How to allow for expansion & contraction in a wide door rail?
I am building a full-light exterior door, with a triple-glazed window unit. I’d like this door to last hundreds of years, so I’ve bought some quarter-sawn 8/4 white oak, and I’m planning on a 12″ bottom rail to take the weight of the glass.
My plan is to give the bottom rail a double tenon, 3-1/2″ per tenon, with a full haunch, and pin both tenons. Single pinned tenon on the top rail.
What’s got me worried is thinking about expansion and contraction of the bottom rail, and how to allow for it in the joints. I’m thinking that perhaps I could just pin one of the double tenons and leave the other one floating. Or perhaps I could relieve the base of one tenon to allow for some movement. Is there a better way, or am I overthinking it?
What did the old-timers do? I haven’t been able to dig up anything on the subject, and it seems that it’s been almost unheard of to use such a wide rail for quite some time.
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The best source I've found is the book "Doormaking and Window-Making" from Lost Art Press. It shows you how the old timers did it. It shows 9 inch wide rails with double tenons, without pins. Triple glazed glass of course is not how the old timers did it, and that glass will likely fog up before the well made door frame fails.
Thanks Davec -- I'll order a copy of that book. Do they include any accommodation for expansion & contraction of the rail in their joinery?
I agree that the sealed window unit will fail at some point, and hopefully can be replaced multiple times before the frame fails. Ideally with transparent non-heat-conducting aluminum. :D
I don't see any such accommodation. They show wedging the tenons in a 4 panel door. Most of the doors they show include a third rail in the middle for strength, which you might be omitting. Keep in mind this isn't a detailed how-to book, but assumes you already know how to do woodworking generally. But I haven't seen a better reference summarizing how various traditional door styles were constructed.
I'd also recommend watching the recent video workshop by Christian Becksvoort on wood movement. It should help you figure out how much movement to expect.
The middle rail might be a real key. I'm not sure I've seen an old frame and panel door that didn't have three rails.
I did some research on this same subject when I built my exterior doors, also with 8/4 quarter sawn white oak. My bottom rails are also 12 inches wide. I don't remember the source, but it was mentioned to account for the expansion in the mortises. I only used a wide single M&T in the bottom rail and make the mortise longer than the tenon. I pushed the tenon tight to the end of the mortise that I didn't want to move and only glued about half of the tenon. This allow the rail to expand and you can control the direction of expansion. I don't have a full piece of glass in the front door, so my rails expand into the solid panel areas which have gaps to allow this. This keeps the bottom of my door even. With full glass, you may have to allow the rail to expand away from the glass or provide an expansion gap around the glass. My doors are not 100 years old yet, but have been installed for 5 years without issue. I live in the Midwest and we have a fairly large moisture swing during the year.
It's a good point about old doors having more rails -- they definitely didn't have triple-glazed full-light sealed window units in the old days! Still, it seems like with modern understandings of wood properties, it's got to be possible to make it work.
@davec_5 Thanks for pointing me to that video workshop by Christian Becksvoort. Although I don't know exactly what my moisture extremes are going to be, it seems like I may be facing somewhere up to 1/8" of expansion and contraction through the seasons. That seems like enough that I will have to account for it.
@Jake_W Great to hear that you made it work, and it seems like my thinking is not too far out there. Since the most important weight-bearing part of the bottom rail tenon will be at the top, I think I'm going to have to pin it there, and allow for the rail to expand outwards at the bottom of the door. Not the best for aesthetics, but better than the rail splitting if it's over-constrained.
I don't think your 12" bottom rail will cause any problems. Take a look at Nancy Hellers piece in this article. https://www.finewoodworking.com/2020/02/07/arts-and-crafts-chest
She uses quarter sawn white oak and joins wood against the grain, in panels much wider than 12". If your white oak is kiln dried, I bet you will not have any problems.
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