Should You Fill Gaps Before Floor Sanding?
Old floorboards are rarely perfectly even. They shift over time. Some wear faster than others. A few may have been lifted years ago for pipes or wiring, then put back with varying levels of care. Once sanding removes the old finish, the gaps suddenly stand out. And that is where people can get too enthusiastic with filler.
Not every gap needs closing.
Some are caused by normal seasonal movement. Others come from long-term shrinkage, old repairs, loose fixings or missing tongues. Moisture changes can open them up too.
A small, stable gap may take filler well. A wider permanent gap may be better repaired with a matching timber sliver. But if the gap opens and closes with the seasons, leave it alone. Filling a moving gap can create a new problem instead of solving the old one.
During floor sanding, filler is usually applied while the floor is bare and before the final finish goes down. Once the room has been sanded, filled, coated and put back into use, discovering that the filler is cracking out every winter is an irritating way to learn that wood still behaves like wood.
So, before filling every line in sight, work out what kind of gap you are looking at.
A gap is not automatically a defect
Timber changes dimension as it gains and loses moisture from the surrounding air. In more humid conditions it takes on moisture and expands. In drier conditions it loses moisture and contracts. That movement is normal, even after the boards have been in a house for decades.
35% to 65% relative humidity is a normal residential and commercial range. Seasonal moisture changes can produce occasional minor gaps, particularly during the winter heating season. Irish homes experience the same basic timber behaviour: heat the house, dry the indoor air and boards can contract.
The important word is seasonal. A joint that appears in a dry period and tightens again as indoor humidity rises is doing something quite different from a 6 mm opening that has remained unchanged for years.
Seasonal gaps can close again
Normal seasonal gaps should not automatically be filled. To diagnose, start with indoor temperature, relative humidity and the moisture content of the flooring, then compare those readings with the season and expected environmental conditions.
If a dry winter floor is filled tightly from wall to wall, the timber may expand when humidity returns. The boards still need somewhere to go. Filler does not cancel seasonal movement; it merely occupies the space movement was using.
Small seasonal openings are often most obvious beside wider boards because wider pieces show more dimensional change across their width. They can look untidy for part of the year and almost disappear in another season.
If you are planning floor sanding in the driest period of the year, that timing should be part of the inspection. The gap you see today may not represent the floor’s year-round position.
Permanent gaps
Some gaps do not close. Old softwood boards may have shrunk gradually over many years. A room may contain timber from different periods. Boards may have been relaid with extra space after plumbing work. A tongue may have broken. Edges can wear. One section of floor may have moved because the joists or fixings beneath it changed.
Permanent does not automatically mean cosmetic either. A wide or irregular gap deserves a look beneath the surface before somebody hides it with filler. If the board is loose, unsupported or moisture-damaged, repair the cause first. There is little value in producing an immaculate beige line over a board that still moves every time somebody walks past the sofa.
Old Irish suspended floors
The gap question becomes particularly relevant in older Irish homes with suspended timber ground floors. These floors were designed with a ventilated void beneath the joists. They can be cold and draughty, especially where air from the subfloor finds its way through gaps between boards and around the room perimeter.
Suspended timber floors can be insulated from above or below. And ventilation must be maintained so excess moisture and ground gases can escape. Historic boards can be brittle, jointed in unusual ways and easy to damage when lifted.
That is a useful reminder when gap filling becomes part of a renovation. Sealing visible joints between boards is not the same thing as blocking the air bricks or vents that ventilate the structural void. The first can reduce unwanted draughts into the room. The second can increase moisture risk beneath the floor.
Draughty does not always mean defective
Gaps in suspended timber floors can contribute to heat loss and draughts and that draught-proofing can improve comfort. But the floor still has to be understood as a building element, not merely as a decorative surface.
If there is easy access below, insulation between joists may deal with the cold floor more effectively than trying to turn every floorboard joint into an airtight seam. If access is from above, the insulation work should usually be planned before final sanding because boards may need to be lifted and reinstated.
For a floor that is simply being restored, though, stable permanent gaps can often be reduced without stripping away its character. The method depends largely on size and movement.
Floor sanding can make gaps look worse
Sanding does not normally create large new gaps between sound boards. It does, however, make existing ones much more obvious.
Years of dirt, old polish, dark varnish and previous filler can disguise the edges of individual boards. Sand the timber back to a pale clean surface and the black line between two boards suddenly has contrast. A gap that was always there can appear wider simply because the surrounding wood is now lighter.
Old filler can also break out during sanding. Brittle material that looked reasonably intact under several coats of finish may disintegrate once the sanding machine reaches it. That is not necessarily bad news. A restoration is a good time to remove failed filler and decide whether the joint should be refilled at all.
The temptation is to trowel filler across the whole floor because a freshly filled surface photographs beautifully before the finish goes on. But is each type of joint stable enough to hold it?
When floor filler makes sense
Floor filler works best in relatively narrow, stable joints and small defects where there is enough surrounding timber to support it.
For instance, you can use a waterborne binder mixed with sanding dust from the floor. This is typically for joints and cavities up to 3 mm wide. Conventional trowel fillers are designed for small gaps, not miniature trenches.
Mixing a binder with clean sanding dust from the actual floor helps the colour relate to the surrounding timber. It will not become invisible. Filler has no grain, reflects light differently and can take stains or pigmented finishes differently from wood.
Narrow and stable
A 1 or 2 mm permanent gap in an old pine floor may be a sensible candidate for filler if the boards are dry, secure and not moving seasonally by a similar amount. Small nail holes, edge chips and fine splits can also be dealt with during the same preparation stage when the repair system is appropriate.
The floor should be sanded to the stage recommended for the filler system, vacuumed properly and filled before the final fine sanding. Once cured, excess material is sanded flush so the surface is ready for finishing.
This is one reason dustless sanding and good extraction matter. The dust used in a mixed filler needs to be clean wood dust from the appropriate sanding stage, not a cocktail of old lacquer, plaster dust and whatever the renovation produced on Tuesday.
Why filler cracks back out
Failed filler is often blamed on a bad product when movement was the real problem.
If adjacent boards shrink apart every winter and swell together later, a hard filler is being asked to stretch and compress repeatedly. Some products tolerate a little movement. None can convert two independently moving floorboards into one static slab.
Very deep voids are another problem. Filling a large gap from top to bottom can create a thick plug with poor support. As the boards flex, the filler can crack, crumble or detach at the edges. More filler is not always more repair.
Bigger gaps usually need timber
Once a permanent gap is too wide for the filler system, matching timber is often a more convincing repair.
Retain old boards where possible, so slips of matching timber can be used to fill floorboard gaps. This approach is particularly useful on old floors where the joint is too large for a paste filler but the surrounding boards are still worth keeping.
A timber sliver is a thin tapered strip cut from compatible wood and fitted into the gap. Done well, it becomes part of the floor visually. The repair still shows on close inspection, as an honest old-floor repair often should, but it does not have the flat, grainless appearance of a wide band of filler.
A sliver should not glue the floor solid
When installing timber slivers in large gaps, cut a tapered strip of the same species and apply adhesive to one side rather than both. That way the sliver is secured, but the neighbouring board can still function naturally rather than being glued rigidly to its neighbour.
Once the adhesive cures, the proud part of the sliver can be trimmed and the whole area sanded flush. Using wood of a similar species, colour and grain helps the repair settle into the floor after finishing.
Slivers are not appropriate for every gap. A joint that opens and closes dramatically with the seasons can still create trouble. Timing, moisture and the cause of the gap need to be factored in.
Sometimes the board is the repair
A huge irregular opening beside a badly split board may be telling you that the board itself needs attention. So might a gap caused by a missing tongue, broken edge or an old plumbing patch made from a narrow piece that never matched the original width.
In those cases, local board repair or replacement can be cleaner than manufacturing an increasingly elaborate filler solution. Reclaimed or well-matched seasoned timber is often useful in an old floor because dimensions and appearance can be closer to the existing boards.
The objective is not to remove every sign of age. It is to make the floor sound, comfortable and visually coherent without turning a decades-old surface into an imitation of laminate.
Gaps you should not rush to fill
- Seasonal gaps. If they close again as relative humidity rises, filling them during the dry season can restrict natural movement or leave cracked repairs later.
- Expansion gaps. Purpose-made perimeter or internal expansion spaces allow the floor to move. They are normally concealed by skirting, thresholds or other details rather than rigidly filled.
- Moving joints. If two boards flex against each other or one board is loose, secure the floor before deciding what to do cosmetically.
- Moisture-related gaps. A floor that has shrunk after abnormal drying, flooding or another moisture event needs diagnosis and stabilisation first.
- Structural openings. Gaps created by broken board edges, missing support, joist movement or failed repairs need carpentry, not cosmetic paste.
Timing can decide whether a repair lasts
A floor restored in February can be in a different dimensional state from the same floor in August. That does not mean you need to postpone every sanding job until some mythical perfect week. It means the contractor should read the indoor conditions and the floor rather than treating the gaps as fixed measurements.
Temperature, relative humidity and timber moisture content are huge factors. If the home has just had new plaster, a leak, prolonged vacancy, unusual heating or dehumidification, the current gap size may not represent normal living conditions.
A sensible repair is carried out when the floor has stabilised within the conditions in which it will normally live. Trying to force an old floor into perfection while the building is still drying after renovation is an excellent way to create a second job for later.
Check the house, not just the joint
If gaps appear suddenly, ask what changed. Was a new heating system commissioned? Has the house been kept much warmer? Did a dehumidifier run for weeks? Was insulation added beneath the floor? Was there a leak? Has a blocked vent changed moisture conditions below a suspended floor?
The answers can explain why a floor that sat quietly for years has started moving.
For old suspended floors, keep the subfloor inspection in the picture. Ventilation is important here because trapped moisture can lead to decay in structural timber. Filling the visible joint should never distract from damp or airflow problems underneath.
How much character should you keep?
There is a point in many floor restorations where repair turns into cosmetic over-editing.
Imperfections can contribute to the historical interest of a floor and do not automatically need to be eradicated. That principle applies beyond listed or formally protected buildings. An old pine floor can look better with a few honest joints and old fixing marks than with every line packed until the surface resembles a new engineered board.
The right level of filling depends partly on how the room is used. A draught through a 5 mm gap in a living room is a comfort problem. A tiny stable joint that simply catches your eye when the light hits it at 4 pm may be something else entirely.
There is also a visual difference between random deterioration and consistent age. One broken edge beside a doorway can look neglected. A pattern of narrow old board joints across the room can be part of the floor’s identity.
Good restoration is selective. It fixes what is failing, improves what is uncomfortable and leaves alone what is merely old.
Do not fill a floor by reflex
Gap filling can transform a tired wooden floor. Small stable joints become quieter visually, wide permanent openings can be repaired with matching timber, and a draughty old room may feel noticeably more comfortable once the obvious air paths are addressed.
But the floor has to keep moving after the restoration is finished.
Seasonal gaps are part of that movement. Expansion spaces have a purpose. Loose or moisture-affected boards need repair before filler. Large permanent openings often need timber rather than an ever-thicker smear of resin.
The best time to make those decisions is during floor sanding, when the old finish is gone and the real condition of the boards is finally visible.
Fill the gaps that are genuinely defects. Repair the ones that need carpentry. And if a narrow old joint is stable, harmless and part of the floor’s character, it is allowed to remain a gap.
