Mastic/Sash Windows

A friend has timber sash windows in her tenement flat. The mastic (I think it's burnt sand and linseed oil) is breaking up and falling out. I thought she might be better with a silicone sealant. However, a closer inspection has revealed huge gaps between the windows and the masonry. Even if I fill the gaps with expanding foam there will still be a large area to bridge with the sealant - maybe 15 -20mm. Would I be correct in thinking this is too much for the sealant to be effective? Another point is, even if it's ok to fill the gap with silicone sealant is this the most cost-effective solution?

THanks.

I'd fill gaps that size with lime mortar. 3 parts sand 1 part lime. I'd use sand/linseed oil mastic for smaller gaps.

Lime mortar? I hadn't thought about that. Thanks.

Be careful with silicone sealant on a window that's had mastic applied. Because mastic uses oil it absorbs into the surrounding stone or brick and the silicone tends to not stick properly. It's a common problem around my area where the houses are mostly stone and have had the old mastic removed, the silicone just peels off. I had new windows fitted last year and got a local builder (my old man!) to mix up some mastic to seal them.

>Because mastic uses oil it absorbs into the surrounding stone or brick and the silicone tends to not stick properly.<

That's a good point! In Edinburgh, where I live, hundreds, if not thousands, of tenement flats have had new windows fitted and all the ones I'ved paid attention to have silicone sealant. Makes you wonder how long it will be before we see problems on a grand scale because of this!

Thanks for the information.