How to Fix a Dip in the Floor – DIY Guide
To fix a dip in the floor, identify the cause — rotten joist, loose board, or subfloor movement — then repair or sister the joist, refasten or replace the affected board, and fill any remaining low spot with self-levelling compound before relaying your floor covering.
- Pry bar
- Cordless drill and drill bits
- Circular saw or jigsaw
- Spirit level
- Notched trowel
- Flat-bladed scraper
- Rubber mallet
- Tape measure
- Self-levelling floor compound
- Structural timber (for sistering joists)
- Wood screws (structural grade)
- Construction adhesive
- Replacement floorboards or subfloor sheet
- Flexible floor-repair filler
- Primer for self-levelling compound
- Sandpaper (80 and 120 grit)
Step-by-Step Guide
Identify the Cause of the Dip
Walk the area slowly and press down firmly — a springy dip usually means a failed joist or loose fixings, while a hard but uneven low spot suggests subfloor movement or a sunken board. Use a long spirit level to map the extent of the dip before you lift anything. Knowing the cause determines the fix; skipping this step leads to patching the symptom rather than solving the problem. If the floor above is laminate or vinyl, check our guide to fixing lifting laminate edges before deciding whether to lift the covering.
Expose the Subfloor or Joists
Remove the floor covering — lift laminate planks from the nearest joint, carefully peel back vinyl, or lift carpet at the gripper rod. For solid timber floorboards, use a pry bar and work along the board run to avoid splitting. Number each board or panel in pencil as you remove it so you can relay them in the same order. Once the subfloor or joists are visible, probe any discoloured timber with a screwdriver — soft, crumbling timber is rotten and must be replaced entirely rather than reinforced.
Repair or Sister the Affected Joist
If the joist has deflected but is structurally sound, cut a new piece of matching structural timber to the full joist span and bolt or screw it tightly alongside the existing joist — this is called sistering. Apply construction adhesive to the mating faces before fixing to eliminate any movement. If the joist is rotten, cut out the damaged section back to solid timber and fit a new section bearing at least 75 mm onto each support point at either end. Check the repair is perfectly level with adjacent joists using your spirit level before moving on. For related timber repairs, see our guide to fixing a broken stair tread, which uses the same sistering principle.
Refasten or Replace the Subfloor
Once the joists are level, re-fix the subfloor sheet or boards directly. Pre-drill pilot holes to avoid splitting and use structural screws rather than nails — screws won’t work loose over time the way nails can. If a floorboard is cupped, warped, or split, replace it with new timber of the same thickness rather than trying to flatten it under fixings. For sheet subfloor, any gap wider than 3 mm between panels should be filled with flexible floor-repair filler and allowed to cure fully before you apply any levelling compound on top. You can also find advice on floor prep in our complete flooring guide.
Apply Self-Levelling Compound Over Residual Low Spots
Even after structural repairs, a shallow residual dip of up to 6 mm can remain. Prime the subfloor with the primer recommended by your levelling compound manufacturer — skipping this causes the compound to dry too quickly and crack. Mix the compound to a smooth, lump-free consistency and pour it into the low area, spreading it with a notched trowel to a feathered edge. Most compounds are walkable within 60–90 minutes but should cure for a full 24 hours before you lay any floor covering on top. If you’re planning to relay tiles, read our guide on how to lay floor tiles without lippage to ensure a flat finish.
Refit the Floor Covering
Once the subfloor is level and fully cured, relay your floor covering. For vinyl flooring that previously bubbled, make sure any adhesive residue is scraped back and the surface is clean before you press the new sheet down. For laminate and engineered boards, check the manufacturer’s minimum substrate flatness tolerance — most require no more than 3 mm deviation over a 1.8 m span. Work from the original reference point and leave the correct expansion gap at all edges.
Frequently Asked Questions
How do I know if the dip is caused by a joist problem or just the subfloor?
Press down firmly on the dip with your foot — if it bounces or flexes, the joist beneath is the likely cause. If the floor feels hard but just sits low, the issue is usually the subfloor panel or boards themselves rather than the structural timber below.
Can I fix a dip in a floor without lifting the floor covering?
For a very shallow dip in a solid concrete subfloor, self-levelling compound can sometimes be applied directly over hard floor tiles or vinyl without lifting them, provided the existing surface is fully bonded and level enough to allow access. However, for timber subfloors with structural movement, lifting the covering is always necessary to repair the root cause.
How much of a dip can self-levelling compound fill?
Most standard self-levelling compounds are designed to fill depths of 3–50 mm in a single pour, but always check the product data sheet for the specific product you are using as maximum depth varies. Deep pours over 20 mm may require two separate applications to avoid excessive heat build-up during curing.
Is a dip in a floor a sign of serious structural damage?
Not always — dips are commonly caused by a single failed or deflected joist, a loose board, or decades of subfloor movement rather than widespread structural failure. If the dip is large, worsening, or accompanied by cracking walls or sticking doors nearby, have a structural engineer or building surveyor assess the property before carrying out any repairs.
Can I fix a dip in a laminate floor without replacing boards?
If the laminate boards themselves are undamaged and the dip is in the subfloor beneath, you can lift the boards, repair the subfloor, and relay the same boards provided they click back together cleanly. Boards that have buckled or swollen due to moisture should be replaced — attempting to relay damaged boards will result in gaps and raised joints.
When sistering a joist, apply a bead of construction adhesive along the full length before screwing at 300 mm centres — the adhesive does the long-term work of eliminating micro-movement that screws alone cannot prevent. A floor repaired this way should be as stiff as the day it was built.
Sources
- HSE — Domestic floors: construction, insulation and damp-proofing — hse.gov.uk
- Historic England — Practical Building Conservation: Floors and Flooring — historicengland.org.uk
- TRADA — Timber in floors: technical guidance for domestic construction — trada.co.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
