Gap Filling: Resin or Timber Slivers?
The two main methods, and why gaps can return
Most period floors develop gaps over the years as boards dry, shrink and shift. Apart from the draughts, gaps collect dirt and can spoil the look of a freshly restored floor. Filling them makes a real difference, but it is important to know what to expect.
Resin and sanding dust
With this method, a clear resin is mixed with fine dust collected from sanding your own floor, creating a paste that matches the timber. It is spread across the floor, worked into every gap and sanded smooth once it has hardened. It is ideal for narrow gaps, small cracks, splits and nail holes.
Blends well, since the filler contains your own timber.
Efficient and affordable for larger areas.
Suits narrow gaps rather than wide ones.
May crack when the floor moves with the seasons.
Method two: timber slivers
For wider gaps, thin strips of timber, usually reclaimed pine, are cut to shape, glued on one side and tapped into each gap. Once dry, they are planed and sanded flush with the boards. Because the slivers are wood, they take the finish in a similar way to the floor and can look very natural. It is labour-intensive, so it is priced higher than resin filling.
Wood keeps moving
This is the part many people do not expect: wooden floors are never completely still. Timber absorbs moisture in humid summer months and releases it when the heating is on in winter, so boards swell and shrink with the seasons. Structural movement in joists and the building adds to this, particularly on suspended timber floors.
That movement means fillers of any kind may crack, shrink or eventually loosen. It is a normal characteristic of timber floors rather than a fault in the work. That is why gap filling is excluded from our service guarantee, while the sanding and finishing themselves remain covered. We will always explain this before you decide.
Reducing movement
Try to keep humidity and temperature reasonably stable.
Fix loose boards before filling so they move less.
Choose slivers for wider gaps; resin for fine cracks.
Consider fitting insulation under suspended floors where possible.
The order of work
Gap filling is normally carried out as part of a full sanding and finishing project, rather than on its own. We begin by sanding the floor to level the boards and strip off the old coating. After that, gaps are cleaned and filled, and the floor is sanded again once everything has set. This means the repairs are sanded flush with the boards and take the finish in the same way as the rest of the floor.
Filling at the right stage gives a much neater result than trying to fill a floor that has already been finished. It also allows us to check each gap closely and choose the right method for it, rather than treating the whole floor the same way.
Draughts and energy efficiency
Beyond the look of the floor, filling gaps can make a room feel noticeably warmer. Older homes frequently have ventilated spaces under the floor, letting cold air rise up between the boards. Closing those gaps reduces draughts and helps keep heat in the room, although it is important that the void beneath the floor remains properly ventilated to prevent damp.
In summary
Resin mixed with your floor's own sanding dust is ideal for narrow gaps, cracks and nail holes.
Wider gaps are better filled with slivers of reclaimed timber, which look natural and hold well.
Seasonal and structural movement affects every timber floor, so all fillers can eventually crack or work loose.
That natural movement is the reason gap filling is not covered by our service guarantee.
Stable heating, secure boards and good ventilation under the floor all help filler last longer.
Choosing a method
If most of your gaps are narrow, resin is often the best value. Where gaps are wider than a few millimetres, slivers give a stronger, more natural finish. Many floors benefit from a mix of both. Find out more about our gap filling service, or ask us to assess your boards during a free survey in Harrow.