Stabilizing Details for a Mahogany Island Top

      Concerned about wood movement, pros consider options including threaded rod reinforcement, sliding dovetails, or thin-sawing the wood. December 10, 2008

Question
I have a customer that wants a mahogany island top built. Specs are 108" x 42" x 2" thick. I've got a source for some really nice 8/4 African mahogany and I want to use 5-6" boards to do my lay-up. This top, however, will sit on an island base that measures 84" x 27." So the front working edge of the island will have a standard overhang, say 1.5", and the back and two ends will have around a 12" overhang on those three sides. I am wondering if the top will remain flat and true with just the few corbels along the back side of the bar with the overhangs I have. I've never done a top of this size with overhangs on 3 sides.

I was considering drilling through each strip on edge, say every 12" or so, and installing a long threaded steel rod and actually bolting the entire top together. Then just plugging the holes on the worktop edge. Could even be a contrasting wood to make it stand out. I have a bunch of the Go-Bolt material left over, so it's no expense to me, as it's slowly been used for jigs. Any input? I hate to test new techniques on customers, but it seems like it should work.

Forum Responses
(Furniture Making Forum)
From contributor N:
I wouldn't bolt it together. I'd think that restraining it that way would invite eventual compression failures and cause cracks to develop. My bet is that it will be fine with corbels; just make sure whatever fastening system you use allows it to move seasonally.



From contributor J:
I'm no expert, but I don't think the bolt idea is so hot either. Even a 1/2" steel rod will flex fairly easily over 42". Basically if the top warped, the bolt would warp with it.

Of course if you knew which way the top might cup, you could route a curved channel into the top and have the threaded rod act like a truss rod in a guitar neck. With access to a nut which would increase or decrease tension on the rod to counteract movement. Personally I think the engineering involved in something like that would be overkill and more work than it's worth.

I think it's more important that the wood is properly dried and that the installation is done correctly. The top will rest on corbels, but it still needs to move. So you've got to figure a way to fasten the top to the corbels and still allow it to expand and contract seasonally. I would fasten the other end with the 1-1/2" overhang in place.

On a positive note, I made a 1-3/4" thick SA mahogany top for my kitchen over a year ago and it has been more stable than I expected. No detectable cupping or anything between seasons. I just fastened the back edge in place and the front edge is fastened through slots allowing it to move. Seems like a very stable wood from my experience.



From contributor C:
Contributor J, I am always looking for some new ideas for my clients.
How long is the counter? (Did you use single length boards, or are there butt joints in the counter?)
What did you use for a finish material on the wood?
Is it near the sink and water?
Any problems with normal wear?


From contributor J:
The counter is just over 11' long, no butt joints. It is made up of 4 pieces. I think the front is 9" wide, then a 6", an 8", and a narrow strip less than 3" at the back, mostly underneath the b/s.

The c-top surrounds the sink. The finish was a CV finish from Target which came highly recommended. Having used it I would not recommend it, though. We were entertaining guests shortly after the kitchen was installed and had an accident. A placemat that had been used under the blender (to protect the c-top) was left overnight. What we didn't realize was, although our guests helped us clean up and the counter was clean, there was rum spilled under the mat that hadn't been cleaned up. The next morning when we resumed cleaning, we found the finish pretty much destroyed. Lesson learned.

It's now been over a year since the install and for the most part the rest of the finish has held up very well. However there are a couple very slight haze marks in the grain directly under the faucet and under the drip tray. Most people would never notice but you know how that is! Surprisingly the end grain around the sink is still like new, not a bit of wear showing there. Obviously the alcohol was more than the finish was designed to withstand, but the thing I'm bothered by is the hazing under the faucet. Next countertop I'll be looking for a more durable finish.

The last thing I'll say is that you really cannot see the beauty of the top in these pictures. One of the things I love about the SA mahogany that some of the other mahoganies don't have is a depth to the grain - with finish, it's slightly iridescent. I used a couple coats of amber shellac over the stain to really pop the grain. And when you look at the counter, (especially when wet), it's just amazing. I'd do it again in a heartbeat - I'd just use a different finish next time.

The first picture is in the spray booth.


Click here for higher quality, full size image

This picture is the counter installed. The radio on the left is now a permanent fixture hiding the damaged area. Obviously still have some work left to do to finish the kitchen, someday!

In case you're wondering why I didn't re-finish the top, it was just too much to do at the time. The stain was a custom mix applied in three coats, followed by 3 coats of shellac to add depth and a slight amber tint, and finally 4 or 5 coats of CV. Basically it took me a solid week to finish the first time, so the thought of removing that finish and re-applying again, not to mention actually moving a top that big, was just too much. Not to mention having to deal with the wife not having a counter again!


Click here for higher quality, full size image



From contributor C:
Thank you! Your explanation and photo are very helpful. The counter looks gorgeous.

One comment I have about the CV that you used, and was damaged. You said that it was shortly after installation. Was it within the 30 day window after the final coat of CV? I have heard that a number of products will take a full 30 days to cure. Since you said the rest of the finish has stood up pretty well during the past year, I wonder if the CV wasn't completely cured when the accident happened.



From the original questioner:
I saw this top when I did a search on the site, however, the cabinets are completely supporting the top - no overhangs like the one I am doing. I'm also a bit leery of the long bolts, but it seems like I have seen it done before, just can't place where. There is a top company that does a similar type deal for their overhangs. They call it "steel reinforcing to prevent a non-supported work top from bowing." I'm just worried the corbels won't keep it from cupping upwards without attaching them too tight for the top to expand and contract.


From contributor J:
I think it was a couple months out of the booth when it got damaged. Totally my fault though - I believe the damage was because the placemat covered the liquid overnight, otherwise it would have evaporated and been fine.

I'm curious about the steel rod myself now. I still think it's problematic, as if it's bolted tight for strength, it will restrict the movement of the wood. If it's not bolted I don't see how it adds any real strength, but then again I'm no expert so this is just my thinking out loud.

As for the corbels, just like any other top, you have to allow just enough slack in the fasteners to allow the wood to move. You don't want them to be loose, just not tightened too much. The trick is going to be figuring out just how to make that attachment.



From contributor Z:
Consider quarter sawing your 8/4 planks into 2x2" strips and placing the quarter sawn side up. The movement should then be more oriented up and down, which will be insignificant in 2" thickness. The wood should all be well seasoned and at the desired moisture content before gluing, so let it equalize after resawing and before final glue joint planing. Sticker it with weights if you think there might be movement in the 2x2's. After gluing it up, give it a chance to de-stress before flat sanding it, and you can observe whether there might be a need for seasoning further. Be sure to allow it to dry (if it needs to) on both sides. If you sticker the top over a radiant heat floor, you can season it nicely, but turn it over every couple of days.

How will you flat sand this large top?

I've done the steel rod thing before. It's a lot of extra work and only worth it to me if the part is structural or at risk of coming apart. This shouldn't be at that kind of risk at all.
I've done a top with a Waterlox Tung Oil finish and it was easy and turned out looking good, but I didn't do the rum test on it mentioned above.


Click here for higher quality, full size image



From the original questioner:
Customer wants the look of the plank top, so no 2" strips, otherwise I would do it. Will be smoothed on a 50" widebelt sander. What did you do when you did the steel reinforcement? I know it will be extra work, but for this customer I would rather overbuild than have problems later on. It will lead to a lot of other work if done correctly.


From contributor L:
Contributor N has the right idea. Don't bolt it together because it will expand and contract a tad bit seasonally. Proper preparation of your wood will prevent future problems. When preparing your raw stock, take the time to let it acclimate before shaping and assembling. After it has acclimated to your shop environment, assemble with glue, and shape it to your finished size. If you will be finishing it on site, go ahead and get it to the location where it will be installed and stage it there to allow it to acclimate to its new environment. If it will be finished in the shop, do so and then get it to the job site and again let it acclimate onsite. When you are ready to install it, do so allowing the mechanical fasteners to adjust for seasonal changes. Install your corbels and step back and admire your job! Mahogany is a resilient wood and curling won't be a problem if you slow down and properly prep your wood up front, especially since it is 2" thick.


From contributor K:
I have often used sliding dovetails on the bottom side to give stability to such a top as you describe. You may want to use an offset dovetail to make it easier to drive home. I would suggest dovetails at least 1 1/2" wide at base and at least 3/4" tall; 1" would probably be better. They can also be done so as to decorate, or they could be done as blind dovetails if you glue the last and first board as such. No glue necessary for dovetails if properly sized. Can also be contrasting wood or same. Mechanical joint that has been around several thousand years. It works!


From contributor Z:
Though I don't think it's needed in a top, my method was to drill and insert stainless steel threaded rod with washers and nuts on each end. Inset them about 1/2" and finish with face grain plugs you make from the same material with a plug cutter. It's a lot of drilling and lining up. It seems most of the other forum members are also saying the extra work shouldn't be needed.


From contributor W:
As yacht builders, we use a lot of mahogany, and it's one of my favorite woods. Why not slice the 2" stock into thinner pieces and laminate them on to a core material such as Nida-Core or even plywood? It would save quite a bit of valuable rainforest hardwood, be less expensive, and the saved material can be made into more furniture. Such a construction will solve your stability problems as well. With a lightweight core it will stay flat and will be very stiff. It will save your back during installation as well. Years ago we used to through bolt 44" wide maple kitchen bar tops. I don't think it was of much benefit to do that. We glued the outer bands on after bolting to avoid the holes and bungs on the edges. While I agree wood needs to acclimatize, 2" mahogany is going to need months to do so. If it is well seasoned, I wouldn't bother.


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

Comment from contributor A:
I would heartily agree with contributor W - to make a work surface of this size out of solid stock is both a waste of premium material and a recipe for problems resulting from wood movement. If the client wants a planked look, choose your widest board and resaw it into less-than-1/8" thick veneers. Build the work surface as a torsion box, then veneer it with your show wood. Be sure to cover both sides with the same species for balance. As contributor W said, it'll save your back, your material, and will be dimensionally stable. As an added bonus, a torsion box will easily handle the 12" overhangs with little or no additional support.



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