What screws to use for floating vanity frame?

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Forum topic by Kate posted 12-08-2017 02:15 PM 8551 views 0 times favorited 28 replies Add to Favorites Watch
View Kate's profile


5 posts in 1221 days

12-08-2017 02:15 PM

Topic tags/keywords: question

This may not be the right forum, and I am new to the site and the world of woodworking, so forgive me.

I am building a floating shelf vanity measuring 50” x 24”. My intention is to create a simple frame using 2×4s that the edge-glued panel countertop sits on, and the 2×4S will of course be attached to the studs in the three walls around the vanity. I am not sure how best to join the frame to the studs- the last thing I want is for the sink to be too heavy and everything come crashing down. I Have (#8) 2 1/2” screws but am concerned they will not be strong enough, even if I use two screws per stud (there are 4 studs in the wall behind the vanity and one student on either side). Should I use lag bolts?

Thanks all!

28 replies so far

View UpstateNYdude's profile


965 posts in 3034 days

#1 posted 12-08-2017 02:20 PM

I’d make a french cleat set it in and then lag into the studs. I’d also add some bracing underneath 24” is fairly wide to be unsupported.

-- Nick, “I am always doing what I cannot do yet, in order to learn how to do it.” – Vincent Van Gogh

View CampD's profile


1825 posts in 4537 days

#2 posted 12-08-2017 02:37 PM

2 1/2” screws are enough. Build the frame, screw it in place predrilling holes, than attach the top with screws from underneath. You could even make a false bottom with just a few screws holding it in place.

Good luck!

-- Doug...

View MrFid's profile


908 posts in 2955 days

#3 posted 12-08-2017 02:39 PM

Welcome to the site!

I also think 24” of hang is probably too much for even a lag bolt, especially considering you’re dropping a sink into the coutertop. No matter how strong your connection is between the counter/frame and the wall, it will feel wobbly if it’s not supported further out. Which is not a feeling someone wants when they’re washing their hands or leaning on the counter. You can probably find fairly non-obtrusive shelving brackets at a home center that will suit your purposes. Or, if you know how to weld (or know a local welder) they could easily fabricate something that would provide much more support underneath the shelf.

Let us know if you need more help with your project. There are lots of smart people on here that I’ve learned a lot from. We look forward to seeing what you come up with. Again, welcome to Lumberjocks!

-- Bailey F - Eastern Mass.

View Kate's profile


5 posts in 1221 days

#4 posted 12-08-2017 02:42 PM

Thank you both! Doug, I was doing some more thinking last night – Would it be better to use 3 1/2” screws? I’ve got to go through 1 1/2” of 2×4 and 3/4” of drywall before I even hit the studs.

View Kate's profile


5 posts in 1221 days

#5 posted 12-08-2017 02:44 PM

Great idea with the shelving brackets! That’s one of those, “oh, duh! ” moments.

View jonah's profile


2136 posts in 4350 days

#6 posted 12-08-2017 04:53 PM

Welcome to LJs!

If I’m understanding correctly, this floating vanity has walls on three sides around it.

If that’s the case, screwing it into the framing on all three sides will be plenty to support a 24” front, so long as the whole assembly is sufficiently stiff. Having a strong frame and cross bracing will help there, but brackets in the corners couldn’t hurt either.

Your instinct is right that you need 3.5-4” screws. Personally, I’d use beefy 1/4” lag bolts rather than wood screws. Be sure to pre-drill the holes to avoid splitting the wood.

View jerkylips's profile


495 posts in 3621 days

#7 posted 12-08-2017 05:06 PM

I missed the part about “3 walls” at first, as I suspect some others did too based on the responses. If the shelf is supported on 3 sides – back and each side – I would do a cleat into each wall, and fasten the counter top to the cleat from below. This picture shows 2 of the 3 sides. I would use 4” lag bolts into the stud – be sure to pre-drill.

View JBrow's profile


1368 posts in 1971 days

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


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…

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.

View TheFridge's profile


10863 posts in 2537 days

#9 posted 12-09-2017 04:38 AM

A dozen screws 1” into the stud will hold up just about anything. I usually use spax screws or #10 screws for jobs like this.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View clin's profile


1128 posts in 2047 days

#10 posted 12-09-2017 07:44 PM

Since it is supported on three sides, the loads are pretty small at any point. Your worst case load is the 300 lb guy that will sit on it.

The cleat that jerry shows is the way to go. You don’t need a 2×4 frame up against the wall. 1×2 would be enough. The front end, of the two side cleats, are where the greatest load will be and two screws there is not a bad idea. Though one good size screw like a #14 should be plenty.

Elsewhere, one in each stud is certainly enough. As for screw length, you will probably have 1/2” drywall and then the thickness of the frame/cleat.

Assuming a 1” thick frame/cleat, + 1/2” drywall, + 1” screw into stud, as TheFridge suggested, you need screws 2.5” long. If you use 2×4 or similar frame/cleat, then you need another 1/2” so 3” long. A bit longer doesn’t hurt, but unless you know for sure there isn’t wiring running through the studs, you don’t want to just run 2” deep into the stud.

#8 screws are probably plenty big, for all but the front end of the frame. You won’t break (shear off) the #8, but they could bend. The bigger the screw, the less they will bend and the more wood the forces are spread over. So maybe just go with #14 or similar fro those front ones. You’ll want to pre-drill for those to make sure you don’t split the frame/cleat or stud. I’d pre-drill the frame/cleat no matter what, makes it easier to pre-load the screws when mounting. For #8 screws in the stud, you don’t need to drill the stud.

You didn’t describe how the shelf was constructed. At 50” long, you will need some strength along that front edge. If the top is something like 1.5” hardwood, then that’s likely plenty. But if the top is laminate covered particle board or similar, then you need to support that.

This of course may be why you have described a frame. And certainly a 2×4 frame going around all 4 sides is more than enough. Smaller would still be okay, even for the 300 lb guy who will sit on it. You just want the shelf to not bend too much under an extreme load. And of course not bend or sag at all under it’s own weight and the weight of the sink.

Often a smaller frame is desired for looks. If so, you could consider a steel tube long the front.

-- Clin

View MrRon's profile


6005 posts in 4295 days

#11 posted 12-10-2017 08:00 PM

You have to realize the weight of the sink is equally distributed around the lip of the sink, which is closer to the walls. The shelf itself won’t sag, as the weight is not concentrated at the center of the shelf. Besides, 2” thick maple is pretty stiff all by itself. I would recommend 4” star drive deck screws, 2 per stud is adequate. Anymore and it is overkill. Just make sure the supports are screwed tightly against the wall. You want as much tension applied to the screw to reduce the shear on the screw. You do not want to introduce bending in the screw.

PS; Don’t use stainless steel screws. 1/4” lag bolts are also OK

View clin's profile


1128 posts in 2047 days

#12 posted 12-10-2017 10:22 PM

Shelves stills sag with evenly distributed loads. just not as much as if the same total load is in the center. Also, I don’t think the OP said anything about a 2” thick maple top. That was someone else’s example used in a weight calculation.

Though I agree that if it were a 2” thick maple top, that alone would be plenty stiff enough.

While I see no big advantage to stainless screws, why NOT use them? Is there some reason they wouldn’t work as well?

-- Clin

View jonah's profile


2136 posts in 4350 days

#13 posted 12-10-2017 10:34 PM

There’s no reason I can think of why stainless steel screws wouldn’t work perfectly fine.

View JBrow's profile


1368 posts in 1971 days

#14 posted 12-10-2017 11:47 PM

clin and jonah,

The only reason to avoid stainless steel screws is that the heads of ones I have used strip easily. But if the hole is predrilled to the correct size and care is used when driving the screw, I would think stainless would hold weight like a construction screw in this application with no problem. But then since I am not a metallurgist, the loading capacity of stainless compared to other metals is a guess.

View Kate's profile


5 posts in 1221 days

#15 posted 12-11-2017 12:53 AM

Thank you for all the responses. This forum is a wealth of knowledge and I appreciate you all taking the time to respond and explain!

You had me sold on using cleats instead of a frame, but I’ve run into a new problem. When I went to measure out 24 inches from the back wall, I discovered that the stud on one side started at about 23 3/4” from the wall…I wouldn’t be able to screw the front of one cleat to a stud. After much swearing and head scratching, I figured I could either:
1. Use heavy duty toggle bolts to attach that end of the cleat to the drywall
2. Support the countertop on that side with a bracket ( as suggested above). I know Lowe’s carries a 22” bracket with 500lb capacity, and I figure I could attach it to the stud along the back wall.


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