Horizontal load capacity of 3/4 inch birch plywood

[drillbit79]

Hey y'all.

New to the forums here and I've got a technical question. Hoping this is the best avenue for it; if not, please point me in the right direction and I'd be happy to repost elsewhere.

I'm hoping someone can shed some light to my problem. I need to install a granite countertop (3cm thick), which will measure 65" long by 59" wide. The cabinet it will rest on is only 34" x 28", constructed of 3/4 inch, birch plywood, and is secured to the floor. Atop the cabinet lies a 3/4" thick, birch plywood spanning 50" x 44" (horizontal). The only real support currently in place is the cabinet (34"x28"). The rest is floating.

This means that the plywood has an overhang of 16" evenly distributed along the 4 sides of the cabinet. And, the granite countertop would have an additional 7.5" overhang throughout on all 4 sides from where the plywood subtop ends.

I'm wondering if anyone knows whether the plywood top will be strong enough to support the weight of the overhang. Since it is a piece of granite, the weight distribution will be nice and even with the bulk of the weight resting center of this island (cabinet).

Apologies for the long post and thanks in advance!

 

[NutBucket]

Remember, it's not just the weight of the granite but anything you put on top, people leaning on it (kids standing on it?). Anyway, 3/4" ply as a base is pretty common? The 7.5" overhang beyond the ply seems a bit much. Wouldn't it be pretty easy to use a large sheet of ply for less unsupported stone?

What does your installer say?

 

[herm0016]

It may carry the weight but it will certainly not be rigid enough to keep the stone from cracking. I would not overhang something with granite more than about 8 inches and I would always use steel supports if cantilevered with no posts

you will need to do something else, posts and an apron like a table maybe?
The cabinet should be fine for holding it up if it is a well built unit.

 

[drillbit79]

Hey NutBucket.

Thanks for the response! You are absolutely correct! I am not as concerned about the dead weight of the overhang as I am about it under stress and how a dynamic impact may damage the stone and potentially hurt someone.

3/4" is very standard for a subtop. I'm concerned about the load resistance being able to support the full weight of the overhang. Since the plywood is horizontally placed atop of the cabinet, I am unsure about its load capacity. I am potentially looking at overhang over cantilevered 3/4 plywood.

 

[drillbit79]

It may carry the weight but it will certainly not be rigid enough to keep the stone from cracking. I would not overhang something with granite more than about 8 inches and I would always use steel supports if cantilevered with no posts

Hey herm0016. Thank you for your input. That's what my gut has been pointing me to. Any suggestion on steel supports to accommodate this project?

 

[drillbit79]

Another thought....

What do you guys think about a hidden steel support under the subtop? I think that would provide ample support for the 7.5" overhang past the ply. Don't you think?

 

[herm0016]

3/4" is very standard for a subtop. I'm concerned about the load resistance being able to support the full weight of the overhang. Since the plywood is horizontally placed atop of the cabinet, I am unsure about its load capacity. I am potentially looking at overhang over cantilevered 3/4 plywood.

3/4 is pretty standard when you are doubling the edge thickness of 3/4 slab material and need to fill in that space, and when that slab is put on top of cabinets and is fully supported.

on my last island, we used 2 3/4 in thick maple butcher block and reinforced the 14 in overhang with steel angle in a slot on the bottom and a screw into each piece along the steel. steel extended 24 inches into the 5 ft x 6 ft island to catch the face and back of the cabinets on that side.

if you double the edge thickness, you could do something similar with steel. otherwise i may use soemthing like 1 in square tube to go at least the size of the current plywood. it's hard to have something thin and rigid.

 

[drillbit79]

@herm0016 Definitely a sound solution to my problem. Was curious whether anyone else underwent a similar situation without the added expense of steel supports.

I do, however, see the value since the last thing I want is for anyone to end up hurt due to a poorly supported countertop.

 

[Sukhoi]

In my kitchen Greenman installed 11-3/4" overhang of 2 cm granite with 5/8" plywood covering 10-1/2" of that overhang, and it has had no problems.

 

[drillbit79]

In my kitchen Greenman installed 11-3/4" overhang of 2 cm granite with 5/8" plywood covering 10-1/2" of that overhang, and it has had no problems.

Thank you very much for the feedback, @Sukhoi. I've thought about installing a larger piece of plywood and installing a 2cm countertop with a laminated edge to cover up the plywood subtop.

 

[Humpy]

The rule of thumb in my area was 12" of unsupported overhang for 3 cm. We would support with plywood to the extent possible at the request of the installers. Not sure what good it did, because as mentioned, supporting a brittle material with a flexible material doesn't make a whole lot of sense.

I don't recall seeing it after installation but I have seen granite break when carrying it from the truck. That's what's so exciting about natural materials, you just never know.

 

[Greenman]

No way on earth the plywood will support 23.5" of load without deflection. The stone is ridged and has little tensile strength (depending on the stone), it will almost certainly crack.

You're going to have to get creative in your approach. I'd look at having a steel square tube grid made instead of using plywood, then attach the stone to the steel with a flexible adhesive to allow for a bit of differential movement. You'll also want to make sure the cabinet is anchored very well to the floor, someone leaning on that huge overhang is going to put some serious uplift on the opposite side of the cabinet.

It's going to cost you a few bucks to make it work, but it will cost a lot more to do it again if it fails.

 

[Greenman]

The rule of thumb in my area was 12" of unsupported overhang for 3 cm. We would support with plywood to the extent possible at the request of the installers. Not sure what good it did, because as mentioned, supporting a brittle material with a flexible material doesn't make a whole lot of sense.

I don't recall seeing it after installation but I have seen granite break when carrying it from the truck. That's what's so exciting about natural materials, you just never know.

I had a fellow installing a huge top and it shattered as they were setting it into the adhesive. I thought the guy was going to cry.

 

[Micrornd]

I think I'm missing the point here.
Why would you want a table you can't move?

 

[Greenman]

I think I'm missing the point here.
Why would you want a table you can't move?

My hunch is it's an island.

 

[drillbit79]

Hey @Greenman @Humpy @Micrornd . Thanks for your valuable input. I believe I may not have explained myself thoroughly in my original post, so here goes:

It is in fact an 'island'. The placing of it is odd because it is where a breakfast (dining) table would normally be. It's well anchored to the floor. The overhang on each of the 4 sides spans 15.5" from the cabinet, of which 8" rests on cantilevered plywood (tied to the cabinet) on all four sides. So, in actuality, only 7.5" of stone overhangs and floats past the cantilevered subtop.

I hear what others have said and am definitely leaning towards steel support underneath the subtop. This would ensure that the plywood is less forgiving over time and the overhang has a sound structure to lie on.

Thoughts?

 

[Greenman]

Hey @Greenman @Humpy @Micrornd . Thanks for your valuable input. I believe I may not have explained myself thoroughly in my original post, so here goes:

It is in fact an 'island'. The placing of it is odd because it is where a breakfast (dining) table would normally be. It's well anchored to the floor. The overhang on each of the 4 sides spans 15.5" from the cabinet, of which 8" rests on cantilevered plywood (tied to the cabinet) on all four sides. So, in actuality, only 7.5" of stone overhangs and floats past the cantilevered subtop.

I hear what others have said and am definitely leaning towards steel support underneath the subtop. This would ensure that the plywood is less forgiving over time and the overhang has a sound structure to lie on.

Thoughts?

That's a tough one. You're out there just past the edge of what I would consider reasonable. At that point, it depends on the stone you choose. That call should be made by the fabricator.

 

[SevillaRepara]

Muchas gracias from Spain