sealing between window and wall

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Halo Jones

Established Member
2 Aug 2010
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Fife, Scotland
Hi All,

Making the most of time at home and repairing/ repainting wooden windows. In some places, where the window meets the concrete surround the filler that was previously used has cracked and come away. It looks like a mixture of a concrete skim and silicone was used. Hopefully you cans see in the pictures. What should I be using to reseal these? We are striping the windows back and using linseed oil paint so something that is compatible with that would be best


I suspect you'll have to hack quite a bit of that render off, once it begins to come away from the wall it's a losing battle until you remedy it properly.

Lime cement would probably be just as good as anything else for it.
Thanks Trevanion. A little more research reveals that something called burnt sand mastic is the traditional thing to use. Luckily, I have a local supplier and they are still doing click and collect! It's not too expensive for an experiment so I am going to give it a go.
I tried burnt sand mastic, it was tricky to use and didn't last well, the modern equivalent is called trowel mastic,I found that to be much better.
I'm a great fan of burnt sand mastic. It's the traditional approach and lets the joint breathe. Especially as you are using linseed oil paint. (Have you used it before ? There are 'gotcha's). When applying BSM you only need a thin coat as any gaps behind are traditionally filled with a weak lime mortar.

Are you getting it from Masons Mortar by any chance ? Good company.
I'd be really cautious about mixing materials here. If that is cement render, then I really wouldn't be using a lime based product for gap filling.

The normal thing with render junctions is to consider them to be for looks only, and to make a good seal between the window (or door) frame and the wall as your primary protection. When repairing something like your example, though, it's hard to know what that structural junction is like, so I'm at a complete loss as to what to suggest. I'm pretty sure that I'd be taking out that outer frame and replacing it if I were in your shoes, and that would reveal both the nature of the problem and the the best solution. If you don't want to do that, then maybe just patching with a bit of cement render is the best you can do.
Intrigued as to why your rationale, Mike, mixing materials.

In my experience working on old windows, it was always the cement joint around the frame that locked in any water and so leading to eventual failure and rot of the timber - especially behind the cement render.

Putting lime mortar there instead always allowed that joint to breathe.
Then you've got a junction between lime and cement, and the failure will be there rather than at the window. Another great reason to despise cement render.
I think perhaps "seal" was a poor choice of word. As far as I can tell the windows are well sealed in terms of draughts, water penetration etc. I cannot tell how it was fixed and sealed but the window frame sits something like 40 - 50 mm into the rebate behind the concrete window surrounds and will be sealed beyond what I can see.

Perhaps I should have said how do I "Finish" the transition between was the cement and window. As far as I can tell the burnt sand mastic or the more modern equivalent of the mastic Doug referred to are linseed based with additional hardners. Both are breathable, in comparison to silicone type sealants that I am trying to avoid. If there was a deeper void I saw something called oakum being used which I think is a flax rope type product mixed with pine tar that is tamped into the space before being finished with the mastic.

Doug - I cannot find the trowel mastic in stock anywhere with at least a 10 day lead time, otherwise I would have been tempted if it is easier to use.

Roger - Masons Mortar HQ are based just a few miles from me and happen to have stock available. I have never used them before so good to hear you have had good experience of them. I did a test door with the linseed paint last year and didn't have any major issues with it. Applied a primer diluted with linseed oil and turpentine and then applied 2 thin coats of paint. It is still looking good a year later. I'd be happy to hear any pointers that you have because I have 19 windows to get round to and would prefer to do them only once!
Halo Jones":34t3pr0y said:
... Applied a primer diluted with linseed oil and turpentine and then applied 2 thin coats of paint. It is still looking good a year later. I'd be happy to hear any pointers that you have because I have 19 windows to get round to and would prefer to do them only once!

That's the trick...and you've got it ! Thin, thin, thin, thin coats. I found that if I applied a too thick coat then the brush marks would be visible and that a skin formed over the brush marks but underneath it never dried properly. After the last brush stroke to ensure all excess was taken off and used elsewhere, I make one pass incredibly lightly to 'flatten' any seriously proud brush marks. Definitely a case where 'Less is more' !

this may help, the first seven paragraphs - I've used this method for all 16 of my windows down in the Borders, and it seems to work. The mastic is unusual stuff to work with, but not too difficult.

Oh, and the newspaper replaces your more traditional oakum.

Btw this book was a godsend when renovating my flats in the New Town. Not sure if it is still in print.

New Town Sash Windows 3.jpg


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Halo Jones":2c8s931a said:
Thanks very much Phlebas. That looks interesting. I don't suppose you have the title and author of the book?

Sure. Should have said that, shouldn't I? SIlly me.

Care and Conservation of Georgian Buildings. multiple authors.

I've just looked for it:

Care and Conservation of Georgian Houses: A Maintenance Manual for Edinburgh New Town ISBN-10: 0851397875

I think I have a first or second edition - very hapdash typesetting, as you can see.

Doesn't now appear to be available except at ridiculous cost. It is good though. Erm. You are in the Kingdom of Mordor (aka Fife, sorry, work joke from my previous partnership.) The National Library on George IV Street has a copy if you can get yourself across the Forth.

I happened to have those pages (about windows) photographed because someone else asked on another forum.

Oh, and Masons are good. But it helps if you know what you want when you go in. Unless you bypass the front desk (in Edinburgh a portakabin) and speak to the chaps getting you the stuff.

Hope that helps.
You are in the Kingdom of Mordor (aka Fife)
Dundonians hate us, Edinburgers hate us. I think you are all just jealous of the beautiful Kingdom :lol:

Masons are only doing click and collect at the moment so I will have to give them a ring. Only problem is that chief controller noticed the mastic comes in lots of colour variations so I now have to wait on approval!

Many thanks all.
Hi there.

I'm also in Fife, and have slowly been working my way through my sash and case windows during lockdown. I'm in a Victorian semi, 3 stories high, and the windows were desperately needing attention. Most of them were painted shut. I'm on sash 20 out of 32, freeing them up, sorting the odd bit of rot, stripping the old paint, replacing ropes, pulleys lifts etc , inatalling brush seals and converting the lower sashes to the simplex system that allows them to hinge inwards for painting, maintaining them. Been using Reddiseals who have been excellent. I have also been using traditional burnt sand mastic. Judging by the newspapers used to fill the gaps previously this job was last done in October 1946. I've got the hang of doing the mastic, but am struggling to get it to stick to the sandstone lintels. Working above your head is always more challenging, especially at heights. Should I prime the lintel with the boiled linseed oil first ? Should it be allowed to go off before applying the mastic ? Any hints or tips from anyone who has done this would be appreciated. Thumbs up for Masons Mortar in Cowdenbeath.