Granite Countertop Underlayment

[craigwatanabe]

Re: Granite Countertop Underlayment

When it comes to sheer strength support you can go down to as thin as 3/8" for walls. For countertop support I'd go 3/4" as it's the thickest ply you can get. When it comes to sheer strength in walls you're looking at the lateral strength so thickness isn't as important as tearability. But in countertops thickness does count as weight can vary, such as a 250-lb carpenter standing on that granite counter with no underlayment or supports too far apart to handle that load.

For granite you can seal it to protect the underlayment. If you're using tiles or non-continuous slabs with grout then you should seal at least the grout annually. For marble countertops sealing is a must as marble is porous.


Escondido100 when you're in the Hilo store come visit me I work at the Hilo 8453 store. You must know John Bey. He does the estimations for our store for price quotes. You may also know Mark Kitagawa of Big Island Countertops who imports granite countertops from China by the container.

[cezanne]

Re: Granite Countertop Underlayment

after reading escondido100's post, my conclusion is that the "underlayment" is for structural/support?

Correct. Thanks for the info Escondido.

[craigwatanabe]

Re: Granite Countertop Underlayment

I'm toying with the idea of making my own cabinets someday. Just learned how to make the doors with the mortise & tenon joints and floating panels. I was lucky and someone gave me the necessary router bits.

I enjoy doing things like this but don't know if I could do it as a job. Besides, which, I'm way too slow!

Whatever you do make sure you have an ample supply of cabinet hinges. I deal with many customers who come into our store asking for direct replacement compound-joint cabinet hinges only to find out that because of the location of the forsner-bit mounting point was proprietary to that particular hinge, they cannot get one of our "Knob Hill" or "Liberty" brand hinges to fit that mount location.

The 6-ply is a 3/4" cabinet grade plywood we sell for I think about $30 (I think) for a 4x8 sheet. It's not treated as Escondido100 indicated.

There is a green hardi-backer board that is sold in the Flooring department (and not in the Building Material department) of HD. This is definately not the drywall or greenboard but is a solid cement-fiber board that weighs a lot more than the greenboard.

In any case I strongly believe using an underlayment is good. It helps strengthen the cabinet the countertop is sitting on as well as strengthen the subsurface for which that countertop is attached to. Remember structural strength is defined by it's frame and not by it's facade. In home building some building codes require the use of sheer walls under the T111 exterior panel facades to hurricane-proof the walls of a home for lateral support.

[salmoned]

Re: Granite Countertop Underlayment

Nuts, if you develop a sagging frame, then you must've gone el-cheapo on your cabinets. I've never seen such a problem, since the framework is always entirely supported by the cabinets.

[Beachboy]

Re: Granite Countertop Underlayment

Nuts, if you develop a sagging frame, then you must've gone el-cheapo on your cabinets. I've never seen such a problem, since the framework is always entirely supported by the cabinets.

who are you talking too? I don't recall saying anything about sagging framework. I started this thread to better understand the plus & minuses of MDF use in Hawaii. To me there doesn't seem to be much difference between particle board & MDF....both are nightmares in my opinion, and now I have my granite counters sitting on top of MDF underlayment!!!

Plus I must worry about my two boys(ages 2 & 9yrs old),and the effects of out gassing from the MFD. The builders never sealed the MFD that was used on my counters!!!

[kani-lehua]

Re: Granite Countertop Underlayment

being that i don't know much about this topic, i'm wondering if the granite slab is somehow adhered to the underlayment or does it just sit on top of it? if it is adhered to the underlayment, would that somehow prevent, "the effects of out gassing from the MDF"? if so, could you then seal the other support areas to prevent the out gassing? is there any way to get the contractor to do it the "correct" way?

[WindwardOahuRN]

Re: Granite Countertop Underlayment

As per the instructions of the granite company from which we purchased our slab kitchen counters we used 3/4" plywood sheets as a base for the countertops. The company installed the countertops so I'm not quite sure whether they used an adhesive or not.
We did the rest of the kitchen ourselves---a true test of the solidity of a marriage . Both the marriage and the countertops are doing just fine six years later.
Another thought---MDF is available in exterior grade. Is it possible that this was the kind of MDF that was used in the counter application? If so, it's supposed to be extremely water resistant. I believe that it's so resistant to the elements that its used for road signs. There is also a low-formaldehyde version of MDF.

[escondido100]

Re: Granite Countertop Underlayment

when installing granite on a plywood sub base i use a high quality polyurethane construction advesive.....you can also use a high quality silcone as an adhesive. as to MDF ...many mdf products have a skin of melamine on one or both sides..... if this is what you have then it would be considered sealed in those areas.... the edges remain unsealed. i rather doubt that an exterrior mdf would have been used in an application such as this since sign board is rather expensive...unless the contractor has adopted this as a technique that he feele is superior to other techniques.....if so that means he has thought this out and would be willing t o explain his logic. and offer a gaurantee for his work beyond the minimal as required by law.
since the granite is extremely stable by itself i would be too worried about the use of MDF ....other than the out gassing and there are different schools of thought on this as well.......i had a client that was allergic to the formaldehyde and when we built a house for them they did a lot of research on products that dont out gas......there are some but it wasnt easy to do..... one of the reasons i got the job is that i was willing to work with them on it......it was alot of extra work.....formaldehyde is in darn near everything..... maybe not so much any more but 15 yrs ago it was.

[craigwatanabe]

Re: Granite Countertop Underlayment

I wouldn't worry too much about the gassing as homes built here in Hawaii are pretty open (meaning not sealed like homes in the mainland). There's lots of cabinets and even those computer desks and entertainment cabinets you by and assemble have some level of MDF. Lots of today's speaker cabinets (your subwoofer in your living room) are made of MDF as well as those sub boxes in cars.

As for countertops, I don't care how sealed it is, once the surface is breached, water can seep in and the damage occurs, that's why I don't like those laminated countertops that have the backsplash and lip that you can buy at home improvement centers (such as Home Depot, Lowes, City Mill) because they're made of particle wood.

[craigwatanabe]

Re: Granite Countertop Underlayment

being that i don't know much about this topic, i'm wondering if the granite slab is somehow adhered to the underlayment or does it just sit on top of it? if it is adhered to the underlayment, would that somehow prevent, "the effects of out gassing from the MDF"? if so, could you then seal the other support areas to prevent the out gassing? is there any way to get the contractor to do it the "correct" way?

When it comes to adhering using a construction grade adhesive, one that most people recognize is Liquid Nails but there are quite a few others out there with differing adhesive strengths and bonding to different material.

But in the case of an adhesive, typically one would incorporate a lazy S pattern of laying down the adhesive so there are many areas where potential gassing could escape any adhesive beading. Plus there's the underside of that particle wood that is the inside "ceiling" of the cabinet the countertop is attached to that is fully exposed to the storage area.

[WindwardOahuRN]

Re: Granite Countertop Underlayment

I wouldn't worry too much about the gassing as homes built here in Hawaii are pretty open (meaning not sealed like homes in the mainland). There's lots of cabinets and even those computer desks and entertainment cabinets you by and assemble have some level of MDF. Lots of today's speaker cabinets (your subwoofer in your living room) are made of MDF as well as those sub boxes in cars.

I was thinking the same thing, Craig. Unless you live in an air-conditioned totally sealed house (and I do know some people who live this way in Ewa Beach and Kapolei) I think the dangers of inhaling the formaldehyde gases would be minimal here.
Windows open, trades blowing...should be okay, I would think.

[craigwatanabe]

Re: Granite Countertop Underlayment

Nuts, if you develop a sagging frame, then you must've gone el-cheapo on your cabinets. I've never seen such a problem, since the framework is always entirely supported by the cabinets.

It doesn't matter if you went cheap or expensive. Heavy weight on any framework will take it's toll on a cabinet. When you put heavy weight on the top you develop a high center of gravity. That weight will put stresses on all joints of a frame. On most cabinets you don't see cross-bracing. It's assumed that the countertop will act as the sheer support for ANY cabinet despite it's price and quality of construction. If the granite slabs are providing that sheer support, they will separate at the grouting or butt points in time. If you have a solid granite top, the stresses imposed on the countertop will crack that counter in due time.

And it's the opposite way around, the framework supports the cabinets as the cabinet is the framework and facade.

[salmoned]

Re: Granite Countertop Underlayment

Huh? The countertop only provides support for what's on top of it. The cabinets are self supporting, as well as supporting the countertop. The countertop lies dead on the spacer frame, which lies dead on the cabinets. That is to say, the only forces on the countertop and frame (or underlayment) are from gravity and both are supported by the cabinets alone. There are no sheer forces in this situation [unless you're imagining a countertop with a large, unsupported overhang (which is NOT recommended)].

[cezanne]

Re: Granite Countertop Underlayment

The only shear forces I can think of in this situation would be if it were an island type of installation. Where there could be a twisting motion viewed from the top or even a lateral shear (since there are no side wall(s) that it's attached to as in a typical wall to wall countertop).

[Beachboy]

Re: Granite Countertop Underlayment

I was thinking the same thing, Craig. Unless you live in an air-conditioned totally sealed house (and I do know some people who live this way in Ewa Beach and Kapolei) I think the dangers of inhaling the formaldehyde gases would be minimal here.
Windows open, trades blowing...should be okay, I would think.

I understand all that, yet in some places MDF is outlawed! For what? because it is deemed unhealthy for humans. Sure most of us don't plan to go to sleep every night with a piece of MDF for a pillow. But I do have small people in my house, and they are everywhere in this house! They tend not to wash hands very often, yet love to touch everything they see! So, yes this could be a problem.
If just one country outlaws MDF, it's enough for me to consider the ramifacations of using MDF. I wish I would have been given the choice here with regard to using toxic building materials on my house!