When we meet with a customer to start a door refinishing project we always review everything in detail. This includes taking a close look at the door, hardware, frame, weatherseal and so on. For wood doors it’s not at all uncommon for some of the veneer to have begun to peel away from the core, especially at the very bottom. When this is pointed out to the homeowner the typical response is something very close to, “Oh no, my door is solid wood, there’s no veneer.” The sentiment seems to convey that a door with veneer is inferior and that theirs is a single chunk of mahogany, walnut, cherry, or whatever. We then go on to explain that their door is very high quality, it’s well constructed and that nearly all doors have veneer. This usually sets the homeowner at ease. It got us thinking, though, about what the average homeowners understands about how a door is constructed. Since Door Renew is in the door restoration business we thought we’d give a short primer on door construction, a topic of, we’re sure, immense interest for most Americans.

First of all, a wood door is not a solid slab of oak, or mahogany, etc. Just think for a moment of what would happen if it were. Even kiln-dried wood contains moisture and will continue to dry over time. Wood is absorbent. When humidity is high the wood will absorb some of the moisture, causing the door to swell. When the humidity drops this moisture is expelled. If water hits the door in the form of rain, sleet, snow or, heaven forbid, a sprinkler head that was hit by a mower and is now pointed directly at the door (yeah, we’ve seen that) the amount of moisture absorbed can be even greater. Temperature fluctuations, often in conjunction with humidity levels, cause the door to expand in the heat of summer and contract in the cold, dry winter months. All this is on the exterior surface of the door. What’s happening on the inside? Nuthin’. There’s no rain or snow hitting the inside of the door. The temperature fluctuation is very minor, regardless of the season. Due to air conditioning even the humidity level is relatively constant. So, on the exterior surface the door is experiencing the full assault of Mother Nature’s ravages. On the interior side, a mere 1.75” away, the door is in a constant, steady state. With such extremes a solid hunk of wood would quickly twist, bend, crack, bow and warp. Within a very short time it wouldn’t even fit into the door frame.

Yikes, that’s somewhat alarming, isn’t it? That’s why doors aren’t made that way. Let’s take a quick look at basic door construction. Here’s a graphic of the major components of a pretty common wood door.

The rails are the horizontal pieces, the stiles run vertically and panels are the inset pieces. What we don’t see is what’s under the skin of the door. This diagram will help.

This give a great view of how things work together to keep a door straight and prevent warping & twisting. First of all, notice the ‘hardwood core.’ A mahogany door is 100% wood, but it’s probably not 100% mahogany. Mahogany is expensive, and there’s less of it than there was in years past. The core of our mahogany door is a different, less expensive wood. The diagram says it’s a ‘hardwood’ core but on some/many doors it could be a softwood. Notice how the core is not a single slab of wood. It’s made up of many strips of wood, like a laminate. This further mitigates twisting and warping. The ‘Thick Hardwood Face’ is the veneer. This is what we see when we look at the door. In our example this is the mahogany. In the diagram it looks pretty thick. It isn’t. Veneers are usually between 10 – 22 mils thick. A business card is around 13 mils. Of course, veneer can be thicker, but that would make the door more expensive.

Here’s a more detailed diagram that portrays much of what we just discussed.

Let’s shift our attention to the panels. In the diagram it says Solid Hardwood Panel. It doesn’t mention a core or face. That’s because panels are often a solid chunk of the hardwood. They’re thinner than the door, especially if the edges are beveled. There’s just not enough thickness to have a core and then a veneer. Having said that, we’ve seen panels where the face, not the contoured beveled edge, is a veneer. We’ve also seen 2-part panels. That is, there’s one panel facing toward the interior and a separate one on the exterior. Next, notice the Expansion Spacer between the edge of the panel and the core. As the name implies, this allows room for the wood to expand. If it were tight against the core the panel, being the weakest member, would bend & crack when the wood expands. Panels are usually not nailed or glued into the rest of the door. This allows them to ‘float’ as the door expands and contracts as dictated by temperature & humidity. If you have a panel door you might be able to move the panels ever so slightly during cool and dry periods. This is ok. Finally, if you have a panel door, whether it’s painted or stained/varnished, look at the corner where the panel enters the rail or stile. Chances are good that the paint or varnish is cracked in that corner. This is an unavoidable consequence of expansion/contraction. When your door is refinished or repainted such cracks will recur.

Fiberglass doors are constructed differently as the diagram on manufacturer’s this site shows.

The fiberglass is a veneer, of sorts, but it’s usually put atop a foam or polyurethane core. This reduces weight considerably and enhances the insulative qualities. The bottom rail, the lock stile and the hinge stile are wood, or a wood composite. This is because hardware, hinges and weatherseal will be attached to these edges and need something solid for the screws. Ok, we can’t avoid a couple editorial comments on fiberglass doors. They’re wonderful, we love ‘em and we refinish a bunch of them every year. Many homeowners chose a fiberglass door because they thought it would be maintenance free. Well, the fiberglass portion certainly is maintenance free. This part is never going to warp, crack or twist. The finish that sits atop the fiberglass is NOT maintenance free. In fact, it has about the same maintenance cycle as a wood door since it’s exposed to the same ravages of Mother Nature. The last comment involves the trim around windows, and sometimes panels. We often find that the trim is made of plastic or vinyl, not fiberglass. The plastic/vinyl breaks down over time, especially if there’s a storm door that, through the greenhouse effects, creates very high temperatures between the two doors. So, if you’re upset that your ‘maintenance free’ fiberglass door needs to be refinished in the first place, and then are told that the trim has deteriorated please don’t blame us!

As mentioned earlier, this is just a primer. To learn more just Google ‘wood door components,’ ‘diagram of a wood door,’ or something similar. Regardless of how a door is made, or if it’s wood or fiberglass, the appearance is what concerns most homeowners. Check out some of these before-and-after projects that Door Renew handled. Here are a couple links we found particularly helpful.