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I am building a trestle style farmhouse table base out of cedar for outdoor use. The tabletop is 3/4 inch granite on 3/4 inch plywood subbase. The dimension of the base is 33 x 72 and the table is 36 x 77. How can i know if the base will support the top. I am using 2x6 and it will be like the attached photo in design.
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I am concerned about setting a granite table top on a base of this design. The issue is not so much the weight of the granite which is 12.8 psf, but the brittleness of the granite and its inability to span any distance between supports. Granite does not deflect, it breaks and it has many natural fault lines in the its matrix that are weaker and could break under smaller forces. The most common application of granite tops is found in kitchens in which the span is usually less than 24" and there is still a risk of it breaking if someone sits or stands on the surface. In any event, while the plywood below is a very good idea, it will not provide adequate support for any span. It needs additional support below, much as a floor of a house has a framing supporting the plywood. Ideally you should consider constructing a top frame at least 3-4: thick with span widths no greater than 12"-18" in one direction and 24" in the other. Once you have a support platform for the granite top, the type of table base shown in the photo should be more than capable of supporting it.

One last consideration, will you bullnose the ends to hide the plywood?

  • Strongly agree with the above. As an experiment, put a sheet of ply on 6' centers. Then go stand on the middle of the ply. Don't be surprised when it deflects about 4". I'd suggest (at a minimum) an apron and a center strut running parallel to the apron. Beautiful design, btw. – Aloysius Defenestrate Apr 24 '18 at 3:03
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Weight wise your table will easily handle the load. I am more likely to be concerned about the span but the plywood will help. A stringer running between the trestles would be a big help.

  • A couple of stringers (hardwood, not plywood) would be even better. – WhatRoughBeast Apr 30 '18 at 19:10

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