I recently built 4, 5 piece doors 16" wide by 60" tall with 4 1/2" stiles and rails out of Sapele. I Pre-milled all the pieces and then stickered them for a week before milling them to final dimension. I didn't check but I'm sure they were flat when I built them. When I went to instal them they had all warped so badly I couldn't use them. They are bifold doors mounted with butt hinges so not much room for adjustment. I've never had much trouble with doors warping but I am very careful if they are large and always pre-mill and sticker the stock for a week. I'm concerned that if I build them again the same thing will happen and I'm thinking of building them with a stave core and then edging them and glueing a 1/8" face veneer on. Any thoughts?

If you don't have a full understanding of what went wrong the first time, there is no guarantee that building another set will improve things.

Doors of any size must have joinery that is extremely square and coplanar with all the parts. You can have very nice, perfectly straight parts, solid or stave core, but when put together the door will rock on the bench like a potato chip.

The larger the door, the more effect these problems have, and the more likely they will cause problems. I would review all your joinery and question where the first doors went wrong. If the rails and stiles are twisted and 'warped', then that is the problem - bad wood. If they are still flat ans straight, then it is the joints.

Flatsawn or Quartersawn (ribbon striped)?

It is Quarter Sawn. I cut the doors apart and all the stiles are now twisted. They weren't when I built the doors. I have built many large doors and now and then one moves a bit but I have never had them move like this. My lumber supplier suggested I build them again but cut them out of boards only a little wider than the finished size. We checked the moisture content and it was 6%.

Don't have a reason for you then. I know that flat sawn is notorious for not being stable. Which is why you see it QS so often.

Sounds like luck of the draw and you pulled the short straw.

Andrew,
On thinner doors, less than 1 3/8", instead of stave core you could try 3 ply for your stiles. It can be a hard sell on stain grade doors but , is an excellent way to build stable stiles.

One lesson I keep pounding into my own head is, the only reason wood moves is because of change in moisture content.

Wood moves in response to changes in moisture content. So....that explains it, somehow.

If you know how to make doors, then the joinery is not the problem. If the stiles are twisted once free from the assembly, then that is the problem. But exactly how that happened is the question. Again, until you have some idea as to how that can happen, you could face it all over again if you rebuild. Even stiles made of stave built construction will not be immune if change in MC is the risk.

Andrew, I find that slightly bowed door stiles can normally be used, if carefully positioned, but twisted ones cannot, [unless perhaps slightly twisted if it can be paired to one which mirrors it]. You will probably know most of this, but something following might be informational: Firstly your timber needs to be of consistent moisture content throughout, which is unlikely if you have just purchased the timber, it is likely to be different in the centre than at the outsides, which can result in the wood bowing as soon as a plank is put over an over-hand planer, and then maybe bowing again in the opposite direction [possibly cancelling out the previous effect] when thicknessing the other side, so to avoid this, best let the wood acclimatise in your workshop at least a week, preferably a few weeks for m.c. to even out before planing it. Don't leave planks on the floor to pick up moisture from the ground. If you are forced to use it straight away try to take off an even amount from both sides, [although that is not possible with bowed planks]. If starting from sawn timber much thicker than the final thickness, as you have done, it is good to plane over-size, leave in stick a few days before finishing to required thickness. When you have planed to thickness, leave it a couple of days, again if you can, in stick or on edge so air accesses both sides, then match tall stiles in such a way that the worst bows happen at the hinged edge, so the centre hinge flattens the stile. Match remaining stiles to each other for grain match if not painted, and so that most importantly, the stiles which meet in the middle of the opening, line up with each other, so bows match, making sure that any bulge is outward, so doors close fully at top and bottom. This way, slightly bowed stiles can be used, but twisted ones must be discarded. Number the stiles on the fronts with pencil so you keep the sequence. The gluing up of the doors is important. I like to use good quality T-Bar cramps which deflect less than sash cramps. If the doors are thin, twist the cramps so that the pressure exerted by the threaded cramp head is exactly in line with the centre of the door stiles. It is important that you look carefully along the front of the cramped up door while the door is in cramp to check it is not being glued up in winding, the door must be completely flat as the glue is drying, other than that caused by bows which you have already made allowance for. When the doors are all glued up, you can make a final check, putting the sets of four doors to make the bifold pairs next to each other on a large bench or the floor to ensure that they meet each other well and that the outer hinged stiles are parallel. The important thing I am trying to say is pair and mirror irregularities to achieve compatability to minimise wastage because a few bowed stiles are inevitable with tall doors.