The winter gaps between the boards on this barn door will close back up when the boards swell in summer's humidity.
We’ve all seen it. Every winter, a few weeks after the heat comes on, the doors in our houses start to shrink. If you have traditional 4-panel or 6-panel doors, the panels show white edges, or even bare wood on both sides where the panel has pulled back from the frame. It happens every year, and it’s all perfectly normal. It means that your frame and panel doors are functioning as designed. They are compensating for the seasonal movement of wood.
I learned about wood movement the hard way. When I was in high school, I was fortunate enough to take four years of woodshop. We learned all about safety, joinery, hand-tools, machine use, and finishing. Amazingly enough, wood movement was never touched on. One of my first projects was a plant table, made out of mahogany. It was glued, screwed, and tattooed, and built so strong it could have held a full-size tree in a cement container. My mom graciously accepted it and parked it directly over a hot air vent. After less than a week, the top cracked almost directly down the center. I asked my shop teacher what had gone wrong, and he replied, “You didn’t let the wood move. Guess you learned your lesson.” Well, not really, since I still didn’t have a clue what I’d done wrong. That was the beginning of an ongoing battle to predict, anticipate, and deal with the seasonal and inevitable back-and-forth journey of solid wood.
Early in my woodworking career, I took a few wood technology courses, which cleared up the nagging mystery. What it boils down to is that wood expands and contracts across the grain (side to side), but barely at all in length. The culprit behind this movement is water. Like a sponge that is thin when dry and puffed up when wet, wood cells contract and expand in a similar manner. To oversimplify, imagine a hardwood board as a tightly compressed bundle of soda straws running lengthwise. When you look at the end of a board, you’re seeing the hollow ends of the soda straws. When the wood is wet, the straws swell such that the board gets thicker and wider. Hence, there’s a lot of expansion and contraction across the width of a board, as millions of cell walls take up and give off moisture. But the board will not get appreciably longer when it’s wet: only about 1/16 of an inch in a 16-foot-long board.
Just to throw a monkey wrench into what seems like a fairly straightforward situation, the wood movement across the width of a board is uneven, depending on how the board was sawn. This disparity is caused by ray cells, which emanate from the center of the tree outward like the rays of the sun. They act as restraints between the other cells and reduce wood movement considerably. A log that is sawn into four quarters through the longitudinal center of the tree will yield four quarter-round timbers. The first boards sawn off each face of the quarters will be called “quarter sawn.” The growth rings will be perpendicular to the wide face of the board, and the grain on the face will appear as parallel lines. The board will not experience much movement side to side, and it will also stay flat.
On the other hand, a flat-sawn board will have growth rings that are more nearly parallel to the wide face of the board, and the grain will appear as wide, overlapping spires, sometimes called cathedral grain. Flat-sawn boards tend to cup, and they do so away from the center of the tree, since the ray cells hold the wood in radially (perpendicular to the center of the tree), but not tangentially (parallel to the center of the tree). The tendency for wood to cup is also why it is virtually impossible to dry a log in the round, without severe cracking, or checking, all the way to the pith (center) of the tree.
To make matters even worse for the woodworker, the wood of each tree species moves a different amount. As a general rule of thumb, the heavier or denser the wood (having more cell wall material), the greater the amount of movement. There are exceptions. Teak, for example, is very dense but moves only a little. Of the native North American woods, the oaks, hickories, beech, and rock maple are the worst offenders, moving about 10 percent or more between green (wet) and dry conditions. Better behaved are walnut, cherry, butternut, and soft maple, which move in the neighborhood of 6–8 percent. White cedar and white pine are the best behaved, moving only 5–6 percent on flat sawn boards. Quarter-sawn white pine is the North American champ for minimal movement at 2.1 percent between green and dry.
For the woodworker, this means that if you ignore wood movement, your furniture will self destruct. If you encased your wood in plastic or kept it in a constant, humidity-controlled environment, movement would not be an issue. That, however, is not how things work in the real world. Going back to my mahogany plant table: I had glued the top to the stretchers, the cross pieces where the legs connect to the table. The top shrank as it dried, and since the stretchers prevented it from moving, it cracked.
Doors In History
That brings us full circle back to the frame and panel doors. Until the Middle Ages, doors were made of one or more boards nailed to perpendicular battens or braces to keep them flat and rigid. Long nails were driven all the way through the boards and battens and clinched over in the back, or “killed,” hence the phrase “dead as a door nail.” A door made of a single unfinished board – let’s say red oak, 14 inches wide – would be 3/8-inch narrower in winter than summer. Since the door would be hinged on one edge, the gap between the door and the door jamb wouldn’t change and the contraction would appear only on the knob edge of the door. That single board would fit tightly in the summer, and have a 3/8-inch gap at the knob edge in the winter.
By modifying the design and using several smaller boards instead of one wide board, the movement could be minimized by leaving small gaps between the boards. Thus, the boards could move independently, much like narrow floor boards that shrink and show gaps in the winter.
Somewhere along the line, probably first with the Egyptians and then popularized during the early Middle Ages, someone had the smart idea of frame and panel construction for doors. The carpenter made a narrow wood frame with the desired outside dimensions and joined the boards at the four corners with mortises and tenons. Grooves were cut along the four inside edges of the frame. By setting a single board panel into those grooves, and not gluing it to the frame, the carpenter allowed the panel to move independently within the grooves of the frame. The overall change in the outside dimensions of the door is thus minimized to the combined width of the two side frame members, no matter how wide the panel. The wider the panel, the deeper the grooves must be cut to allow for the predicted movement.
Further refinement to the frame-and-panel concept increased the number of inside panels, so that today, most doors have four or six panels. The principle is still the same: the panels expand and contract seasonally inside the frame, but with two smaller panels side-by side, the seasonal change of each panel is smaller. When winter comes and you see the exposed edges of the panels, it’s because the panel shrinks as it dries out in indoor heat, and you’re seeing wood that heretofore has been hidden.
I have three tricks that I use in all my door construction to minimize, or even eliminate, this unsightly display. First, I use quarter-sawn wood for the frames to cut frame movement in half. Second, I apply the paint or clear finish to all my panels before putting them in place so that no bare wood is exposed when the panels shrink in winter. Finally, I center the panel in the opening and nail in tiny brads, top and bottom centered, on the back or inside of the frame. This keeps the panel centered and ensures equal movement on each side.
What about painted doors? Ideally, the panels should be painted before they are assembled. But if you are painting or repainting already assembled doors, take special care not to let paint sneak between the frame and the panel, because when the paint dries it will act like glue and cause irregular paint cracks as the panel shrinks. Worse yet, by holding the panel along the edges, it could split it down the middle. Painting requires a steady hand or, lacking that, a thin sheet-metal eraser shield, like those used in drafting. If you plan on repainting your doors, the ideal time is in mid-winter, when the panels are at their smallest. That way, as they expand, the old colors won’t show up next winter.
C.H. Becksvoort is a furniture maker from New Gloucester, Maine.