Glue & Sawdust
The DIYers sawdust resin, but don’t knock it, its been a good solution for a long time.
Mix sawdust with good quality pva or wood glue into a paste & trowel into
gaps, leave to dry and sand off flush with the boards.
Pros
Non smelly, slightly flexible so tends to stay in even if there’s a little movement or shrinkage, aesthetically pleasing. excludes drafts
Cons
Hard to trowel in as very sticky, Slow drying especially in big gaps, Shrinks a little when dry so needs to be filled again, excess is hard to sand of the boards, cannot be stained ( beware using cheap pva, it can make the filler go black once dry)
Although if you persevere with this method it’s probably the best gap filling solution, from a contractors point of view it’s just not feasable to wait 24-36 hrs for filler to dry and then re-filing and waiting a further day.
Waterbased resin gap fill, Pretty much a modern take on the pva/sawdust method, does the job, but is not flexible and takes a while to dry.
Pros
Non smelly, Aesthetically pleasing, stainable once dry (at least overnight) excludes drafts
Cons:
Slow drying, Shrinks so needs refilling once dry which extends job times, can fall out as doesn’t expand and retract, expensive
Gap Filling Tape
In truth i’ve never used it, but i’ve seen pictures and had a few questions unanswered and a sample blankly refused when i contacted the manufacturer a few years back. The theory is the rope solutions except they use a rigid tape with a crease running down the centre which you push into the gap thus making the outside edges expand & cling to the board edges. A bit like a stint thats put into heart valves to widen arteries. Great in theory but when dealing with small and large gaps on a single floor i dont think the tape would fill the smallest ones, leaving draft spots.
I’ve never known a professional floor person to have used the tape method, which has to tell you something. But i shouldn’t knock it until i try, anyone out there used the tape?