Glass Panels in Mahogany Exterior Doors

Wood movement and weathering are the chief concerns. December 14, 2009

I have a customer that's set on wood front doors with a vertical glass window offset nearer the center of each door, approximately 8" wide by 48" high, with obscured glass. The home has a southern exposure with a bit of an overhanging roof over the doors (2 foot eave, maybe).

I don't have time to make them, but have contacted a millwork shop that will do them. His construction method is as follows: Solid rails/stiles and mahogany ply center panel (two pieces of ply, 1/2" thick glued together to get 1" thick panel). I asked him if the center panel would float and he said no, it would be glued in.

What do you think of this construction method? I would be finishing/installing but I would like to hear other opinions on this construction method. I've already informed the homeowner regarding the maintenance/refinishing that will be needed in the future to maintain the look of the doors.

Forum Responses
(Architectural Woodworking Forum)
From contributor M:
Get a second quote. Based on the questions you asked, I think you know the right answers.

From contributor T:
So, the panel is 1" thick. How thick is the door? If it's the typical 1.75" thick, it seems like the panel is too thick. Also, I don't know anyone that would glue in the panel and not have problems later on. Especially on an exterior door.

From contributor C:

I have seen all sorts of different glued up cores for panels and door stiles and rails up to 3" thick by 10 ft tall. It seems that for true longevity and ultimate stability in a less than ideal compass setting (undeniably the single most important factor to be considered), staved and multiple ply cores with good thick veneered surfaces and edges hold up the best. On an engineered door with glued up cores, it is acceptable for the panel to be glued in solid, especially on a thick assembly.

From the original questioner:
Thanks for the replies. I'm not sure how thick the veneer is on the center panel, but I'm assuming it's probably .020-.025". Door is std 1-3/4" thick. Spoke with customer today and he may be changing his mind on the design, to a more traditional multi-panel design.

As an option, what is the consensus on fiberglass doors, pre-finished from the factory? How does their finish compare longevity-wise to real wood with premium finish?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
One more comment: Make sure that the glass is floating or has space balls so the glass can expand. The wood and glass move differently (wood moves only with MC changes; glass moves with temperature changes) so there must be room to accommodate this difference.

I saw some beautiful doors that I think were similar to your description made of solid sapele. The company was close to Charleston, SC.

From contributor J:
Gene, as a master woodworker that is also a juried glass artist and had a business for 8 years doing, among other things, custom entryways, I wanted to mention that glass really does not expand in any real measure within the temperature ranges of real life. Yes, there is a measurable expansion, but unless one is actually jamming the glass into the lite opening, it is a non-issue. And if you do fit it that tight (not recommended) it is invariably the wood that will cause breakage due to movement. The COE (coefficient of expansion) of glass is several orders of magnitude less than wood of any species. It is physically impossible for the glass to expand enough to cause any problems.

As long as the glass is cut at least 1/8" less in each direction (1/16" on each edge) it really does not matter if you use space balls or anything else. It does not even matter if you center it, to be honest. And 1/8" is actually overkill, but accommodates any kind of possible wood movement.

A few more notes of interest: Wood moves predominantly with moisture changes. Everything expands or contracts with temperature. No exceptions. And keep in mind that the moisture in wood is also in a symbiotic relationship with temperature. As the moisture heats it also expands - that's why we can sweat out a dent in wood. The moisture alone won't sweat it out; you need to apply heat too.

As to the door construction: in my opinion, you should always construct with an odd number of plies for even distribution of movement.

Panel thickness: Most of the 1 3/4" doors I have made with raised panels have 1 1/2" panels. Flat panels are usually 3/4". If you are using a veneer panel, go through the extra trouble of having (or doing) an Extera (waterproof MDF) panel. Plywood just does not cut it for exterior doors. Ever.

The finish: The absolute best finish for a door with a bad exposure is a teak oil. Tung is a little too thin and mahogany oil will work if you can find it. It takes some serious attention to your customer due to the fact that you need to re-coat at least every 6 months for a couple of years, but I look at this like a chance to solicit new work (and if they see you as so dedicated to customer service that you come out every 6 months to coat their door, they will have more work for you, trust me), and a way to document the process and photo the door as it ages. It looks so much richer after 2 years. This finish will not peel, crack or shed and once you have gotten through the first few years it can be reapplied as needed (usually every two years).

I always give my customers 2 years on the house and then offer to re-coat for my cost thereafter. It has gotten me more business than I can relate, and some customers discover it's not that hard to do, and actually enjoy caring for their doors!