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We all hear the warnings about not going across the grain when fitting together boards, but obviously doors are constructed cross grain, with the grain of the rails going one direction and the stiles going the other. Why don't they split apart as the wood moves?

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We all hear the warnings about not going across the grain when fitting together boards, but obviously doors are constructed cross grain, with the grain of the rails going one direction and the stiles going the other. Why don't they split apart as the wood moves?

Those warnings generally mean "don't join wide pieces with grain running crosswise without accommodating seasonal movement," not that you can't ever join parts with crosswise grain.

Almost everything about a typical solid-wood exterior door is designed to allow seasonal movement without splitting:

  • frame and panel construction: Much of the width of the door is made up of panels that float within their frames and have room to expand. The rails are narrow enough (4-10") that they don't move much.
  • mortise and tenons: Rails are mortised into the stiles, giving a good mechanical connection. If you think about it, mortise and tenon joints always involve cross-grain connections, but they're one of the strongest joints around. Very wide rails will have two tenons instead of one wide one specifically to avoid a large cross-grain glue connection.
  • finish: A few coats of varnish or primer and paint significantly reduce seasonal movement.
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Normally wood rails are only several inches in width. The expansion over such a short width is not significant. As the width increases the overall expansion becomes more significant. Wood expands and contracts at different rates for each species and differently for width and depth. However, it does not expand in length. The amount of movement will differ between species and the direction of the grain being considered (does the grain run across the width or across the depth). Also note that the width expands and contracts due to temperature and humidity changes. If the wood is exposed to a constant environment there will be little change. If the immediate climate changes significantly then the expansion and contraction will be more pronounced. I would caution you that while the piece may be in a stable environment now, it may not stay there for long, so it is advisable to be conservative in your design. You can find plenty of information on the rates of species online by searching 'Calculating Wood Movement'.

I have not found any reference materials recommending a maximum width for this type of joint, but I would certainly recommend keeping it below 3" (although paneled doors will often have wider pieces in their frame). Once the joint width is is beyond this size, I would recommend other joint options such as breadboard end pieces etc. where wood movement is better accommodated.

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    The bottom rail of a traditional door is in the vicinity of 8 inches. Joinery techniques vary, but a fairly common production technique is to use dowels. The trick here is that the dowels aren't 8" apart; they're more like 4 or 5. Alternatives for wide joints where you expect movement are double tenons with a bit of room for movement (preferably haunched) or as noted, breadboards. – Aloysius Defenestrate Sep 18 '20 at 1:01
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    Oh, and since doors are frequently painted or at least varnished, that slows the uptake of moisture seasonally, so expansion/contraction are slower and perhaps less pronounced. – Aloysius Defenestrate Sep 18 '20 at 1:03
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    Even so, you'll notice that traditional doors, especially exterior doors, will eventually tear themselves apart right where the rails and stiles meet. That's case in my home, anyway. – jdv Sep 18 '20 at 2:26
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Why, solid wood frame-and-panel doors do split apart, especially big exterior doors or gates. Usually a stile splits around drawbore pins or there's a crack in the middle of a rail. You don't see it very often though because of climate controlled houses, new materials and sealants, because some cracks are painted over, etc. Also an experienced joiner prefers stable woods and assembles a door in such a way that wide rails can still move a little, very similar to how a wide panel is bread boarded.

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