This is a series of diagonal timber braces fixed between joists at mid-span to prevent lateral movement and share the load between neighbours.

Cut 50 × 38 mm treated timber at 45-degree angles and nail them in pairs, crossing between each joist from top on one side to bottom on the other. Solid strutting — a full-depth piece of timber cut tightly to fit between the joists — is quicker to fit and works just as well for most domestic floors.

4. Sister the joists if they are undersized

If the existing joists are simply too small or too far apart for the span, sistering adds a new joist alongside each existing one, bolted or screwed together. The new joist runs the full length and bears directly on the wall plates at each end.

Use the same depth as the existing joist or one size up. C16 or C24 regularised timber is correct for structural use. A single sister per joist, properly fixed, doubles the effective stiffness for relatively modest cost and effort.

5. Tighten loose subfloor boards

Even well-supported joists can cause a bouncy feel if the boards are loose. Old floorboards fixed with cut nails will gradually work free as the timber moves through the seasons.

Screw the boards down using 65 mm or 75 mm wood screws driven into the centre of each joist below. Pre-drill to avoid splitting. Tongue-and-groove boards that have sprung apart at the joint can sometimes be tapped back together and re-fixed before the gap becomes a trip hazard.

6. Fix any point loads or problem areas above

Heavy items — a freestanding bath, a large fridge, a stack of books on a bookshelf — concentrate load onto a small area of floor. If the bounce is directly under something heavy, consider whether spreading the load across more joists is the answer rather than stiffening everything.

A 25 mm plywood sheet screwed over the subfloor in a bad area distributes load effectively and can be done from above without any access below. It also levels minor unevenness at the same time, which is a bonus if you are about to lay sheet vinyl or LVT.

7. Test and check clearances

Walk the floor again after any repairs. A solid result should feel noticeably firmer underfoot, with no visible flex in the boards. Check that any new strutting has not fouled any pipes or cables running between the joists — easy to miss if you are working from below in a confined crawl space.

If the bounce is worse rather than better after adding strutting, the joists themselves may be the issue and a structural engineer's view is worth getting before you go further.

When to call a handyman

Call Richard if you cannot safely access the floor from below, if there is visible rot or structural damage, or if you would rather have the diagnosis done properly before committing to any repair. A bouncy floor is rarely as expensive to fix as it feels — but getting the cause right first matters.