Is Poplar Good for Exterior Door Cores?
From the original questioner:
What would be a good core?
From contributor L:
For an exterior door you will need something that will be able to stand up to the weather, especially humidity. Just because you have a skin of another species and a finish on it doesn't mean it will withstand the rigors of exterior life. I think the pine is a much better choice than the poplar. Of course you will need to consider cost in the equation. I am not qualified to tell you what would be a better core, for a respectable price. But I do know that poplar is not the way to go.
From contributor R:
You would be better off simply making the door out of solid stock rather than using inferior materials for the core. Careful selection, milling, joinery and assembly methods will yield a high quality product that will last a couple of lifetimes and with less labor.
From contributor M:
I ran a pair of doors just as you described a couple of months ago. I took plain sawn white oak and used that for my staves with the quatersawn for my skins. Perfect balance and not really that much more in materials if you look at the big picture. I still sealed the ends with West epoxy.
From contributor T:
Excuse me gentlemen, but you're giving yellow poplar a bad rap here. I started building stave core doors with 5/16 skins in 1984. Built every one of them with 4/4 laminated yellow poplar cores, unless architect specs called for other specie, and used National Casein adhesives. The exposed edge of staves always was of skin specie. I have at least 1500 to 1700 doors out there. At least 150 to 175 of them are exterior doors, 2-1/4 thick, built the same way. Not a single one of these doors has been reported as having a problem with cores or skins. Every door is in northern Michigan. Summer sun and heat to 95 degrees plus, winter snow and rain to 20 below, lakeshore homes, in the woods, along swamps and rivers and out on open fields; I have never had a problem with these doors because of the core or skin. I can, however, confess that a few doors, less than 5 or 6, have had to have a service call because of weather strip and/or hardware. They do have to have same specie wood/skins on both sides, however.
You may be skeptical of above claims, but those are the facts. I sealed every door part with Nelsonite before machining mortise and tenon and every panel was dipped in same before assembly. Early on as I was designing these doors, I sent door samples in 1984 to testing lab and results came back... integrity good until wood destruction.
So I based my line of custom doors on this method of construction and maintained this until I retired two years ago from the heavier millwork. There is no reason why you cannot use yellow poplar in stave core construction. I've read where they even build flat bottom fishing boats out of yellow poplar in the southeast US.
From contributor V:
If you have to use oak, I would use solid oak. If you want to use a different core material, consider Spanish cedar. I have a friend that makes a lot of doors and he uses Spanish cedar almost exclusively. I believe a second wood of choice would be cypress.
From contributor V:
I will counter the recommendation for poplar with a solid recommendation to steer far away from poplar. Despite one individual's experience, I have seen many examples of failure of poplar in exterior use in the last 35 years. Door shops, window shops, and many individual projects have all failed. Even a few threats of lawsuits. Several of the shops started with poplar on the recommendation of a state university. This recommendation appears to have been based more on historical observation than current science.
When examining 100 to 150 year old structures in this area, it is not unusual to see a lot of poplar in exterior use - much of it with no protection. The dark green heartwood is very dense, part of the virgin forest, and has a very high ring density. Current cuttings of poplar are 2nd or 3rd growth, and are grown a bit more like corn. The ring density is not there, and there is little heartwood. This makes for a completely different wood than what was used 100 years ago.
While cladding the wood with a better exterior species is a workable strategy for using poplar in a demanding environment, it is still a distant choice for cores due to the attraction of rot and the wide range of movement. There are better woods for this use, and working with solids is an even better strategy than gluing up lots of parts.
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