Internal Window Sill Boards and Moisture

[aockenden]

Hey all,

We are preparing to install our internal oak sill-boards into our stone window reveals. Unfortunately I had no good place to store the raw oak board stock so they sat in a humid polytunnel in their packaging for months, which has caused them to “cup” noticeably due to the relatively high moisture levels out there.

Since then, we have cut them to fit and have brought them inside to dry out and most of the cupping has subsided. However, I’m concerned that they may re-cup once installed into the reveals for a couple reasons:

1. I’m planning to bed them into some lime mortar, so they may temporarily absorb moisture from that bed of mortar.

2. The lime surrounding the window frames tends to wick moisture through from the outside during periods of driving rain from the south. Eventually the lime around the window frames dries back out but it takes a while, meaning that there will be moisture near the window sill boards during those periods.

Is there a way to mechanically restrict them from cupping (maybe with some back bracing strips or something)? Is a wooden sill board just a poor choice for an old stone house? Will the render that will seal the board into place coating the sides of the reveal be enough to hold the board flat and prevent cupping or would it likely just crumble the render that touches it?

Thanks for any thoughts.

 

[Toby Newell]

You stored them in a humid polytunnel? Can I get a lol...

You bought cheap multiplank boards... hmmm, not ideal. Are they constructed grain up grain down to minimise warping? If they are cupping you may want to:

1. Fit dpm underneath the cill.
2. Glue (MS hybrid) and screw and pellet to fix eg 9 screws evenly spaced.
3. Rout out relief grooves at back, say 6mm depth at 25 or 50mm centres (with the grain)
4. Consider that thicker one piece timber may be more stable, especially if you can choose quarter sawn pieces.

If you don't have a router you can use a circular saw or tablesaw with a thicker 3.2mm kerf blade.

If they want to move you can't stop them. I don't like the multi plank effect tbh, personally I would search ebay for large dimension oak tables, you can occassionally find bargains, look for farmhouse or refrectory tables then look for large, wide thick section with quarter grain figure (medulary rays) the wood will be well seasoned and kiln dried and cheaper than buying quarter sawn from a timber yard. You might need to make one biscuit join. I think the uber rustic nature and coarse texture of your house would match much thicker, natural looking cills, these look a bit weedy imho.

If anyone ever posts a proper photo I might be able to identify some wood...

 

[Toby Newell]

Nice scribing though... satisfying isn't it?

 

[Toby Newell]

Sorry, forgot, leave an expansion gap where the cill meets the window, you have none, the cill is going to ride up where you have blocked it's movement. Either leave, fill with cork strip or flexible filler like Bona gapmaster. Wood expands primarily accross it's width, about 0.25% per 1% change in moisture. You can expect a 10% change in moisture, so 2.5% x width, in mm, so quite a lot. I would leave 5mm at least.

Quarter sawn timber cups less and moves less overal as more of the fibres are orientated vertically and so the expansion occurs more accross the thickness and less along the width, so typically the width movement can be halved.

Pretty sure those are medullary ray flecks in your lintel which means they are oak. Seeing as you have new oak cills and new oak floors whya do you not want to match up the lintels by sanding them clean?

 

[Toby Newell]

Sorry, to answer more questions, no, bedding in lime mortar I would expect the cill to detach within months, or possibly weeks. Lime vs wet oak expanding 10mm, oak wins. And wooden cills are perfectly possible if you fix them properly and leave expansion room. As long as they remain in place I would argue you would want them to move and warp a little to blend in with the rest of your very prettily non linear house.

 

[aockenden]

You stored them in a humid polytunnel? Can I get a lol...

You bought cheap multiplank boards... hmmm, not ideal. Are they constructed grain up grain down to minimise warping? If they are cupping you may want to:

1. Fit dpm underneath the cill.
2. Glue (MS hybrid) and screw and pellet to fix eg 9 screws evenly spaced.
3. Rout out relief grooves at back, say 6mm depth at 25 or 50mm centres (with the grain)
4. Consider that thicker one piece timber may be more stable, especially if you can choose quarter sawn pieces.

If you don't have a router you can use a circular saw or tablesaw with a thicker 3.2mm kerf blade.

If they want to move you can't stop them. I don't like the multi plank effect tbh, personally I would search ebay for large dimension oak tables, you can occassionally find bargains, look for farmhouse or refrectory tables then look for large, wide thick section with quarter grain figure (medulary rays) the wood will be well seasoned and kiln dried and cheaper than buying quarter sawn from a timber yard. You might need to make one biscuit join. I think the uber rustic nature and coarse texture of your house would match much thicker, natural looking cills, these look a bit weedy imho.

If anyone ever posts a proper photo I might be able to identify some wood...

Yes I knew the polytunnel was a bad idea from the start but our infrastructure here is so remote and minimal and we are living in the house while renovating it from the ground up so there’s literally nowhere else to put stuff that’s even remotely dry. Delays in work schedules also meant they were in there longer than we had originally hoped. As for them being multi-plank boards that was a surprise to us too, as we thought we were buying solid oak, and we were expecting listings to say “engineered” if they were actually made from multiple pieces. We deliberately bought the thicker stuff available to try to prevent this warping issue in the first place.

Now that we have them and they’re cut, I feel like we might as well try to get them to work as we won’t be getting a refund either way. If it still warps in-situ we’ll just need to start from scratch with new material.

So your point #2 is suggesting screwing down through the board with some kind of self-tapping masonry screw into the underlying stone of the reveal, and countersinking the screw-heads into the topside of the sill board and then plugging the countersink holes with tapered oak plugs? Or did I totally misunderstand?

 

[Toby Newell]

Yes I knew the polytunnel was a bad idea from the start but our infrastructure here is so remote and minimal and we are living in the house while renovating it from the ground up so there’s literally nowhere else to put stuff that’s even remotely dry. Delays in work schedules also meant they were in there longer than we had originally hoped. As for them being multi-plank boards that was a surprise to us too, as we thought we were buying solid oak, and we were expecting listings to say “engineered” if they were actually made from multiple pieces. We deliberately bought the thicker stuff available to try to prevent this warping issue in the first place.

Now that we have them and they’re cut, I feel like we might as well try to get them to work as we won’t be getting a refund either way. If it still warps in-situ we’ll just need to start from scratch with new material.

So your point #2 is suggesting screwing down through the board with some kind of self-tapping masonry screw into the underlying stone of the reveal, and countersinking the screw-heads into the topside of the sill board and then plugging the countersink holes with tapered oak plugs? Or did I totally misunderstand?

Engineered wood means it has a cross ply backing, multistrip boards are different. Check on the edges you want the grain to run up, then down in semi circles, this helps even out the cupping. Next time keep timber in the house, flat, stacked with blocks in between, all moot really if you are fixing them to a sponge.

Yes, I would apply a dpm, a thick bed of Wakol MS 260, Bona R848 or Quantum then screw and plug as you said (plug as in wood pellet to cover the screw) . I prefer masonry screws to plugs and screws as they expand less, so less cracking. I have hundreds of them.

 

[Toby Newell]

Glue mostly holds the cill day to day, those adhesives are strong and flexible, but the screws do the heavy lifting when the forces get too great.

The adhesive will hold a few hundred kilos, the screws will hold a few thousand. You want to make a pyramid? gouge a small hole in a 200 tonne stone, put a bit of dry wood in and add water, the expansion of the wood will easily break apart the stone, it's hard to imagine the power of expanding wood.

I was on a site once near St Pauls in the early 1990s, they had fixed the beech or maple skirtings then comissioned the heating. One section of the several km had obviously been replaced and instead of screwing and pelleting it they guy botched it with gripfill. Well, the skirting shrunk and the timber decided it wanted to go on holiday, I heard a small explosion behind me and an 8 foot section of skirting exploded off the wall, with half of the bricks attached.

Timber movement is a form of criminality, ie if someone wants something bad enough, they will find a way to pinch it, you just make it as difficult as possible for them. If timber wants to move, you can reduce it, but in the end it will win and so you have to make accomodation for it.

Over time it will happily bend even metal bracing. Remember weeds can break though concrete and try and forget bamboo can do the same to animals. Quite amazing really.

 

[aockenden]

Bloody hell, I just checked the board again and it’s still badly cupped. I thought it had mostly recovered. I think we’ve botched this one.

I also hadn’t fully thought through that expansion gap issue… so thank you very much for pointing that out. I definitely do NOT want the sill expanding against and torqueing our nice new window frames which we painted and installed ourselves and had made for us by a local joiner.

I really have no idea where to get any thick, super wide, single piece oak slabs. We might need to pivot and put in some slate sills instead. Less expansion problems, can mortar it straight in, no screws needed. Downside will be the big thermal bridge it’ll create but we could probably bed it into some insulating perlite mortar to improve things slightly.

 

[Toby Newell]

Bloody hell, I just checked the board again and it’s still badly cupped. I thought it had mostly recovered. I think we’ve botched this one.

I also hadn’t fully thought through that expansion gap issue… so thank you very much for pointing that out. I definitely do NOT want the sill expanding against and torqueing our nice new window frames which we painted and installed ourselves and had made for us by a local joiner.

I really have no idea where to get any thick, super wide, single piece oak slabs. We might need to pivot and put in some slate sills instead. Less expansion problems, can mortar it straight in, no screws needed. Downside will be the big thermal bridge it’ll create but we could probably bed it into some insulating perlite mortar to improve things slightly.

Lol, see above. Ebay. Set searchs for 'large oak table' 'rustic oak table' and 'oak refrectory table' sort by distance. Most of them will be silly money but you will get the odd gem. Look for ones with thick, wide planks with lots of medulary rays (quarter sawn figure) will probably be cheaper than you can buy new.

You don't need one piece, but two, maybe three max.

Mmmm, slate is going to be a condensation magnet near the windows and oak cills would tie in with the lintels and flooring, I would still go for oak.

I wouldn't necessarily worry about a little warping, your entire house is wonky, just make sure you fix it down properly, laying into lime it will curl like leaf and peel off.

 

[aockenden]

Lol, see above. Ebay. Set searchs for 'large oak table' 'rustic oak table' and 'oak refrectory table' sort by distance. Most of them will be silly money but you will get the odd gem. Look for ones with thick, wide planks with lots of medulary rays (quarter sawn figure) will probably be cheaper than you can buy new.

You don't need one piece, but two, maybe three max.

Mmmm, slate is going to be a condensation magnet near the windows and oak cills would tie in with the lintels and flooring, I would still go for oak.

I wouldn't necessarily worry about a little warping, your entire house is wonky, just make sure you fix it down properly, laying into lime it will curl like leaf and peel off.

Ok so how does it work to do it with 2 pieces? Do you just butt them up against each other? Or would I need to lap-joint them?

 

[Toby Newell]

Ok so how does it work to do it with 2 pieces? Do you just butt them up against each other? Or would I need to lap-joint them?

You can buy a table and cut from that straight. Otherwise you would need to biscuit joint them.

Honestly if you screwed and glued what you have it would stay stuck. A little warpage isnt so bad. Cutting the relief on the underside can help.

 

[aockenden]

You can buy a table and cut from that straight. Otherwise you would need to biscuit joint them.

Honestly if you screwed and glued what you have it would stay stuck. A little warpage isnt so bad. Cutting the relief on the underside can help.

No dude, I think you’re right. They are too thin and don’t fit the aesthetics of the house. Tbh my wife wanted to get thicker stuff but I thought we’d get away with the 25 mm thick stuff. A live edge would also probably be preferable.

 

[Toby Newell]

No dude, I think you’re right. They are too thin and don’t fit the aesthetics of the house. Tbh my wife wanted to get thicker stuff but I thought we’d get away with the 25 mm thick stuff. A live edge would also probably be preferable.

Send me the dimensions. I don't think you need a live edge, just maybe a little thicker, 30 to 40mm and wider boards. Also quarter sawn will be more stable.

Quarter sawn oak is around £300sqm and then you have to biscuit them, so if you can find a nice table it can be cheaper, the wood is better seasoned and they are already biscuited. I was working in Suffolk and I needed some reclaimed oak for infills on a 1407 - 1550 oak floor, we visited a few local yards but nothing. I found an amazing table on ebay down in London and won it for £175. I felt really bad destroying it but it was made from 3 x 15 inch wide, 1 inch thick, 100 year old quarter sawn English oak boards with beautiful medulary rays. You simply could not have bought the timber, if you could of found it, for 3 times the price, so it can be done.

It does help if you know what you are looking at! You want listings of stained, dirty or perished finish tables with not the best photos so that they aren't enticing for people to bid but which you can see enough medulary rays to know the timber is stunning, then sometimes you get lucky.

I found a company selling quarter sawn English oak boards 250mm x 40mm they work out about £310sqm but then if you don't have a biscuit jointer and clamps it's a no go.

 

[aockenden]

Send me the dimensions. I don't think you need a live edge, just maybe a little thicker, 30 to 40mm and wider boards. Also quarter sawn will be more stable.

Quarter sawn oak is around £300sqm and then you have to biscuit them, so if you can find a nice table it can be cheaper, the wood is better seasoned and they are already biscuited. I was working in Suffolk and I needed some reclaimed oak for infills on a 1407 - 1550 oak floor, we visited a few local yards but nothing. I found an amazing table on ebay down in London and won it for £175. I felt really bad destroying it but it was made from 3 x 15 inch wide, 1 inch thick, 100 year old quarter sawn English oak boards with beautiful medulary rays. You simply could not have bought the timber, if you could of found it, for 3 times the price, so it can be done.

It does help if you know what you are looking at! You want listings of stained, dirty or perished finish tables with not the best photos so that they aren't enticing for people to bid but which you can see enough medulary rays to know the timber is stunning, then sometimes you get lucky.

I found a company selling quarter sawn English oak boards 250mm x 40mm they work out about £310sqm but then if you don't have a biscuit jointer and clamps it's a no go.

I definitely do not have a biscuit joiner. The dimensions are two sills around 1200 x 520 and two sills around 1000 x 520.

 

[fernicarry]

You can make biscuit joints with a router.

 

[aockenden]

You can make biscuit joints with a router.

Yeah not happening. I’ll just go with slate sills and cut them to shape with an angle grinder before I go down that route. I’m not an advanced wood worker.

 

[Toby Newell]

Ah. Well, you could try putting a dpm underneath the wooden cills you have already scribed and just drilling and plugging them with no glue so quite easy to remove if they peel badly. I just think half a square metre of slate next to a window in an old stone wall is going to act as a mini dehumidifier... You did such a nice job scribing them I would give them a go.

Maybe run an experiment, get some slate and place it on the cill and see how much condensation it creates, you don't want to be mopping your cills every day in the winter.

 

[Toby Newell]

I honestly dont think aesthetically a little bit of warpage is going to be deleterious, perhaps the contrary, it's only a functional issue if it pulls itself out or your coffee cup slides off.

 

[aockenden]

Ah. Well, you could try putting a dpm underneath the wooden cills you have already scribed and just drilling and plugging them with no glue so quite easy to remove if they peel badly. I just think half a square metre of slate next to a window in an old stone wall is going to act as a mini dehumidifier... You did such a nice job scribing them I would give them a go.

Maybe run an experiment, get some slate and place it on the cill and see how much condensation it creates, you don't want to be mopping your cills every day in the winter.

Thanks, but scribing and cutting was quite easy for us with our current skill set, and it will be a simple matter to trace those onto another material to redo the scribe cuts. So I’m not worried about that part. I’ve just come to agree with your assessment that they’re too thin to match the aesthetic of the house and that installing will be a total pain in the butt, as I will definitely be wanting to “pull” the warp out of them with the masonry screws but I’ve rebuilt the stonework underneath the sill with many layers of small stone and softish lime, meaning getting solid fixings down into that will be VERY challenging.

The slate would solve the fixing problem, the warping problem, the expansion gap problem and would be slightly thicker as well (30 mm instead of 25 mm of the current boards I cut). I grant they will be colder and more prone to condensation. Though none of that is relevant as I apparently cannot convince my wife to go for them… idk what I’ll do. Lost.