Large-Dimension Ipe Exterior Woodwork

A woodworker puzzles over some troubling issues with a 12-inch by 3-inch Ipe rail cap detail. July 30, 2009

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.

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Forum Responses
(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.

From contributor H:
I agree with contributor B. The pressure treated lumber will be too wet. The ipe being dry will wick any moisture out of the PT and cause your rail to warp/cup upward. Spaced blocking of ipe itself may be a better option. I would not warrant any of this job due to its exterior use and design.

From contributor J:
I'm wondering if there is a plastic material, something like a Trex decking type of thing, that could be used instead of the PT lumber? Or maybe 2 layers of exterior ply epoxied together.

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...

From contributor F:
I am pretty sure that will cup hollow to the face side. The drawing looks like the boards are whole and then dadoed out to make the channel. If that is the case and edge pieces are not being applied to make it appear thicker, as you know, leaving it full thickness will make it less prone to cupping if the pressure treated framing plate could be somehow eliminated or designed around.

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.

From contributor M:
I don't like flat top exterior wall caps. I like to push for either a radiused or peaked type of scenario, because you are going to get far more cupping from water and snow sitting on the flat surface than the wet wood under it. As far as the wet treated lumber I feel it is going to go primarily one way and that's to shrink down and the seasonal movement from there will be minimal once equalized. I guess the question is, can the ipe withstand the strain of the initial shrinkage without cracking the rail? Perhaps running the screws down tight, then back off a turn and allow room under the plug. A good dose of "mule snot" along the top off the pt lumber should keep things from rocking.

From contributor J:
I forgot to mention one last thing... I hope you have some really big helpers, cause moving 3" x 12" sticks of ipe around is not going to be fun work. Probably not a lot lighter than moving steel I beams around!

From contributor P:
Some time back I milled well over a hundred ipe 3 x 12 stair treads. The weight alone nearly killed me. After a season of the elements, I did get to see how the whole affair survived. The treads that were kerfed did really well. The ones that were left solid did move. Some twisted and a few broke the 1/4 stainless machine screws attaching them to the steal cleats. What I learned: When ipe wants to move, it will. And it will break anything trying to restrict its movement. I did have the contractor sign a waiver detailing all my concerns.

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.

From contributor D:
Some excellent advice above - the questioner's instinct is correct. There is no historic precedent (or even less than historic) for flat caps in wood, or even stone (as in balustrades). They are always designed to run water, as all good exterior wood should do. There is a reason this isn't seen - it doesn't work.

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.

From contributor G:
I wouldn't even think about touching that job unless everybody that could come back on you signed off on it. As others said, it will not work. You have in essence an unbalanced panel, uneven moisture content between the PT and ipe, wide board exterior construction, poor construction methods, yet another architect who knows nothing and probably another half dozen problems I didn't think of. Unless you are very hungry, pass on this one.

From contributor K:
Poor design. It should be all ipe construction.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

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.