solid wood internal window cill question

[fobos8]

Hi all

At work we're doing a re-furb of an old granite farmhouse. We saved all the old rafters which are oregon pine (a.k.a douglas fir) and we intend to use them to make a load of internal window cills. The depth (from window to front nosing) of the cills will be around 400mm which is about the thickness of the granite walls. The boards have been planed/thicknesses to about 24mm and they look really lovely. Each one is is around 160mm wide and so three will be edge jointed together to get the 400mm needed.

My question is about how to keep the cills flat and how to limit the effects of cupping and shrinkage. My boss's suggestion is to glue and screw the whole cill to some 18mm plywood. I'm not sure about this idea though as how will the wood be able to shrink/expand? But then what about veneered plywood which is the same thing surely - wood glued to plywood.

What about screwing the cills to the ply with fixed holes at the back of the cill (where it meets the window) and then have elongated holes in the middle and at the front of the cill. This will allow the pine cill to expand contract as it wishes.

Anyone else got some suggestions? Will it work to glue the whole thing to ply.

All the best,

Andrew

 

[Rob Platt]

Joint them, cut to size, tongue into groove in external cill and fix down after varnishing all six sides. Pellet holes and varnish again. Final topcoat after plasterers have tried to saturate them as they have a habit of doing. Probably how I`d do it dependant on moisture content of timber.
All the best
rob

 

[Chems]

I asked the same here if its of any help:

window-sills-internal-t45613.html?hilit=window

 

[mtr1]

They sound like they might be dry! I would do as you suggest, and joint them. Take even amounts off both sides, and let them sit awhile where they are to be sited(1 week/2 weeks). See if there is any movement left, and shrinkage gods willing???

 

[fobos8]

many thanks for the replies

I need to point out that these cills are 400mm deep (from window to nosing). They will MOVE AND SHRINK. I reckon about 5mm.

I cannot simply screw rigidly to masonary underneath cos they will crack as they shrink.

Can I glue and screw the whole thing to ply (as you do when you veneer something) - what will happen?

Andrew

 

[Chems]

I think even if you glue it and screw it, if it wants to move it will move. You have to allow it movement space. As your gluing it up from multiple boards it should be better, you can alternate the grain and all that. When you veneer the veneer is only mm's or less thick, it has no movement but your working with solid wood.

 

[fobos8]

When you veneer the veneer is only mm's or less thick, it has no movement but your working with solid wood.

why does solid wood that is 1mm thick not move, whereas wood which is say 20mm thick will move. DOES veneer not move or is it restricted from moving cos it is glued to ply/mdf? If so why can't you glue 20mm solid wood to Ply and expect the same results.

 

[knappers]

Because 1mm thickness doesn't exert enough force when it moves to overcome the glue or substrate its fastened to, whereas 20mm of wood exerts a **** of a lot of force.
Si

 

[fobos8]

Because 1mm thickness doesn't exert enough force when it moves to overcome the glue or substrate its fastened to, whereas 20mm of wood exerts a **** of a lot of force.

Thats very interesting. Roughly what thickness is the max you could use for veneering before the wood exerts too much force.

On engineered wood flooring I notice that there is normally about 6mm of wood glued to ply and that is very stable. Can you go thicker than that?

 

[SNight]

Ive seen them fixed with expanding foam before. Might be an idea! Lot less work to it also.