Replies

1. 
   hammer1 | Dec 06, 2012 07:53am | #1

   The issue isn't the thickness of the stock you use, it's the characteristics of the particular pieces of stock. You have to understand the individual pieces of wood and if they might have a propensity to warp. You look at end grain to see where the piece was cut from the log and you look along the edge and face to see changes in grain. By and large, poplar doesn't have a lot of swirling, changing grain patterns like many other species.

   Many doors today are made using stave core construction. There are several reasons for this. The main one is to provide stability. When manufacturing doors by the thousands, finding nice straight grained stock that is less likely to warp isn't easy. It would mean rejecting a very high percentage of the lumber. If you have enough quality stock to use, then use it. If you use pieces that show grain changes that are indicative of a twisted tree trunk, out growth of branches, changes in growth direction, then the possibility that these pieces will warp is much greater. Door manufacturers have to stand behind their products and if they have a percentage of doors that twist or warp, they suffer losses.

   Most manufacturered doors come with finishing instructions. This is an out growth of the large percentage of doors that do warp and customers want to return. These instructions require that the tops and bottoms of the doors get finished. Lumber is the stalk of a plant. The job of that stalk was to transfer moisture and nutrients to the plant. The celular structure of the stalk is very good at doing it's job. You can chop it down, slice it up and dry it out but those cells will still absorb moisture.  The cells are particularly absorbent on the end grain. Sealing the tops and bottoms helps reduce the ability of those cells to absorb moisture out of the air. It doesn't stop it but it does reduce it.

   Wood moves because of moisture absorbsion and loss. It can't be completely stopped. This is why museums have sophsticated systems to monitor and control humidity. Most of us can't do that in our homes. Cabinet and furniture makers use FAS kiln dried lumber at about 8% moisture content and handle and store our lumber carefully to maintain this content as much as possible. This gives us the best chance that our products will stay the way we built them and not have to deal with call backs or other issues.

   If you use air dried lumber, or lumber with higher moisture content, it's more likely to move, especially indoors, particularly when the heat goes on. Since each individual board will move depending on it's particular characteristics, face gluing two boards together can cause several issues. One can move more than the other, breaking glue lines and/ or one can make the other move along with it. The longer and wider the boards, the more potential for problems.

   The people that are telling you to glue boards together to get the thickness probably haven't built many interior passage doors. Ask to see them, it will shut them up quickly. That's not to say there can be some success with that method but the odds are against it. If you are milling your own door stock, flattening on the jointer before planing to thickness, taking equal amounts off both faces and not ripping narrower boards from wide boards, you should have pretty nice, stable pieces to make your doors from. After milling boards will show their propensity for movement right away. If the stock you have stays nice and straight and flat, it probably will remain that way as long as you take care of it, keep in off the concrete floor, out of the rain, etc. Of course, you have to build them flat, straight and square. Get the doors finished as soon as possible and don't forget the ends. The longer you leave them unfinished, the more chance of problems.

2. 
   c1802362 | Dec 06, 2012 10:09pm | #2

   thanks for confirming my suspicions

   everything you said is the way I've been building things for the last 25+ years. I flatten and thickness my stock equally on both sides, all my joints are tight, flat, & square, and I finish my projects within a few weeks.

   And, I haven't had any problems...

   thanks

   Art