Best Tenon Length for a Large Door

Here's a good discussion of joinery for frame-and-panel entry doors. April 4, 2011

I do mostly cabinetry and furniture but on occasion get asked to do architectural millwork items. I have been asked to fabricate three doors 1 3/4" thick x 42" wide x 89" tall. Each door will have four panels (five rails between 5" and 7" wide). I have done entrance doors before but none this wide. I will be using Spanish cedar. The doors will have a 3/4" thick flat panel so I can cut the mortise for the rails and the dado for the panel at one time on my table saw. My table saw can cut a depth of around 3 7/8". Are five tenons (per side) 3 7/8" long enough for the structural integrity of the door?

Forum Responses
(Architectural Woodworking Forum)
From contributor M:
Spanish cedar is not as heavy as many other woods, and considering that on each side you will have 5 tenons either 5 or 7 inches x 3 7/8", I would say yes. I am not familiar though with using tenons in thru dadoes. We count on the strength of having the mortise shoulder against the tenon. But I do not see how you could really increase the size of your tenons either, so I believe your sizes are fine.

From contributor A:
There would be a minimum tenon length required. I believe that would be below 3". You should be fine with 3 1/2" tenons. Keep in mind no one else has a method for cutting longer tenons besides a bandsaw, which is really not accurate enough.

From contributor J:
I'm sure I must be missing something here, but are you planning on cutting a full length 3 7/8" dado in both the stiles for both tenons and panels? I don't think that is going to work. I would probably make the top rail and kick rail tenons at least 4 1/2" long (depending also on stile width) as these are large doors. Certainly there are other ways than a band saw to cut long tenons. I have a choice of vertically on a tablesaw with a 14" blade, flat on the tablesaw with a dado setup, or on one of two single end tenoners. I'm sure there are other ways as well.

From contributor G:
When I make tongue and groove style entry doors I use a 1" long tenon. No problems yet.

From contributor V:
I, like others are not understanding what you are really trying to do. The panels should not go further than about 3/4 inch into the rails and stiles, whereas the tenons should go as far in as you have the technology to do so accurately. These doors are not going to be heavy as say an oak door but the mechanics are the same. Also there should be enough room for the panels to move. We use space balls, not the movie. Space balls are 3/16 in diameter rubber balls and when put on the sides and bottom of a panel allow it to always be centered. Do not glue the panels into the rails and stiles as this will allow the panels not to move and they may split. Be sure to use a good glue which is moisture resistant.

From contributor H:
Generally the rule is that the tenon is 2/3's of the width of the stile (6" stile = 4" tenon). I like to make the mortise 1/8" deeper than the tenon length. The inset of the dado should be 5/8", the bottom rail on a 89" door is at least 11", usually 12", this rail gets 2 tenons, spacing is 2" up, then 3 1/8" tenon, 3 1/8" gap, 3 1/8" tenon, 5/8" gap (dado), use a "pin" at 1/2" in each tenon at 3 1/2" and 3" to create a nice pattern. The pin should only go through 1 side, then the tenon then 1/4" to lock it in place. The intermediate rails should be at full with minus the dado, top rail should be 1 1/2" down from the then begin the mortise. By having these solid areas you get several benefits, you have a bit of a "haunch", you have a solid area for sizing, and you prevent the rails from rocking. Dry assemble first to get your order of assembly, then go to work. Also, instead of superballs for rattle prevention try the 1/8" x 3/8" silicone cabinet door bumpers. They already have glue on them, and if you want 1/4", just stack them and shoot them with micro nails.

From contributor R:
Your tenon length is fine, but using a dado in place of a mortise is not.

From contributor H:
Yep, I should've pointed that out as well - thanks Contributor M for doing so. A dado would only serve to weaken, even if you placed "filler blocks". So get the drill press or hollow mortiser ready. A drill press works well with Forstner bits and a chisel cleanup.

From the original questioner:
The majority consensus is that 3 7/8" is long enough. Yes, I know that I would not want a panel groove depth of 3 7/8". I planned on and see no reason why I could not fill that in with the same species that was removed (glued in place of course). The stock say 3 1/8" wide leaving a groove with a depth of 3/4". I am confident that I could glue this in place accurately, thus essentially creating a mortise deeper than I am currently capable of creating otherwise.

From contributor D:
The go to for us is 2" and we've built several thousand doors, with very few problems. Still, the longer the better I think. I occasionally have to reproduce very old doors exactly. In some cases the adhesive has completely failed and just the mechanics of a square-pegged through tenon has kept the door serviceable for over a hundred years. All material was quartersawn. Craftsmen back then knew what they were doing.