Joining bay window boards

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ObservantGround28

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Hello.

I have an L-shaped bay (2.5x0.8m with a single 90 deg internal corner) and am using 200mm wide by 25mm bull-nosed MRMDF window board.

I know the standard practice is to mitre joins for window boards in a bay. Is there any merit in joining the 2 boards using a butt join that's usually used on worktops?

In either case, should I use biscuits or dowels to align the 2 surfaces? Should I glue the join?

This is my first bay window in the house to contend with. Any advice is much appreciated!
 

Cabinetman

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First of all let me say that I’ve never actually done one but I’ve seen a lot that are grotty, I think you might be on the right track trying to reinforce that mitre joint with the metal joint pullers from kitchen worktops but I don’t think you’ll have enough thickness of the timber to be able to do it successfully.
Perhaps a glued lap joint, easy on mdf as it follows the layers. Glue and G cramps then put into position. Ian
 

Argus

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I suppose that from a practical point of view of the joint itself, you could do either.

What would be most appropriate from an aesthetic point of view would be to choose the method that fits with the age and style of the house.

If you choose a mitre at about 45 degrees, anticipate the potential of movement at the meeting point with heat and humidity through the seasons.

In any case, from experience, I would not expect either of the inner corners that the builder left you to be right angles......!

Good luck
 

ObservantGround28

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First of all let me say that I’ve never actually done one but I’ve seen a lot that are grotty, I think you might be on the right track trying to reinforce that mitre joint with the metal joint pullers from kitchen worktops but I don’t think you’ll have enough thickness of the timber to be able to do it successfully.
Perhaps a glued lap joint, easy on mdf as it follows the layers. Glue and G cramps then put into position. Ian
I ruled out using a connecting bolt method because, as you suggested , the material is just too thin. Interesting idea about the lap joint and I'll have a play with that with some scrap.
 

niall Y

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Hi ,
In the past the standard practice of Joiners was to use a butt joint on an internal corner such as this. With MDF the long miters do not open up as they would with ordinary timber. If you want to maintain a traditional look, then the butt joint as used on a worktop is the way to go. Biscuits will help you keep the top surfaces flush, and gluing will help.You might encounter problems getting the two pieces together in the opening,if using biscuits. However, running a groove on one of the mating pieces will allow for a slide fit. Best of luck. Niall
 

recipio

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A mitre will always look better. Assuming you can cut an accurate mitre I would pocket screw them together on the underside. Don't use PVA glue as it will leave a glue line - a slow setting Cyanoacrylate glue would be better.
 

ObservantGround28

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Hi ,
In the past the standard practice of Joiners was to use a butt joint on an internal corner such as this. With MDF the long miters do not open up as they would with ordinary timber. If you want to maintain a traditional look, then the butt joint as used on a worktop is the way to go. Biscuits will help you keep the top surfaces flush, and gluing will help.You might encounter problems getting the two pieces together in the opening,if using biscuits. However, running a groove on one of the mating pieces will allow for a slide fit. Best of luck. Niall

The smaller run of window board is not confined by a wall on the end so it should be easier to get them to mate properly. I would definitely groove along the entire end of the board though as you've suggested. Thanks for the advice!
 

ObservantGround28

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A mitre will always look better. Assuming you can cut an accurate mitre I would pocket screw them together on the underside. Don't use PVA glue as it will leave a glue line - a slow setting Cyanoacrylate glue would be better.
I do have reservations about the practicality of fixing from underneath (I have considered pocket screws), but my concern is around being able to fit the entire piece in one go. I feel that I'd really make a hash of it and am far more confident of getting a better fit and finish if I'm able to install each section separately.
 

MARK.B.

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Leave the shorter piece longer than needed until you are sure you are happy with the fit, then cut to finished length.That way you get a second or third go at getting it right:)
 

recipio

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I do have reservations about the practicality of fixing from underneath (I have considered pocket screws), but my concern is around being able to fit the entire piece in one go. I feel that I'd really make a hash of it and am far more confident of getting a better fit and finish if I'm able to install each section separately.

Fair enough. I wouldn't be concerned about the strength. Cutting long mitres can be a challenge and you would need a dead flat surface to assemble them.
 

Chip shop

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I think I'd Mason's mitre, with biccies / loose tenon / Domino. Apply glue to the tongue/tenon/biscuit. Hoik it all together with some gentle pressure from a pry bar at one end then fire finishing nails through both ends of the joints to keep it all together until the glue cures. Fill the nail hole and add some caulking goodness to taste.
 

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