Bay window repair, exterior sill

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Established Member
18 Jan 2009
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London UK
I have a job to repair a rotten sill nose on a 1930s bay window. The central section (parallel to the front wall of the building) has extensive rot, and has been previously filled extensively, so I'm going to cut it away and glue on a new section with Lumberjack 5 min polyurethane.


1. Do I need to do any joinery where the new section meets the two mitred 'wings' on either side of the bay? I was minded to do a straight butt joint, and use CT1 or similar, on the basis that the joint always fails due to wood movements, and at least the CT1 is flexible and provides a good seal. Is there any point trying to include a loose tenon or similar?

2. Accoya or Meranti? Both are available locally in suitable sizes, very similar price. I imagine the original is softwood, so I'm leaning towards accoya, not least as I need a project to try it on.

Any input extremely welcome.
Although I’m no joiner I’ve repaired a few frames, cills etc in my time . The first thing is photos of the frame , and especially the damaged areas. The repair depends upon the extent of the rot, the type of wood used in the original frame-often the visible damage is just the tip of the iceberg . A frame can easily be uneconomical to repair especially as you say there’s been multiple previous repairs. As for your glue choice I’m not sure that is suitable for exterior use . Post some pics of the frame and I’m sure you will get plenty of advice. Good luck 🤞
Polyurethane is a good choice. I think and is the best option for Accoya - I don’t like to have to rush so why not use 30 minute PU?

I would probably use Accoya because I like it - easy to work etc. - but I don’t have much experience with Meranti so that may be just as good.

How is the replacement piece going to be fixed? The glue may be good enough so long as the wood being fixed to is really sound and true - but I would probably try to add a slot/housing for a tongue. On the corners I would be tempted to screw in some longish (stainless steel) screws from the new wood and plug, but others may disagree.

I don’t know whether CT1 is the best sealant/adhesive (seems to be the answer for many things) - I am using some Timbabuild hybrid polymer glazing sealant for various butt joints in window frames (frame within frame) and it is recommended for that as well as the glazing which is why I have it. CHEMFIX - Total Service

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Here a photo. Despite the missing material its still pretty solid, but too far gone to fill and patch up.

The sides of the bay look OK, but I'll get a better idea once I remove a bit of the rotten material.


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The bottom of your window looks rotten too , especially the right hand side , I think the more you expose the more issues you will find . I wish you good luck 🤞 I think you will need lots .
Thanks JohnnyB, I'd hoped to avoid the repaircare route, but it looks like I might have to go down that particular wormhole whether I want to or not!

On the brightside it suddenly become much less critical to get a good fit on the splices.
Looking at the photo, I can’t see anything that isn’t rotten. Sometimes, the expression, flogging a dead / rotten horse comes to mind. I think practically, remake is the way forward rather than repair.
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I hope the rot isn't as expansive as several posters fear, I guess I'll find out one way or the other....
It may be the photo, but the sash bottom rail, muntins and styles are all rotten where they meet, so it needs to come out to be repaired / remade. The parting bead, jambs and cill of the frame are all rotten, which suggests the box will also have rotted out. To confirm, I’d be having a good poke around with a screwdriver to ascertain just how much and how far the rot has extended to.
If you have a bradawl then try to push it into the timber in multiple places. If you can achieve this in several places other than the bottom Cill then you will have your answer. I was working in an old house recently where I could push a screwdriver up through the joists that were 8” deep. They had all been repaired/bodged up previously to no avail.
Bradawl, ice pick, anything like that; doesn't need a specialized tool.

As with mice, the rot you can see is usually just a fraction of the rot that exists.
Just heard of it but quite like the look of Dryflex
Some years ago a friend was selling his mums house and just needed something to make it look ok for a while, we ended up filling the bulk holes with quick drying cement (it has a fine/smooth surface) and car body filler to finish. It looked pretty good and several years later (we knew the purchaser) was still sound but had some hairline cracks between the joint of wood and filler which the purchaser simply filled
Repair care is the way to go, we use a metabo die grinder with a tungsten ball end cutte around 10mm I thing, you just let this eat at the rot, when it gets to solid wood it stops cutting so rapidly. Its a bit daunting but isnt too skilled a job. You then "replace " everything moved with the repair care resin using silicone bocks, masking tape whatever to make moulds for it. You cast/mould the whole missing mortice and tennon stile/rail joint (or whichever components are damaged).
Its a super expensive system, but it avoids the costs of replacements and Im told that here in the UK some listed building officers are mandating its use to repair rather than replace significant windows etc.
I'm not affiliated just a convert. I got my equipment from county construction chemicals in north london.

good luck with it
in a listed building with important windows it's superb. money is no object. in a normal rotten wood situation it's performance is still great but the bang for buck starts to unravel. most people look blank and glaze when you start explaining the benefits. I no longer use it with what I lovingly refer to as morons. it far to expensive. tbh replacement is the best option in most cases for both parties.( although they may not realise it.)
I've been back today to the customer (my aunt) to look at two other windows, and on that basis I've suggested a sash refurb/repair/replacement specialist gets involved. There are at least two 1st floor windows which need replacement sashes, and 1 very rotten sill, with rot extending into the box. It really needs somebody with the experience to assess what needs replacement, and what is worth repairing etc.

Can anyone recommend a good company based in N.London?
If it's as bad, far gone, as it appears, better to seriously investigate, consider, a completely new frame. And if the window reveal will allow, consider making it double or even triple glazed... The other alternative is of course uPCV sash look a-likes; again double (or if possible triple) glazed. Some of them today are pretty good in terms of appearance compared to the early earlier designs. If going for a new unit, ensure you know actual internal reveal dimensions, especially if inserting uPVC replacements, so as to ensure you get maximum glass and not to much frame - wood or the uPVC than you have at present. Don't simply rely on what you can see externally/internally to determine actual measurements required; which will mean removing the internal framing to determine those measurements. Some of the outfits installing uPVC are reasonably goo/priced; depends on where you live. INstaling one yourseelf may be more than you'd like to tackle; equally for a wooden replacement.

The traditional box windows company (I think that's what they called themselves years ago) charge an arm and a leg for cut out/replace damaged wood; and often far cheaper to gofor a completely new frame etc. and use a more local joiner/installed.... Sadly today most wood sash units are pine all round, whereas in very olde properties it's likely a harder wood or pine sides & top, and if you're lucky oak for the actual cill (and even oak will rot in time as I suspect you have and discovered).

I've not found compound wood filler routine a really successful/long term solution no matter how it's done and painted thereafter.
if I was going to repair it I'd be worried about the structural integrity of the joints, obviously if it's rotten there as well you're buggered, it's also a lot of work repairing stuff can even end up being more work/time than just building a new one from scratch, accoya is likely to be the best bet here. Have heard that trad linseed oil paint is really good for old sash windows, the paint you use is really important.
Looked at the Repair Care products and there are three? different products, what's the difference, can't seem to find any information.
I saw a test kit for £71 and a whole host of other products and the tools reqd . The one thing I see as per some of the above posts is the cost is horrendous. Given the amount of rot that is visible in the photo I can’t help thinking that to spend a considerable amount of cash notes on such a project is imo pointless especially if in a year or 2 the rot has spread to other parts of the frame/ window. Also was it not common years ago for the brickwork to be built directly onto the frame ie no lintel’s. Not saying this is the case here but given the £71 cost for the small starter kit how much of this product will be required to fill this frame again given the previous failed attempts at a repair. Me thinks new frame is the way forward, I’d also be inspecting the other windows for similar issues. Government grants are available in many areas for home improvements that improve the insulation and thermal efficiency of homes in the uk . It may be worth submitting an application.