Engineering for supporting a stone top.

[mrFinpgh]

My sister asked me about making a piece of furniture which could support a soapstone countertop.  I think it will be around 5' wide, 20" deep.  Google tells me this is around 160 lbs. The piece will probably end up looking like some kind of mid-century or modern style credenza.

However, I'm not really sure what additional engineering I need to consider for supporting the extra weight of a stone top. 

For example, let's say I am designing the legs of this thing.  Am I asking for trouble if I don't place the legs at or near the outside corners?  Will the weight of the slab cause any issues?

Any sagging concerns I might want to think about?  I assume it's a rigid and distributed weight, but 32lbs per linear foot is still a fair bit.

Should I be planning to support the underside of the top as fully as possible?  Is a cantilever a bad idea?  Would frameless construction be problematic in this case?

Any ideas, tips, insights, or experiences would be greatly appreciated!

 

[CeeJay]

Aha! I had exactly the same problem 18 months ago for a desk with a 2” x 55” x 30” sandstone slab top.

The top weighed about 220lb.

I built fairly bulky leg frames out of Qld maple, with a beefy back stretcher to avoid racking, and some corner bracing under the frame as you can see.

Anyway 18 months in and my client (wife) loves the desk and no issues at all.

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[CeeJay]

My main concern was the slab cracking in the middle due to unsupported span. Sandstone is very weak in tension.

Hence fully supporting the top front and back, and using the corner bracing, without making the structure look too ‘heavy’.

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[Michael Kellough]

Very nice [member=72399]CeeJay[/member] !

Maybe the corner braces aren’t needed (the sandstone slab effectively prevents racking) but they certainly don’t hurt and they keep the joints tight when the slab isn’t in place.

 

[huntdupl]

I'm not familiar with the particularities of the stone, but if slab is around 3cm I wouldn't be so concerned. I'm more wondering how you attach it to the base...

I've seen some precious marbles etc. be reinforced with mesh and what I assume to be 2 part resin - you could apply alkaline resistant fiberglass mesh screen on the underside. You could also look at mortising or laminating metal support brackets into the legs either for the center section or cantilevers. These are pretty pricey for hunks of metal...

counter brackets

other bracketry

 

[Cheese]

Rodding has been popular for sinks and countertops for at least the last 15 years.
https://gluewarehouse.com/products/get_more_info/rod_view/Rod-It

 

[CeeJay]

I'm not familiar with the particularities of the stone, but if slab is around 3cm I wouldn't be so concerned. I'm more wondering how you attach it to the base

other bracketry

Yes for a kitchen counter for sure. For this desk I didn’t attach the slab to the base, it’s so damn heavy there is no way it’s going anywhere. It was a two man job to lift the slab off a flat trolley onto the base in the study. Pretty glad it is on the ground floor - no idea how to get it up or down any stairs as the most likely time the slab will break is when it’s being lifted and isn’t fully supported over it’s length.

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[afish]

I epoxy rods in like cheese mentioned for wide spans.  Cantilevers are usually supported with 3/8 or better steel supports.  You can find them easy on line as they are popular for kitchen cantilevers.

 

[mrFinpgh]

Aha! I had exactly the same problem 18 months ago for a desk with a 2” x 55” x 30” sandstone slab top.

The top weighed about 220lb.

I built fairly bulky leg frames out of Qld maple, with a beefy back stretcher to avoid racking, and some corner bracing under the frame as you can see.

Anyway 18 months in and my client (wife) loves the desk and no issues at all.

CeeJay, that looks great.  Very substantial looking, but the proportions of the feet to the legs is clever.  Glad to hear it is holding up for you!  How thick are the legs and feet/top?

I was thinking about this some more today and how much weight I routinely put on my krenov sawhorses, which are a trestle style.  They hold up just fine, even w/ 7/8 thick verticals and 3/4" tops. 

i will work on some drawings in fusion (if I can get it to cooperate) and see if I can put them up for critique here.

Thanks,
Adam

 

[CeeJay]

Cheers Adam.

The feet and the upper stretcher are mirror images, 1 3/4” square section with roundovers and tapered ends to lighten them, and the legs are 1” x 7” section, bullnosed in profile.  The leg tenons  are pretty substantial at 3/4” x 5” x 1 1/4”.

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[mrFinpgh]

I did a first pass at a design in fusion360.  I'm still getting comfortable w/ the app, and it has some bugs with the UI when switching in and out of the application, but it's a nice tool.

All the members here are 1.75" square, and I'm thinking joinery will be floating tenons of some sort.  Currently thinking walnut for the primary wood. The panels are veneered MDF glued into a groove or rabbet depending on if they are vertical or horizontal in orientation.  Span is about 60".  The hidden interiors would be veneered in some lighter toned wood - probably depends what I can find.

View attachment 1

View attachment 2

 

[CeeJay]

That looks great! I think there’s plenty of support for the stone top.

Are you planning on fixing the top to the frame?

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[nclemmons]

I think your design is fine in terms of strength.  The inset panels in the legs will help with rigidity and prevent the weight of the top from causing movement.

You could use double stick tape, carpet tape or just caulk to attach the top.  Most kitchen counters are held in place with caulk or construction adhesive.  I'd want the top to be able to be removed if you ever want to move the desk!  Six quarter-size dabs of caulk would hold that top fine.

 

[mrFinpgh]

Thanks [member=72399]CeeJay[/member]  and [member=167]neilc[/member]

What would be the benefits of fixing the top more rigidly to the body? It's not going to be my call, but i will have some influence on what they do with it.

Also - as designed the top is currently 'open' (so a framework of 2x2 walnut but no upper panel.  I was thinking about whether the whole thing would work better if the top is closed - perhaps 56*16 panel let into the framework w/ a rabbet.  Any thought around that?

 

[nclemmons]

I don’t see a huge advantage to fastening the top more securely, given the downside of it being very heavy to move in the future.  One advantage of using caulk or double stick tape beyond the weight consideration is the option of flipping the top should the surface get scratched or stained in the future.  The more securely you attach it with fasteners or epoxy, the less likely you can do that.

To your second question…

Are you thinking about an inset panel that is parallel to and below the stone top?  Right now, you have one panel across the back and one on the right side.  And below that section you have other panels.  That gives it a lot of strength even though the upper level is not fully enclosed.  Those legs look to be 1.5 or so in cross section.  That’s plenty of strength given the way you have it constructed.

 

[CeeJay]

If it’s never going to move I’d leave the top unattached.

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[JonathanJung]

The best people to ask are countertop suppliers and installers. Client's of mine regularly use 1-1/8" thick stone countertops, some soapstone, some with 30" unsupported spans over cabinets.

 

[mrFinpgh]

Picking back up on this.. now the request is to do it in Ash. Which I think will look nice.  I will see if my sister wants to do anything interesting w/ the doors or panels (all veneered or ply). I can see a case for either option.

I was pondering the construction details a bit more and have pretty well decided I'll be doing double loose tenons at the frame joints. Since the d500 can cut a 33mm wide 10mm mortise, I'll mill up some extra wide tenon stock and use a shim to create two mortises per joint.

However, I'm not sure yet how I ought to handle getting this all assembled, and whether i could make some joinery choices to make life easier. Until now, I've been assuming that I will glue up in stages: sides, then the middle part.  But even that makes me a bit concerned, with the inner framework adding some additional pieces to glue together and align. I'd prefer to keep the anxiety levels as low as possible during assembly, and I can't imagine gluing in the back panels, cross pieces, vertical dividers, etc, in one shot is going to have that result.

Here's a detail shot of the middle of the case: View attachment 1

It seems like perhaps another sequence would be easier or more manageable.. I'm just not sure what that is yet. 

Also, debating how to best place the 'shelf' panels in the assembly.  As of now the frame will be 45mm sq wood.  I can rabbet the parts that will have a shelf on them, and notch the shelf around the corners.I could also add 10mm to the width of those pieces and rabbet that, creating an internal ledge to place the shelf on.  That seems seems less risky than notching a veneer panel for the leg, and adjusting the rabbets (if necessary) would probably be a little simpler. 

Of course, I could also.. glue on material after the fact to hold up the shelf and align it to the framework.. kind of like drawer slips. That would be fine for the bottom shelves, but might look a little low-class from some angles on the upper shelves.  At least wood movement shouldn't be a big issue, with the panels being dimensionally stable.