Best way to fill between timber windows and stonework

[DianeEliz]

I own a Victorian house in Bristol - a ‘miner’s cottage’ built in 1900. I recently paid a lot of money for some timber sash windows to replace old plastic glazing.
They are engineered wood with special microporous paint.

The windows themselves are beautiful. But the installation was disappointing. They filled on the exterior side with stone filler right up to the timber frame. It’s all cracking now as the windows have shrunk a little. The sun has been so strong on them.

I’ll need to redo the stone filler. But it seems to me that if the wood will shrink more, I need something with flexibility between the windows and the stone filler.
Reading here, that silicone would not be the answer and I can’t get it between the wood and the stone anyway, what can I do?
I have read someone in Shropshire using corking as used on boats - using rope packed tightly in the gap, but it seems highly specialist and not local.

I’m interested in proper repairs and am frustrated with current standards and methods of repair - chaulk or silicone and expandable foam. Does anyone have any knowledge on this subject please? I spent a fortune on the windows, yet their installation and aftercare is dismal.

 

[Feltwell]

The traditional way was something called burnt sand mastic, but I've tried it and wasn't impressed, it only lasted about 3 years before cracking.

Therre is "silicone" and there is decent sealant. I switched a while back to using CT1 - I've used it on a number of applications, including sealing leadwork to a chimney breast right on top of my roof, where I won;'t be going back to in a hurry. It seems to be very good stuff, not cheap but good. You *defintiely* need to buy the Multisolve spray as well - you will not smooth the CT1 without it, it's very sticky. Put it on with a sealant gun, very light spray of Multisolve over the top, then smooth - after trying many tools with this stuff I find my finger works best! You can paint over the sealant as well, unlike many of the ones out there, which you'll be grateful for next time you come to decorate.

https://www.ct1.com/our-products/

Toolstation sell both CT1 and Multisolve, but not the full range of CT1 colours, you'll have to go online for those.

I am a *huge* proponent of traditional methods but sometimes there are modern alternatives which are better.

 

[noj]

Feltwell beat me to it!

Traditionally you'd use burnt sand mastic at the face, a bit like pointing. Filling a deeper void behind could be packed out first with oakum or just more mastic.

The burnt sand is working for me so far (only 4 years in). It is a cheap and reversible option but modern may well be a better and longer lasting solution. Mr F's experience could save you doing the job more than once!

 

[Cubist]

I have read someone in Shropshire using corking as used on boats - using rope packed tightly in the gap, but it seems highly specialist and not local.

Quite possibly that would be me. Its caulking cotton which arrives in a ball of twisted cotton rope approximately 8mm diameter. It can be braided, like a pigtail, to make thicker strands when needed or applied as it comes to narrow gaps. It is simply pressed into the gaps and hammered gently into place to achieve a 'force fit' within the cavity. It is very easy to use, very cheap, needs no fancy tools and it works.

A fillet of burnt sand mastic or lime filler/render provides the cosmetic finish.

Heres a link to one of the many on-line suppliers.

Davey and Co. Caulking Cotton 1/2kg

Soft, top quality caulking cotton made up of eight strands totaling an uncompressed diameter of approximately 8mm. Used for the caulking…

captainwatts.co.uk

 

[fernicarry]

I had burnt sand mastic put around my timber sash windows at the end of last summer. The wood has moved around enough since then for cracks to appear, not to mention peck marks and foot prints!! I've got the internal finish off in one room and can see that water is being forced under the sill when its stormy so thats not good on multiple levels. I will need to dig out the mastic and go with something modern and flexible to form the weather seal, then a fillet of mastic on top for the cosmetic finish.

The window frame I most recently repaired I put a strip of expanding foam under the sill before putting it in position. Not squirty gun foam, the stuff that comes on a roll and is designed specifically for sealing around the perimeter of door and window frames. The foam has now nicely taken up the shape of the stone sill underneath it to form a weathertight seal and it carries on trying to expand so future movement should be catered to. A nice mess-free and reversible solution but not something you could retrofit terribly easily unless you can open up a bit of a gap to work with. Wish I'd thought of it sooner tbh.

 

[Cubist]

I had burnt sand mastic put around my timber sash windows at the end of last summer. The wood has moved around enough since then for cracks to appear, not to mention peck marks and foot prints!! I've got the internal finish off in one room and can see that water is being forced under the sill when its stormy so thats not good on multiple levels. I will need to dig out the mastic and go with something modern and flexible to form the weather seal, then a fillet of mastic on top for the cosmetic finish.

The window frame I most recently repaired I put a strip of expanding foam under the sill before putting it in position. Not squirty gun foam, the stuff that comes on a roll and is designed specifically for sealing around the perimeter of door and window frames. The foam has now nicely taken up the shape of the stone sill underneath it to form a weathertight seal and it carries on trying to expand so future movement should be catered to. A nice mess-free and reversible solution but not something you could retrofit terribly easily unless you can open up a bit of a gap to work with. Wish I'd thought of it sooner tbh.

I'm surprised its failing so quickly. I started using burnt sand around this old place best part of 8 years ago and have never had to revisit it despite the annual movement each season and that induced by the, frequent around here, low flying helicopters. Am I right in understanding its underneath a windowsill? If so there's a chance that the fit of the window frame was so tight there was no room for the burnt sand to form strong internal bonds after it was applied. It could also be that the builder did not use genuine burnt sand or did not mix it thoroughly.

The only trouble I've ever had with it was when wasps took up residence in a void behind a window frame fillet and pushed the sand out.

 

[fernicarry]

Its out of an Everbuild tub so may not be the real deal. Its all the way around the window frames. Gaps between the frames and masonry are variable. Some places none at all, others quite wide. I take it that it should be pushed well into the gap?

I think the guys who did it were relying on gun foam to waterproof (yeah I know, we discussed how it wasn't appropriate and when I came back a few hours later it was done...) and saw the mastic as a cosmetic finish. The window I noticed leaking doesn't appear to have any foam under it however and it is quite tight onto the stone cill. Its on the list....

There has been an uptick in low flying helicopters around here too recently. Merlins I think. Not stealthy pieces of equipment by any stretch. I don't quite get why they need to circle around a few times before landing.

 

[Cubist]

I've no experience with either of those products and I've always mixed my own. Like baking a cake, there's a good time to quit mixing and a lot of bad times that can produce a poor result. In your case there are several 'unknown unknowns' that could be the cause of the failure.

I understand, though my informants sources are unclear, that helicopters will circle before landing to check for loose materials that could be sucked into their engines.

 

[fernicarry]

I understand, though my informants sources are unclear, that helicopters will circle before landing to check for loose materials that could be sucked into their engines.

Totally off-topic, but that would make some sense. From my viewpoint they disappear behind the tree-line. Very slowly and carefully. There's a training camp up in the hills.

 

[RBailey]

I heard it said that before using the burnt sand / linseed you can prime the area with boiled linseed to help it stick better.
(Not bothered myself as I've always found it stuck okay but I always had the tip filed in my "brain".)

 

[Cubist]

Theres a one inch wide horizontal ledge on the gable end here that I felt I needed to 'prime' using that technique before laying a slope of burnt sand to limit/prevent water standing on the ledge. May have been overkill but it seemed to work.