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I'm currently in the process of renewing an old wooden floor. What would one use to fill in the gaps between the planks?

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I've asked in the local home repair store and they gave me a silicone like mass that resembles the color of the wood.

That seems to work ok for the nail holes, but is kind of soft and somewhat expensive for filling out all space between the boards.

I have seen some youtube videos in which people are mixing sand dust with either wood glue or some special liquid. Does anyone know something on the pros and conns of these approaches?

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  • Another filler alternative is to mix sawdust with varnish, sanding it when dry, and then applying a varnish floor finish.
    – Ashlar ♦
    Mar 3, 2017 at 1:11

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I'm currently in the process of renewing an old wooden floor. What would one use to fill in the gaps between the planks?

Nothing unless there's a draft problem. The gaps aren't generally an issue except visually but they can make for a cold floor depending on the rest of the structure of the house and how much air movement can take place through the floor.

Some argue such gaps are supposed to be there, and if you look at old floors 99 times out of 100 you'll see gaps between boards, sometimes quite wide ones, and in many cases there's no filler and no trace that there ever was any present.

In the past where gaps were filled they used various methods, including the same way gaps were dealt with on ship decks, using oakum and tar (!), as well as with melted wax or wax/resin compositions (also used as a filler material for furniture in this era).

See more discussion on the issue and solutions at these links:
How to Fill Gaps In a Wide-Plank Wood Floor on This Old House.
My home's pine flooring has large gaps in between some of the planks, especially in the kitchen. What can be done to fill or seal them? on Quora.
How to fill gaps in a wood floor? on HomeOwnersHub.
Filler for ancient wood floors? on the WoodenBoat forum.

I've asked in the local home repair store and they gave me a silicone like mass that resembles the color of the wood.

That seems to work ok for the nail holes

That stuff is no good for filling nail holes on a floor. This type of flexible filler is for filling gaps that require some movement to take place without cracking, as in gappy installations of skirting to walls, one wooden moulding piece to the next where some shrinkage and expansion will take place through the year.

If you must fill the nail holes (you don't have to, it's quite normal for them not to be filled) you want a rigid filler and ideally a strong one if it's to take direct foot traffic, i.e. if there's no carpet going on or any rugs being put down.

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  • The only issue I have with this is that my floors have a smaller gap and that gap just gets filled with dirt and looks terrible. A hardwood floor guy I worked with when I was younger would trowel the floor with filler prior to finishing. I do think the particular gaps in question are a bit large for this though.
    – Dano0430
    Mar 2, 2017 at 20:05
  • @Dano0430 Isn't that what regular sweeping or vacuuming is for? :-) Seriously though, if your vacuum isn't capable of sucking the dust from between boards then you might want to consider getting a Dyson, or one of its clones.
    – Graphus
    Mar 2, 2017 at 23:24
  • Haha LOL FWIW I have a Dyson. Sweeping wouldn't help much in this situation as you're just pushing dust into the gaps. Vacuuming might pull some dust out but when it goes the thickness of the board it might not cut it. Thanks for the thought though
    – Dano0430
    Mar 2, 2017 at 23:27
  • @Dano0430 :-D Re. sweeping, you aren't pushing dust into the gaps if you sweep along the boards rather than across. I presume this is a duh but I've learned not to make the assumption because what's obvious to some isn't always noticed or realised by others.
    – Graphus
    Mar 3, 2017 at 8:28
  • I think the duh moment is on the installers who didn't install a modern HW floor correctly. There is no real way to guarantee that you will always be able to sweep along the length of a floorboard. I've installed HW floors for work and think that these gaps are much better filled and sanded smooth overall. (To get back to topic)
    – Dano0430
    Mar 3, 2017 at 16:25
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The old school way I have seen to fill large gaps in wide plank flooring is with rope:

1 Scrape debris from between flooring planks with putty knife or painter's tool.

2 Vacuum floor clean of all dust and debris.

3 If desired, tint rope with wood stain to match floor; let dry.

4 Force rope into gap between planks with putty knife or painter's tool.

This minimizes what can get caught in the gap, stops drafts, while allowing for wood expansion and contraction.

This Old House

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You do not want to fill the joints between the boards. The gap provides room for expansion which normally occurs across the width and depth of wood over the seasons. The amount of expansion can be calculated. (See the tag wiki info under the wood-movement tag for links to tables for how to do so.) A flexible sealant may be used but it will bulge in some gaps as the wood expands. The screw holes can be covered and your proposed methods will work as will some manufactured wood putty products that can be found in hardware or woodworking stores and websites.

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  • How does a parquet flooring manage to go without the expansion gaps?
    – TheMeaningfulEngineer
    Mar 1, 2017 at 23:57
  • @TheMeaningfulEngineer Parquet flooring is usually glued to an engineered substrate (particleboard or plywood if above grade, concrete if at or below) that doesn't move.
    – SaSSafraS1232
    Mar 2, 2017 at 0:13
  • @SaSSafraS1232 How does that prevent the parquet wood to expand? Is your point that it's the absence of an wooden underfloor that allows the parquet floors to have no gaps? Because it's the underfloor expansion that's causing the problem?
    – TheMeaningfulEngineer
    Mar 2, 2017 at 0:34
  • parquet is thin enough that expansion/contraction is not an issue when it's glued to a stable substrate. (something that won't expand such as particle board, ply or concrete). Your problem is that the wood on the floor is solid and thick enough to require movement to be taken into account
    – Dave Smylie
    Mar 2, 2017 at 0:43
  • Parquet floors do have expansion between each tile of the system.
    – Ashlar ♦
    Mar 2, 2017 at 2:09
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I have done the Sawdust/glue approach before a few times. I made a large mixture and tinted it with stain to match the brown color of the floor. Press into gaps with putty knife, let dry and then sand and varnish. It's not a fun process but it works!

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Sawdust and boiled linseed oil mixed into a paste is the way to go for this if you really want to fill them( as others have said you don't have to ). It takes a while to cure, but it stays flexible for a long time and is easy to repair/sand/etc. Best paired with an hard oil finish like osmo, rubio, etc.

I have never tried to poly over it, I imagine it should work once the linseed oil and sawdust putty is fully cured, but I would be concerned about the flex of the paste vs the stiffness of the polyurethane on top causing flaking.

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