Hi all
I have a grante island bench. On one side of the bench I have 470mm overhang with a 1430mm span. There are full length support either end of the span.
I have used 25mm mdf as the substrate. I have also screwed 5 L shaped brackets into the top of the substrate which are then secured into the carcasses behind the overhang.
The stone then cracked on the right edge of the overhang.
I then installed a 6mm thick 25mm x 50mm square hollow steel bar across the front, spanning the entire 1430mm.
With all of this, I am still getting 1-2mm of deflection in the middle of the overhang. This is less than before thee beam was installed.
Before the stone is reglued and patched on the edges, do you think it will crack again?
If so, Is there anything else I can do to reinforce?
here is a photo of the bench:
https://drive.google.com/file/d/1exFDWqHrLDrht6fCz36gEA4n08ckcq8a/view?usp=drivesdk
Replies
No good news here.
Yes it will crack again.
MDF has no strength and is certainly not the right substrate...especially 1 inch thick.
With the span it would need at least 2 inches of good quality plywood glued and screwed together...maybe even thicker.
Some of the variables are the weight of the marble and do people sit on this "bench"?
To make it right you need to start over is my suggestion and consult span tables before you start.
https://woodbin.com/calcs/sagulator/
That appears to be a simple island seating area. From your use of the word "bench" and the metric measurements I am going to guess that you are not in the USA so I don't where to direct you for assistance but I am a professional kitchen designer and successfully do designs like yours all the time so something that I can't see is wrong.
Plywood would definitely have been a better substrate but I suspect your brackets are the main culprit. Did you use heavy iron brackets design specifically for this task like these?
https://ironsupports.com/collections/hidden-countertop-brackets
Was the granite 3cm? If you used 2cm that would also be a problem.
The last thing could be the choice of stone. I don't recognize the stone you used, but some can be very weak and need more support than others. Your stone supplier should be able to provide some guidance with this.
Thanks for the responses. The stone is 20mm thick. It is called tundra grey and is a natural stone. I agree we probably should've used a better substrate, but 25mm thick MDF is fairly strong.
I'm not sure how much the stone is allowed to deflect without cracking the corner. The stone itself is not cracking, it's the join in the corner which is glued and filled.
The brackets we used are similar to those, they are 6mm steel. I don't think they come out far enough, as we only used 250mm brackets. Those were the largest I could find locally.
Just interested to see if anyone has any ideas on how to add further reinforcement? Maybe screwing up another substrate underneath the existing substrate (i.e. laminating it) with ply?
I would never have done that design with 2cm stone, we use mostly 3cm here, without significant steel bracing, but I think it is more common the use thinner stone in other countries.
How much deconstruction are you willing to do? I can't see where laminating a layer of plywood just to the overhanging portion of the substrate would be of any help.
I think the best solution would be to lift the top and install more and better braces, but that comes with risk and a lot of work.
If you can remove the waterfall end panel you may be able to reinforce the miter joint by drilling and epoxying in something like these:
https://screwsandbolts.com/products/90-degree-right-angle-dowels
But that is uncharted territory for me and I can see a fair amount of risk of damage.
Other than that you may need to find a forum for stone workers not woodworker for better advice.
25mm mdf is not strong in any way, it would deflect on its own weight over that span and overhang. It however makes an excellent core material if laminated between two sheets of thin plywood or even better, two sheets of Formica. It then forms a I-beam with the lower laminate resisting tension and the top layer compression .
I don't know where the MDF is positioned but the key is making the substrate rigid. If the MDF is on the bottom of the granite I suggest you create some wide grooves in the MDF and place some 1/8 to 3/16 flat steel strapping screwed into the MDF. I have done this method on a really long maple island top with a 20 inch overhang and there is no sagging.