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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Extension problem https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1582492888 Message started by white hunyer on Feb 23^rd, 2020, 9:21pm

Title: Extension problem
Post by white hunyer on Feb 23^rd, 2020, 9:21pm

My extension was built with a gap down the total wall where it meets the main house, the gap is about 1 inch wide and filled with a mat fibre. We were told that this had to be done as both the main house and extension is built on a concrete rafts. I am now getting damp on the main building and suspect it coming from this gap as the fibre is very wet. Can this gap be filled with mastic or another water proof filler?

Title: Re: Extension problem
Post by woodsmith on Feb 24^th, 2020, 8:38am

Without seeing it it's not possible to say for absolutely certain but I would think you could fill this gap with something more waterproof. A flexible mastic would work but if you have a long, wide and deep gap it will cost you a small fortune in mastic. It may be possible to fill the gap with a foam filler as this would be much cheaper but you may need to use a fire resistant version and it would need some sort of cover as these foams are not usually UV resistant and it would soon degrade in sunlight.

Another option may be to remove the fibre, part fill the gap enough to form a water seal, then put the fibre back.