How do I fill the gaps in my staircase?

Stripped and painted staircases show every gap, and there are more of them than people expect: between tread and riser, between the stair and the skirting, around the strings, and at the top and bottom. Almost all of them move, which is why a beautifully filled staircase can look cracked again within months.

Step by step

1. Check whether the gap is a symptom of movement.Stand on the tread and watch the gap. If it opens and closes as you load the step, the joint has failed and filling it will not last. GuideHQ has a guide on a moving tread.
2. Use flexible filler at tread-to-riser joints.This joint flexes with every footfall. Decorator's caulk or a flexible filler tolerates that; rigid powder filler cracks along the line within weeks.
3. Fill deep gaps in stages, or pack them first.A deep gap filled in one pass shrinks and pulls away. Backer cord or a first pass of filler, then a finishing pass.
4. Use flexible filler where the stair meets a wall too.That junction is between two elements that move independently. It is the same situation as a wall-to-ceiling junction and wants the same treatment.
5. Tool it properly, because it cannot be sanded.Flexible fillers do not sand — they clog abrasive and roll. Get the profile right with a wet finger or tool while it is fresh, because that is your only shaping opportunity.
6. Use rigid filler only in genuinely static defects.Nail holes, small dents and damage within a single piece of timber can take an ordinary filler, and it sands properly. Reserve it for those.
7. Let it cure fully before painting.Flexible fillers continue to shrink after they skin. Paint applied too early cracks along the joint as the filler pulls back beneath it.
8. Accept that some gaps will reopen.A staircase is a timber structure that moves seasonally and flexes daily. Occasional recaulking is maintenance rather than failure.

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