Materials Options for Exterior Gingerbread Trim

Here's an extended discussion of various natural wood, synthetic, and composite materials and how they hold up in outdoor exposures. November 24, 2007

I have a customer that wants some Victorian style fret work redone on their house. I am wondering what would be the best material to use for this that would last. What they have now was either pine or poplar, and didn't last 5 years and was painted twice. I thought about either cedar or that waterproof MDF (Extira)? Has anyone used that before?

Forum Responses
(Architectural Woodworking Forum)
From contributor S:
I haven't used the Extira yet, but I am about to. I have 20 sheets on order to do some work on the outside of the house we are building. I had a sample that I stuck in a bucket of water for a couple weeks, and it didn't soak up any water, really. I think it should work fine. As far as cedar goes, I would go with yellow cedar - it's not quite as soft, and machines pretty well.

From contributor B:
Spanish cedar with Benjamin Moore penetrating primer 100. Wear a dust mask all the time. Machining it is nasty.

From contributor N:
Another vote for Spanish cedar. Extira is very heavy and fibrous, as you'd expect for MDF. For fine detail work, I wouldn't think it's a great choice.

From contributor R:
Save a tree and go with the Extira. I bet it lasts longer and gives a better paint job.

From contributor J:
Try Azek PVC. It comes in sheets also. I use it all the time.

From contributor G:
Gingerbread is our specialty. We stay away from real wood for exterior. It isn't like the wood our grandfathers used. Extira is a fantastic product for exterior trim. We had one architect run it through her dishwasher three times as a test. Be careful - the dust is slippery.

Azek is the expensive choice for material. Sherwin Williams has a new paint formulation especially for Azek (PVC - stinks like dog poo when you cut it). They are saying a 25 year warranty would be a great selling point. How will you be cutting it?

From the original questioner:
Contributor G, is the Extira what you use? The pieces I am redoing go in the gables at the end of the roof. So they are A shaped to match the roof pitch and have scroll work along with dowels that have about a 1 1/2" -2" round ball on them. The balls are something else I am trying to decide what to use.

From contributor G:
We use Extira for all of our paint grade exterior millwork.

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From Gary Katz, forum technical advisor:
Extira is great stuff. Fuzzes a little and has to be sanded lightly/primed/sanded then painted, but boy, it holds up outside really well.

From contributor G:
There is a huge difference between Extira and MDF.

Medium Density Fiberboard - MDF is wood dust and urea formaldehyde glue mixed and spread into a mat about 12" thick, pressed in between two heated plates to cure the glue. If exposed to water, this glue and the structure of MDF will deteriorate.

Moisture Restistant Medium Density Fiberboard - MRMDF (one product name is Medex). Wood dust and exterior grade of glue, original formula rated it exterior. Made the same way as MDF. The heated platens stay in contact with the material longer and that is why you will notice the hard outer shell. Failure in the field and formulation changes make it no longer rated exterior. It will hold up to moisture exposure a lot better than MDF.

Extira. You could call it an MDF as most people do. The problem with that is the non-exterior properties associated with that name. The mat is all hardwood chips\dust\fiber. The binder (glue) is phenolic resin. Zinc borate is added, and wee-beasties don't eat it. When the mat is pressed it is done with the sides contained to keep it from smushing out. Steam is injected into the mat at the time of press and this sets the resin. Phenolic resin will not break down in water. As with any product containing wood, you cannot have it in direct contact with the ground or water.

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From contributor D:
Cut off a piece of Extira (or any MRMDF) and measure it in 3 dimensions with calipers and note the dimensions. Put it into a bucket of water for 48 hours, weight it if you like. Then remove it and measure it again. Ever since MDF first arrived on the scene, manufacturers have been trying hard to expand its market, and calling it "Exterior Grade" expands it greatly.

The glue used in MRMDF is not a type 1 waterproof glue - the only type of glue that should be used in gingerbread, cupolas exposed to the weather, boats, etc. Moisture resistant is a worthless term. Just because you can doesn't mean you should. Experience and close study (not marketing materials) will tell you that there is no substitute for the real thing. "Wood products" that can't be in contact with ground or water, can't be outside.

As to the original poster's request: Make a nice 5 ply plywood in your shop out of Spanish cedar or equal. Use West System epoxy. Cut out your gingerbread as needed. The cross grain wood in the ply assembly will keep your parts together for a real long time. The real thing, improved.

From contributor G:
Contributor D, excellent process and if the project demands and the money is there, that is a very viable option.

I do hope you notice that I described 3 different materials. As your suggested test implies, there would be some dimension and weight change in Extira. I would agree with you. Would you be willing to do the same test with your Spanish ply at the same time? Also, would you recommend any trim on the exterior of a house be in direct contact with the ground or water?

These are not argumentative questions. All input with a positive attitude is appreciated in our shop, as we are professionals.

From contributor D:
Recommendations as to ground/water contact and reality are two different things. Water will sit on a flat 1/8" wide ledge and start its work of harboring fungi, then... A simple 10 degrees is enough to remedy this, but how often do we not see this? Is it due to neglect, ignorance, design or other considerations? Add in snowmelt, dirt and dust accumulation, and you have the same conditions as a mailbox post, but on the side of a house. Wood species and glue types are hardly a consideration (outside of this forum) to many/most manufacturers. The MRMDF has no inherent resistance to Nature's onslaught, as do Spanish cedar, Honduras mahogany, WR cedar, OG redwood, ipe`, and several other woods.

Sure, the Spanish cedar will gain dimension and weight in the same test. Yes, I've done the same test with a 5 ply sample of Honduras mahogany. The wood rotted after 14 years on a splashblock and ground contact, but the glue was still pretty good. Yes, a woodworker can examine closely, find water or ground contact, stand up and announce "Ah ha! Warranty void!" with a triumphant turn on the heels and an offer to replace, at full cost. But what does this serve? Far better to use our knowledge and resources to produce something better, something of lasting - increasing - value.

I'm a Neanderthal that tends to believe the original can be improved upon but rarely, if ever, replaced. So I find solutions that still satisfy the original intent - wood. Some historical commission work will also demand a replacement identical to the original. How about MRMDF deck knees in the Constitution instead of the live oak trunk/limb originals? Start down that path, and we'll have a plastic and sawdust boat leaking badly in no time. The fake stuff is fake stuff, no matter how you cut it.

Today, the craftspeople working in shops all over the world have an unprecedented array of species, adhesives, coatings, joinery, machinery, techniques, and technical evaluation tools at hand to help them earn their daily bread. It is a shame that all these resources go unused in favor of a bit of gee-whiz techno-babble that will be set aside for the next big thing, sooner rather than later.

From contributor G:
Did I say replace everything that was wood with something else? The major point of not having contact with dirt or water is wicking (rot) and a path for termites. If one of our customers wants real wood, they pay the price, in dollars. It costs more to handle and to work. No problem. If you are going to paint it, why not use a composite that will last and fit the budget?

From Gary Katz, forum technical advisor:
Before using Miratec and Extira, I cut a 1 in. x 6 in. net piece of Extira, unprimed, and put it in a bucket of water under a brass weight for two weeks. When I removed it, it still measured 1 in. x 6 in. The ends were moldy looking, but hard. When I shaved a 1/16 in. from one end, I was into raw, undamaged material. We used Miratec and Extira on a Victorian ten years ago and the paint is still good. Try that with any type of wood. The clients wanted a low-maintenance home and we tried our best to provide one. I think we succeeded.