Filling gaps between floorboards

[piersmcg]

Hi,
We've had some old floorboards (straight edge - no tongue and groove) laid onto a concrete floor, where they have shrunk a little, widening the gap between them. The gaps vary from maximum 10mm to min 3mm.
The question is, short of relaying them, what is the best substance/method of filling the gaps between each board?
I've heard of sawdust mixed with glue, paper mache, wood filler etc.
Thanks for any help!

 

[Antwerpman]

I have also heard about mixing the sawdust from sanding the boards with PVA glue to seal between the boards. As some of the gaps are quite large I know it is also usual to pack these larger gaps out with string/rope - sorry I dont know if there is any particular kind which works best as I have never done it only heard about it

 

[WoodYouLike]

Underneath the floorboards (on concrete) did you find insulation and DPM?
Using Bona's Mix and fill (waterbased mixture to mix with fine, clean saw-dust, from grid 80) is a safe way, but wider gaps will need to be filled a few times due to shrinking of the (nowadays) watebased product.