Wood vs metal
Choosing between wood and metal frameby RV Forum member MrOffshore
There are a lot of misconceptions when you compare wood vs. aluminum in the cage construction of an RV. First understand, that almost all RV's have a steel main frame, the part of the frame that is under the floor, it usually consists of two I-beams that run the length of the trailer, inside of the wheels with crossmembers that are welded in between the two I-beams and steel "outriggers" extend from the I-beams to the full width of the RV. Typically there is then a "cage" frame built up from this foundation...it is typically either wood, aluminum or a combination of both. So from this point you are considering the frame to be the following components, each of which can be wood or aluminum:
Floor - which sits above the steel main frame. The steel main frame crossmembers are not on 16" centers so a sub-floor needs be built on top of this for proper support.
Sidewalls - the sidewalls can be built similar to a house and on 16" centers. In this process the exterior walls can be either mechanically attached aluminum skin or a fiberglass that is glued to the sidewall frame and considered "hung-glass". Some manufacturers utilize a laminated wall construction in which case the aluminum studs are not typically on 16" centers, instead they are placed as needed for proper support and strength. The laminated wall gains much of its strength through the lamination process which is the bonding of the exterior walls to sidewall structure, sandwiching the insulation in the middle (typically expanded polystyrene) and then laminating the interior wallboard.
Front & Rear Walls - the frame construction of the front and rear walls can be either wood or aluminum. The exterior can be aluminum, fiberglass or a molded fiberglass cap.
Roof - The frame can be either wood or aluminum. Typically it is trussed which give the exterior roof a slightly curved profile which helps to drain rain or shed snow. The trussed roof is also typically stronger than a flat roof design. The exterior of the roof is usually either rubber or TPO. Rubber roof membranes are a common more affordable roof material, but are prone to UV damage, ozone damage and tend to lend to the black streaks so commonly found on the sidewall of trailers. The TPO roof is a UV and ozone resistant material, typically thicker than rubber and does as easily create the black streaking. TPO is also stain and mildew resistant.
Now...which is better, wood or aluminum? The thought of wood usually scares off some people, but when built properly is really not a bad product to use, it's very forgiving by nature, it has the ability to flex as needed and return to its original configuration continually over time. The construction of the wood frame is what you need to look at. Are they using staples or screws or both? The only way to really know if the wood frame is built well is to see it being built or take the word of someone who has seen the trailer you are interested in being built.
Wood is a product that will rot if introduced to water or moisture. If the trailer was built right it shouldn't allow water or moisture to penetrate the areas where the wood is located. You should be able to inspect a trailers build process well enough to see if it is prone to leaks...but remember, even trailers that leak aren't always easy to find the point of entry for water, so rather than simply looking for good caulk jobs, you should be looking at the manner in which the trailer was built...what did the RV manufacturer do to keep water from penetrating a wall or roof? Ask those questions and make sure you're getting intelligent answers, not just an RV salesmen making up what he thinks you want to hear.
Aluminum is a welded frame, some people fear the welds will break over time. I think this is a mute point as long as the welders know what they're doing and apply more than a "tack" weld when constructing the frame. Most people believe aluminum to be lighter than wood and it is, but most manufacturers don't take advantage of the lightweight characteristics of aluminum...in the end the only thing that matters is the coach you are interested in and its dry weight....you might be surprised that wood and aluminum constructed trailers aren't as far from each other in weight as you might have guessed.
Take two trailers of equal size, features and benefits...if one of them is significantly lighter than the other you can plan on paying quite a bit more for the lightweight trailer, if it's built right. Why? Because it is probably taking advantage of higher cost composite materials. If it isn't more money you can figure the weight savings was possible because it was "cheapened" up by taking out the materials needed to deliver a quality product that will last for more than just a few years.
So, I'm not making this easy...and that's because it's not a cut and dry answer. There are merits to both aluminum and wood...I can tell you after having seen many different RV's being made, I wouldn't mind owning either, as long as the one I chose was built right from the start. I think the best place to get answers is on forums like this one...you can speak with the owners of the RV you're interested in purchasing and find out what problems they've experienced and more importantly, how their problems were handled. I can promise you one thing...there aren't any RV's out there that are 100% trouble-free...you're going to need repairs of some kind at some time...will your dealer and more importantly, will your manufacturer stand behind your RV after you've already purchased it.