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Reply by JBrow

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Posted on What screws to use for floating vanity frame?

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JBrow

1368 posts in 2006 days


#1 posted 12-09-2017 03:02 AM

Kate,

I doubt the vanity top plus even a cast iron sink will exceed the strength of standard 3” construction screws. By my calculation, the combined weight of a cast iron sink (about 30 pounds) plus a hard maple 2” thick vanity top (about 65 pounds) plus miscellaneous dead load will probably weigh no more than 150 pounds. I think 12 (2 screws per stud) 3” construction screws would be more than sufficient to carry this load (about 13 pounds of load per screw).

An alternative to construction screws and fasteners already suggested that would be stronger are structural screws or sometimes called timber screws. My understanding is that these specialty screws offer greater shear strength than drywall or construction screws and lag screws. Structural screws can be found with various heads and differing sizes, some of which are low-profile heads. Although some manufacturers may state that predrilling is unnecessary, I would predrill as added insurance against introducing a split in the framing studs.

Here are some examples of structural screws…

https://www.lowes.com/pd/FastenMaster-50-Count-0-to-x-6-in-Ecoat-Spider-Drive-Structural-Wood-Screw/3295080

https://www.menards.com/main/tools-hardware/fasteners-fastener-accessories/screws/wood-screws/fastenmaster-reg-headlok-reg-3-x-2-7-8-spider-drive-black-flat-head-wood-screw-1-count/p-1446588768376.htm

As to screw length, I think a 2-1/2” long screw is not long enough when using 1-1/2” true thickness material as cleats to support the vanity top. Assuming the 2×4 cleats are screwed over ½” thick drywall, a 2-1/2” long screw would only extend ½” into the framing. I would prefer 1” of the screw in the framing. That then would require a 3” long screw for a 2×4 cleat. On the other hand, keeping the amount of the screw buried into the framing no more than 1” could reduce the chances of inadvertently drilling into an electrical cable that is run through the center of the framing studs while providing plenty of support for the cleats.


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