Trim question: To cope or not to cope 135 degree inside angles

[Foo Man Choo SE]

I'm installing the trim in my basement and am about to start the 3 piece crown moldings. I'm pretty confident about being able to cope the 90 degree inside corners where the walls meet the ceiling but am not so sure the best way to deal with the 8 sided trays. The trays are octogonal with every angle being approx 135 degrees.

This raises some questions:

1) At what angle would I cut to achieve the profile for the cope? For 90 degree inside corners, it's 45 degrees.

2) The back cut with the coping saw would have to be at a very extreme angle. Would this make it much more difficult?

3) Should the non-coped ends also be cut at a slight angle so they meet the wall flush?

4) Would normal mitering be a better option than coping these joints? If so, any advice on getting the measurements right? Also, I'm sure the actual angles will be off by a degree or two -- would it be worth it trying to cut the exact angles?

Thanks!

--Rob

[Matthew S. Whiting]

Personally, I would miter.

Matt

[Joe Bobst]

Go to the libray and peruse the back issues of "This Old House" magazine. Tom Silva had a nice article on crown molding installation. If you don't have a sliding T-bevel for measuring angles, get one now and take out the guesswork and repetitive sneak-up cuts.

Bottom line, no for crown molding. Do it the experts way and your project will be a success. Good luck.

Joe

[Joe Fabeitz]

[Matthew S. Whiting]

This site has the basics, but doesn't address angles other than 90. I've not cut a coped joint for an angler other than 90, but I'm pretty sure that you still cut the ends with the appropriate miter angle for the corner in question and then cut the cope along the exposed profile just as for a 90. The tricky part is that rather than cut at about a 95 degree angle as you would for a tight fitting 90 joint, you will need to cut about 140 (135 + 5) or 5 degrees more than whatever angle you are cutting. This could be tricky to do accurately with a coping saw and is why I've always just mitered most angles.

Matt

[TGphyllis]

I have coped octagon type tray ceilings, I just cut the miter, then lay the piece flat on a table saw, lower the blade, so the top of the blade is about a quarter inch above the material, then move the piece back and forth across the blade, shaping it out as close as possible, this will give you plenty of undercut to get the cope to fit perfectly.

[Foo Man Choo SE]

It looks like there is no obviously correct answer to my question. I suppose I will just try out both mitering and coping to see which one makes sense for me. I will be painting the trim so mitering isn't that bad of an option but I'll give coping a chance first. The suggested coping angles make sense now that I think about it a bit.

[Foo Man Choo SE]

Glad to hear that someone's actually done it with success. Gives coping a good chance.

The idea of using a table saw (luckily I do have one) to nibble off the excess AND achive a high undercut angle sounds like a good one. This would probably be useful for the more typical 90 deg angles. Would it make sense, after cutting the miter to achieve the profile, to carefully score the profile edge (i.e. the one that should be marked with a pencil to make it more visible while coping) with a utility knife so that the leading edge comes out clean?

[zxcvbob]

There's no *need* to angle cut the back before coping; it just removes material and makes it a little easier to cut, and makes sure you get a tight fit at the front. I've made simple coped joints without back cutting first; you just have to saw more.

Calculate the angle the same as if you would use if you were making a miter cut. Then add a few degrees (however much you think your error is) to make sure you don't remove too much wood.

Best regards, Bob