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I've built a work bench by modifying some plans I found, and one part of the modification involved installing a shelf. The bench is 8' long, stable and solid, and I used 4x4x6 uprights in the back corners to mount the shelf onto.

To keep the shelf from flexing I used 2 2"x6"x8' as stringers between the 4x4s and attached the shelf ( 12"w 1/2" maple) to the bottom of one of the stringers with countersunk 2" #8 Robertson screws spaced every 12".

My question is: should I further brace this shelf considering the maximum load on it will likely be less than 100#? My suspicion is that I should install 3 triangular braces, one at each end and one in the center to prevent any sagging.

Edit: Here's the updated picture with both upper and lower. The curve in the small shelf is illusory, just need to touch it up with the router when I bullnose the front of the bench.

The bench, with implemented solution

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Stephen, little hint. I always put a sacrificial top over my workbenches, usually a layer of cheap 1/4" plywood, MDF, melamine or anything handy. This simply acts as a buffer so you can paint, saw, screw, draw, whatever on it and when it gets really beat up and nasty, take it off and replace it. This saves your nice 3/4" top from all the damage. Just my thing. –  shirlock homes Jun 19 '11 at 11:04
    
@shirlock homes I've been doing that on the work horses just to the left ... it was only in the last step that a lot of the clutter got transferred :-) Thanks though. –  Stephen Jun 19 '11 at 11:52
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up vote 3 down vote accepted

I agree, you should use some "L" shelf brackets UNDER the shelves to the vertical supports. There are many you can chose from, but even the cheap gray 10 inch ones would work fine. Remember, if you use 3 supports, the load is divided, so each bracket will only be holding apx 1/2 of the load resting between two supports. If you mount them over the shelf, rather than under the shelf, the load will be supported only by the mounting screws rather than transferring the load to the upright post, for that reason, I'd stick with mounting them under the shelf. Your shelf as pictured without braces will most likely split at the line where the mounting screws are, as the grain of the wood follows that direction.

BTW, nice workbench! Good luck

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So mounting above will only add rigidity and fight sag where mounting below will actually transfer the weight? Thanks! –  Stephen Jun 19 '11 at 11:56
    
It's less to do with weight transfer than with isolating weak points. Mounting the bracket over the shelf makes the screws attaching the shelf to the bracket a critical failure point; if they go, so does the shelf. Mounting them underneath makes the bracket itself the failure point; the screws attaching the bracket to the shelf can rust away and the shelf will still be putting its load on the bracket. Either way, the screws attaching the bracket to the vertical will be another critical failure point, but even then, mounting underneath reduces the screws' load. –  KeithS Jun 20 '11 at 22:25
    
If you're worried about the load on the screws you could always use bolts and fender washers. It just looked convenient to attach at least the center brace from the top. –  electricsauce Jun 20 '11 at 23:27
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I would. It doesn't look like it could hold 100#. You should be able to attach the angle braces from the top and still get the load capacity that you're looking for without sacrificing your workspace.

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