Kitchen worktop.... Quartz, corian, granite even, that will not care about damp. Likely to be "off cut" size too, so make a template and visit your local worktop fabricator.

If you have an old house, please don't fit plastic window boards. 

A timber board in an old house is a lovely thing and worth the effort.

Are they single or double glazed windows?

It _is_ possible to manage damp in old houses, bit it's very much on a cas by case basis. If the ground level outside is that high that it could reasonably be affecting the window boards then I'd be investigating if a french drain type arrangement could work on the outside to help lower the ground contact level. I'd also be checking for signs of water penetrating from leaks etc as they're far more likely to be the cause of damp IME.

I'm not pretending to be an expert, I've just spent the last 15 years trying to come to an agreement with water in a 1780s cottage (Solid brick, flag floors, no dpc)

(FWIW I fitted MDF boards and spend most days regretting that choice) 

Another option is a couple of large slate floor tiles

Posted by: steveb

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Kitchen worktop.... Quartz, corian, granite even, that will not care about damp. Likely to be "off cut" size too, so make a template and visit your local worktop fabricator.

Pretty much what I was going to say. Our fitter was also working on installing a marble staircase in a posh house... so we have several marble windowsills from the offcuts in his van 🙂

Hmm, stone cut to size, that's a possibility. We have some old stone roofing tiles in a pile outside, one of them might be big enough

I'll need to take the old sill off though. Dumb question, how is the existing sill likely to be attached? Do I just smack it from the underside under the lip and then prise it up?

It'll be glued and/or screwed. Run a sharp knife along all the silicone/caulk beads then smack it from underneath. Depending if it was fitted before plasterboarding/plastering or not, you might need to cut it along the ends where it meets the walls with a multi tool. 

I have two pieces of some tongue-and-groove stuff, presumably flooring(?) on each of my very deep stone windowsills in an old cold stone house (ex-church). Wood effect to match the panelling on the walls, not that they do a very good job of that, what with being a completely different colour.

They aren't the smartest part of the house but not the shabbiest either!

I'll need to take the old sill off though. Dumb question, how is the existing sill likely to be attached? Do I just smack it from the underside under the lip and then prise it up?

Chisel/drill a 25mm or greater hole at the back, centrally, next to the window frame. This becomes a stop for sawing with an ordinary handsaw from the front edge

Gently lever up to see if it gives up easily. Beware of plaster over the ends, underneath ^^ and the possibility of a tongue let into the window frame. Don't expect to use plaster as a crowbar pivot, so pry along the sill rather than across until you have some room

MDF is the worst possible choice. A resinous hardwood like keruing, maybe, or a reclaimed piece of pitch pine for a "woody" look

I've had a look at it and can see the edges all round, none of it is recessed into the plaster at the sides or the (UPVC) window at the back.

Assuming one of the old tiles we have is big enough, any STWrs in or around Stockport/Macc/Buxton that have the ability/tools to cut a piece of stone roofing tile to size (it's a shallow trapezium, nothing intricate) in exchange for some beer tokens?

Have you got an angle grinder? I bought a 4.5" diamond disc from Screwfix for about 15 quid that, after a few practice cuts, let me do a good enough job to hide the edges if the cuts behind the plaster

I haven't, but I could probably borrow one

Well, it's off! With some careful hammering of a couple of brick chisels under the front lip, a bit of lifting, then driving three bits of batten gradually underneath it, it lifted off the big blobs of grab adhesive that it was stuck down with. 

And, someone on the local FB thrift site is giving away a large stone flagstone, so bingo!

Now to borrow an angle grinder...

Now to borrow an angle grinder...

They're useful enough and cheap enough just to buy one. If you choose carefully you might even get a disk suitable to cut the stone bundled in.

Aldi had angle grinders in a few days ago

Sounds like you have the MDF as a template, which is great.

You may be surprised how short a depth a 4.5" disk cuts, and how much dust grinding produces!

Good luck, pics of the finished result please 🙂

Posted by: oldnick

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You may be surprised how short a depth a 4.5" disk cuts,

When you say short, do you mean shallow, i.e. it won't get through the depth of a flagstone in one cut? Or short as in length of cut, i.e. before the disc is shagged and needs to be replaced? 

Second question, am I best to cut from the top face or the bottom face, or doesn't it matter?

Posted by: IHN

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Second question, am I best to cut from the top face or the bottom face, or doesn't it matter?

How thick is the slab? You've probably got about 40mm cut depth with a 4.5 inch grinder, but that's the smallest size. You can get 5 inch ones before you jump up to 9. Cut from the top, you can get chips on the underside depending on the material. 

Posted by: tthew

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How thick is the slab?

Just had a look without a tape measure to hand, but I reckon it's about 25/30mm. It's a standard 3ftx2ft stone slab

Keep the stone wet whilst cutting. It'll keep the dust down and cool the disc so it'll last longer. 

I usually set up a hose pipe to trickle water down the stone towards where I'm cutting. 

So, an update. 

I borrowed an angle grinder from a mate (who already has some stone cutting discs), made a cutting 'table' from some piled up pallets, marked round the existing sill onto the stone slab, set up the hose to provide a little trickle of water as suggested above, had a little practice in one corner to get a feel for it, and then got going properly.

Took it nice and steady, just letting the weight of the grinder do the work, and it was all going well, if quite slowly, for about fifteen minutes, when the grinder starting making a funny noise and a load of smoke blew out the side of it. Bugger.

So, now I need to buy myself a grinder and my mate a grinder...

Posted by: IHN

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grinder starting making a funny noise and a load of smoke blew out the side of it. Bugger.

So, now I need to buy myself a grinder and my mate a grinder...

Shouldn't laugh, but... 🤣 Bad luck dude. 

This is a true story about my missis doing a similar thing. Might cheer you up a bit. Was doing some hoovering for her mum while away and it started making horrible noise and smoking. So she brought it to.our house and was disappointed when, (naturally) it made the same noise and smoke. When I asked her why she'd tried this, her response was that she wanted to "see if it did it on different electricity" 🤷

Arg gutted! Lesson learnt - every man needs a grinder. And a tarp

Posted by: tthew

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her response was that she wanted to "see if it did it on different electricity"

I mean, that is genius 🙂

Always worth checking the brushes, if when you pop them out they are short and damaged they may well be the culprit. If you are struggling to get the right ones go to a proper tool hire place (that does maintenance on site) and see if they can help. Penultimate resort is buying bigger ones and filing them down to fit. 

Always worth checking the brushes,

It's an easy check, so yes worth it, but IME worn brushes tend to cause intermittent running or difficult starting and maybe the odd spark if there's an escape point. Noise and smoke sounds more terminal. 

Thread revival time. After the purchase of an angle grinder and a fancy pants diamond disc, I set up my cutting table and my hosepipe again and had another crack at it and.......

ta-daaaa!

There are some gaps around the edges. It turns out that, although I'm impressed with myself for getting it bob-on the same shape as the old sill, the old sill wasn't a great fit. I'm all for leaving them (to let it breathe a bit, cos of the damp issues that got us here in the first place), but MrsIHN would prefer them filled. Rather than bunging it up with silicon, is there a breathable gap-filler that looks decent?

A gap earlier...

Looks nice. 

For the gap filling, I might go with a small wooden bead painted to match the walls if you don't want it sealed. Which sounds like a sensible approach. 

Hmm, beading eh...

You could try for a matching tile grout, just don't smear it everywhere so use some tape

Otherwise, a bead or flat strip (rather than a more trad glass bead) on the wall and painted white. The problem then is that you'll have gaps under the flat bead 😉 https://richardburbidge.com/shop/mouldings/windows

Posted by: timba

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The problem then is that you'll have gaps under the flat bead

Yeah, good point.

What about good old boggo Polyfilla, is that breathable?

I'd leave it as is. You can only see the gap because you know about it; nobody else will notice. White filler will look like a poor paint job 

First time ive seen this thread. looks brilliant, considering you were going to cap some crappy fibreboard.

well done!

If it were me, i would leave that gap. Its hardly the grand canyon, you wont even notice it in a weeks time, and it lets the damp breathe better?

Posted by: timba

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nobody else will notice

Someone has noticed, and I'm married to her

Nice job. Unlikely that any of your surrounding materials are breathable - emulsion and UPVC windows. I think  you are asking a lot of a tiny bead of filler to provide much in the way of moisture transport. What did you bed it in with? I'd just use whatever filler you have. 

Posted by: robola

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I think  you are asking a lot of a tiny bead of filler to provide much in the way of moisture transport.

Oh, don't get me wrong, I'm not expecting this to fix the issue, but if I can avoid trapping any (more) moisture under there then that can only be a good thing.

Posted by: robola

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What did you bed it in with?

Nothing, it's loose, just sitting on the splats of dried grab adhesive that the old one was stuck down with. Well, I say 'loose', it probably weighs 20kg so it's not going anywhere.

If you really want breathable you are probably looking at lime based, you could try a casein filler - https://www.limestuff.co.uk/fillers/volvox-filler-500g-565

Looks really good.  I would run a wooden bead around the edge if it were mine