I've been asked to quote this ipe U-shaped exterior rail cap that will be used somewhere in New England. You can see from the supplied dimensions that it is 12" wide with a 3" overall thickness.
Close examination will show that the architect has designed this to wrap tightly around a pressure treated mounting block. There will be screws inserted from the edges to fasten the cap to the blocking. Plugs will be used to hide the screw holes.
I have some major concerns with this design. First is the wood movement issue. I suspect the pressure treated blocking will shrink as it dries out and move significantly. If the ipe rail does not match this initial shrinkage, then the screws would either pull out of the P/T or cup the rail.
Also with seasonal wood movement, should the ipe rail move at a different rate than the blocking, I'm concerned about complete failure of the rail. Cupping, warping, splitting etc.
Another concern would be the plug holes. Should the ipe shrink more than the P/T blocking with long term seasonal changes, the plugs could be pushed out of the holes by the screw heads.
I haven't done much work with ipe so am far from an expert here. Do others see the same concerns I see? There is a lot of footage of this rail both as straights and radius components.
(Architectural Woodworking Forum)
From contributor B:
I would have the same reservations. Having not worked with ipe there may be things that I am unaware of. But I believe a better detail/look would be to have the top floating between the two sides in rebates. Let's say the top would be 8" vs 12" which would have a 1/4" reveal above the 2" thick x 2-3/4" tall sides fastened securely to the mounting block. The top would have a groove on each side that would fit a tongue of the sides sized appropriately for allowing full movement of the top.
The mounting block, in my opinion, should be something other than pressure treated or treated wood that has been thoroughly dried to workshop status. Having the PT block fit a tight channel is asking for trouble, in my opinion. Even for your radius components I think separating the rail into 3 pieces would also work better for both milling and site installation.
I would have the same reservations as I don't think ipe will move nearly as much as the PT will. When the PT shrinks I think it will try to pull the ipe into itself. What will the end result be? I wouldn't want to find out.
I guess I would suggest an alternate construction detail based on your experience, and if the architect is unwilling to change, make sure you cover your butt! You know how things roll downhill...
12" wide is bad enough without thinning up the middle and laying it in full contact with a pond dry member. If something dry were used that would move equally with the ipe, fastening through the sides like that could work.
In your case, you have two different materials that will move and behave differently. Eventually the cycles will loosen the attached interface screws. If the ipe shrinks more than the cap, it will split.
I would lean toward splitting the cap into two pieces and using barrel nuts and machine screws to attach it, leaving some room for the material to float relative to each other. Additional work, yes. I have found than most designers have no idea about the perils of ipe.
The job now becomes how to come up with alternates that will satisfy the design imperative (sitting plants on the rail, or people) and deal with the problems. My first thought is to suggest an assembly of strips with cross links that will provide the width, be relatively flat, and each move in its own little way, while draining water through it. And dump that PT member - the ipe will be structure enough.
Comment from contributor E:
I totally agree with contributor G. The architect has no clue about joinery in general and ipe in particular. Tell him you will build it as drawn for x price and he signs all waivers or build it your way for your price and you will stand behind your work. By the way, ipe dust is a huge irritant - we haven't worked it for years for this reason. Thankfully, most of our high end clients will go for FEQ Burma teak.