What sort of filler should I use to fill a large gap between skirting boards and wooden floorboards?

The house was built in 1885 and the floorboards have gradually moved away from the skirting over time with the effects of clilmate.

My house was built in 1842. If you want to hide the separation, use expanding foam, trim when dry, and smooth with caulk. When painted, it will blend quite nicely. If you want repair the problem, you should be looking at the floor joists dropping.

Without knowing at all how you define large, who did the install, why the cove base wasn't first removed, or why the flooring wasn't installed to within a 1/4 inch of the bare wall, you have at least one option. ADD a piece of 1/4 round or other decorative molding that will cover.

Steven Wolf

Thanks for adding the details. I haven't been on the planet as long as your house has, but a Shrinkage of 2 inches seems highly unlikely. I'd be looking into other/additional reasons for such a large gap, even at 100 plus years old.

2" horizontal: remove skirt, lay 2" floorboard pre-finished to match the rest of the floor, replace skirt;
2" vertical(a): remove skirt, replace with skirt to cover all but 1/8" of the gap showing;
2" vertical(b): drop skirt to cover gap, finish wall to match rest of wall;
2" vertical(c): install matching skirt price in gap to match rest of skirt

w/o knowing if the gap is consistent or varying, not sure which fo the above to take as the best bet, but offhand I'd take 2(a)