Filling joints in concrete window sills?

[Labsnark]

Hi all,

The joints in our external concrete window sills are a perennial problem. Seven large (ca. 80x20x20cm) concrete sills go all round the base of the bay window in our extension, separated by gaps of around 15mm. When they were installed, the builder used builder's putty to fill the joints - the problem is that we live in a very exposed site (on the 1000' contour in southern Scotland) and the putty dries out over a couple of years, crumbles and then needs to be replaced.

The extension is timber framed, and the wall that the sills sit on isn't load bearing. Should I keep using builder's putty and accept I have to replace it periodically, or would I be better using something else? I'm not clear, for example, why the original builder didn't use mastic.

Thanks for any advice! Andrew

 

[noseall]

Why didn't he butt them up to a mortar joint ffs?

Your next best effort would be a polysulphide mastic. It'll be a right pig. Use masking tape and have some builders wipes handy.

 

[Labsnark]

Your guess is as good as mine, noseall - fifteen years ago, I was much more reluctant to question builders than I am now! It may have been something as simple as avoiding having to buy the next size of sill in order to save a few pounds.

Anyway, many thanks for that - that pretty much confirms my suspicions that there's no especially good reason for builder's putty. I take it mortar out of the question because there needs to be some allowance for expansion?