Frame and panel door construction has changed little in the last 1,000 years. Only during the last 100 years or so have there been changes, mainly with the tools used and the quality of glues. The basic principles of good construction are the same.
Solid wood is in a constant state of movement no matter what type of finish is used. Movement is mainly along the growth rings. There is also movement perpendicular to the growth rings, about one-quarter as much, and almost no movement lengthwise. This movement is compounded when exposed to the outside, such as in exterior doors. For this reason, quartersawn or vertical-grain wood is far superior to plain-sawn wood.
Because of this movement and the natural oxidation process of everything in nature, the joints are working themselves loose over time. The larger the face-grain gluing surfaces, the stronger the joint. End grain (rails) to edge grain (stiles) has almost no strength over time. Dowels have very little face grain and shouldn't be used in exterior door applications. The best joinery is the age-old concealed mortise and tenon. The more exposure the door has, the longer and wider the mortise and tenon should be. To maximize strength, the thickness of the tenon should be less than one-third the thickness of the door.
Another joinery technique used in the frame of doors is a spline. The spline is usually Baltic birch plywood or solid wood - again a little less than one-third the thickness of the door. A mortise is cut in the end grain of the rail and edge of the stile. The spline joint, along with the mortise and tenon, is strongest when concealed, helping to slow the oxidation process by less exposure, starting and stopping the mortises at least 1/2" from the edges. Like the mortise and tenon, the more exposure the door will have, the longer and wider the splines should be.
For a 1-3/4" thick by 5" wide stile and a 1-3/4" by 8" bottom rail in a low-exposure situation, the tenon should be 1/2" thick by 7-1/4" wide by 2-1/2" long. Spline joinery should be 1/2" thick by 7" wide (1/2" less than both edges) by 5" long (mortise 2-1/2" into both stile and rail). In a heavy exposure, everything is the same except that the length of the tenon should be 4" and the spline 8". Also, the depth of the mortise should be 1/8" longer than the tenons or splines for glue.
Don't fit the joints so tight that they have to be hammered together. They should easily slide together with hand pressure. If the joints are too tight, the glue is scraped off during assembly; what is left is absorbed into the surfaces leaving a weak joint.
Cope and stick joinery is used exclusively in production shops. In this type of joinery, everything mentioned above is exactly the same. The only difference is the tooling and technique used. For mortise and tenon joinery, the tenon is cut at the same time the cope is cut. The stick detail is run first and then the mortise is cut. For spline joinery the cope is run first, then the stick is run, and finally mortises for the splines are cut, preferably concealed for added strength.
Making the panels for the joinery is completely different, although the wood's movement still needs to be considered. Again, dowels aren't a good choice in exterior circumstances. Biscuit joinery is a slight improvement, but won't last for a long period of time. Splines in most panels are not satisfactory because the space for glue at the bottom of the groove cut for the spline weakens the full length of the panel and is not recommended.
The best overall performance has been the reversible glue joint. This joint is interlocked and the glue surface is increased by close to 50 percent. Remember to leave space for movement when cutting panels to size to fit into the frame. Plan at least 1/16" per 12" of width. Knowing the relative humidity and equilibrium moisture content (the moisture content of wood equalizes at a specific relative humidity) of the area where the door will be installed is helpful to prevent problems.
The equilibrium moisture content of wood in Southern California is 12 percent at the coast, 9 percent inland and 5 to 6 percent in the desert. If the wood is 9 percent when you receive it and a door is being built for the desert, the panel should be a tight fit. But that same wood at 9 percent, with a door heading for the coast, would use the 1/16"-space-per-12"-of-width rule for expansion. In locations where the outside conditions are very dry during the winter and have a high humidity level during the summer, the high humidity parameters are used for the space size. To make sure your moldings are wide enough for shrinkage during the winter, you should figure 1/16" shrinkage per 12" width. Without this planning, the panels could become too loose or expand and break the joinery of the frame.
Also, when gluing the panel into the frame, seal the end grain and make sure glue doesn't get onto the corners of the panels, which would lock the panels in place, not allowing movement and causing the panels to crack. The panels need to be able to move. If you can, it's preferable to pre-finish the panels prior to installing in the frame. This will prevent an unfinished line from appearing after shrinkage.
The easiest exterior glues to use are the water-clean-up aliphatic resin glues, available rated for exterior use. These glues work very well for most domestic medium-hard hardwoods, if the open time is adequate. For many of the very hard woods, and especially the exotic woods, a two-part urea formaldehyde resin glue or exterior epoxy is better suited.
If exterior epoxy is used, mix the two parts as directed, apply to both surfaces, allow to set 10 minutes, then mix in an extender, such as colloidal silica, to thicken the glue and reapply to all surfaces. Apply enough pressure to bring the surfaces together, but don't overtighten the clamps. Urea formaldehyde has extenders already added. The extenders keep the glue from becoming too thin, which will weaken the joint.
Frame and panel doors are commonly used in most situations in new homes and remodels. They are simple to construct yet can be built to last a lifetime. By using the correct manufacturing methods along with the right choice of moisture content of the wood and the proper glues for the job, your project can be completed with little or no problems, guaranteeing a satisfied - and possibly a return - customer.
David Frisk has been involved with designing and building furniture, wood carvings, sculpture, glass art and metal projects since he opened his Encinitas, Calif., shop 40 years ago.
This article originally appeared in the December 2009 issue.