Slab granite install

[Brandon Whitmore]

I just inspected a flip house where the slab granite countertop was installed directly onto the pre- fab cabinet. I have always thought that a base of plywood or MDF was supposed to be installed first. I checked out a couple of sites where installation instructions were posted, and they indicated the need for a plywood type surface.

Anyone have any ideas as to whether it is allowed/ recommended to install the slab granite directly to the tops of the cabinets?

[Scottpat]

Are you saying that the granite was placed on top of the old counter top and not on a raw wood surface like plywood, etc?

Personally I don't see why it would be a problem. Now, what I have seen is that when granite tops are placed on older cabinets and the cabinets were not designed for the additional weight of the stone, well Houston we now have a problem!

The cabinets need to be reinforced so that the weight of the new top does not compact and bind the cabinet. If not the drawers/doors will not work properly. This is very, very common on retrofits and even on some new construction.

[Brandon Whitmore]

The kitchen was gutted and new cabinets were installed (pre- fab). There was no counter surface, just the cabinets. The slabs are set directly on top of the cabinet frames.

So far it sounds like a 3 cm ( 1.25") slab does not require a plywood base, while a 2 cm (.75") does. This is regarding the strength of the slab itself and not based on cabinet strength. I guess when the granite slab is glued to the cabinets, it should make them rigid. Of course, I didn't measure the granite, but it looked fairly thin, so I bet it was .75".

Anyone have a copy of ASTM ASTM C615 Standard Specs for Granite Dimension Stone?

[Brandon Whitmore]

Here's a pic.

Image Insert:

3.96 KB

[Scottpat]

I have never seen granite installed without a plywood base to sit on. I would think that it would need support other than the edges of the cabinet frame.

[caryseidner]

Why would it need support other than the edges of the cabinet frame? It looks okay to me.[:-thumbu]

[Scottpat]

Originally posted by caryseidner

Why would it need support other than the edges of the cabinet frame? It looks okay to me.[:-thumbu]

Granite is very brittle, it does not take a great deal of stress or force to cause a fracture.

[caryseidner]

It can be, but it's tensile strength is ranked 3rd on product found in the home. Behind Steel and close to Concrete. I do, however, know first hand how brittle it can be (not a good story), but from the photos it looks like there is a very small overhang. Is your concern that it might crack in the center of the cabinet? That seems very unlikely to me. The counter top is designed and "normally" used to carry a dead load. I wouldn't write this up.

[hausdok]

Hi,

I think that a uniform surface for the granite to rest on would be advisable; only because when the carcasses of those cabinets shrink they won't shrink evenly and the slab could end up cantilevered on a sharp edge and might crack. Last year, while I was talking to a client in the kitchen of a flip, his buddy who'd accompanied him decided to sit on the kitchen countertop. He reached back, lifted himself up onto that granite countertop and then plunked his butt down on the counter, at which point we all heard a very audible popping sound. I said, "What the hell was that?" and the buddy slid off of the kitchen counter. There in the granite was a perfectly straight crack that followed the edge of one of the cabinets beneath the counter. The cabinet next to it had obviously been delivered as the same size, but the wood carcass had shrunk and there was about a 1/8" inch height difference. It was enough; when buddy plunked his fat butt on that countertop, it cracked the countertop.

The client bought the house (he would have bought it anyway) and sucked up the cracked countertop. I told him that, when he repaired the counter it would probably be a good idea to remove the counter and put a uniform surface on top of those carcasses in order to support it and make sure it didn't crack again. I don't know how he dealt with his buddy but he looked pretty pissed about it.

ONE TEAM - ONE FIGHT!!!

Mike

[caryseidner]

Good point Mike.

[Home Pride]

Wow, maybe some of you have weak-a$$ granite slabs, or something. It is never installed on plywood around here.

The slab is resting directly on the cabinets in thousands of houses, and holds up just fine. I doubt the dabs of silicone used to install it matter much.

Dom.

[Brandon Whitmore]

Dom,

From what I have read, it is more common on the West coast to use the plywood base, since we use a lot more 2cm material.

3 cm. is supposedly more commonly used on the E. coast.

Any idea what thickness you usually see?

[Tom Raymond]

Brandon

In my experience 2 cm is for comodity tops, like prefab vanity tops or blanks for the small fab shops, but kitchen tops are almost always 3 cm and premium tops are even thicker. No backer is ever used, but additional supports are some times used on larger boxes. I just did a solid surface top (polyester) 1/2" thick material that required at least 2" wide supports every 18" but was not recommended for aplication over a solid substrate.

Tom

[Home Pride]

Brandon, around here the slabs are 3 cm and up, except for some flip specials, where they occasionally use the 2 cm stuff.

They do install cleats along the back walls between wide spans and dishwasher openings.

Dom.

[charlieb]

Originally posted by Brandon Whitmore

The kitchen was gutted and new cabinets were installed (pre- fab). There was no counter surface, just the cabinets. The slabs are set directly on top of the cabinet frames.

So far it sounds like a 3 cm ( 1.25") slab does not require a plywood base, while a 2 cm (.75") does. This is regarding the strength of the slab itself and not based on cabinet strength. I guess when the granite slab is glued to the cabinets, it should make them rigid. Of course, I didn't measure the granite, but it looked fairly thin, so I bet it was .75".

Anyone have a copy of ASTM ASTM C615 Standard Specs for Granite Dimension Stone?

I'd love to see a reply to this. The MIA manual is 7 or 8 hundred. I've not found a tile guy in my area who has it or will let me get a look at it. I'm also hunting a reference for the max unsupported over hang.

[Brian G]

Granite counter tops for the masses...jeez. Someday soon we're gonna think we can't poop in anything less than a gold-plated toilet. Yeah, I know, granite has some nice qualities, I just get sick of the endless spiralling-up of Average Joe & Jane's expectations. I wonder how many foreclosed houses have lovely granite counter tops in them right now. [:-crazy]

Micro-rant over. [:-taped]

Brian G.

Americans Can't Get No (bah-da-dum) Satisfaction [:-irked]

[Brandon Whitmore]

Charlie,

While researching this, I remember some granite expert writing that the max. is 8" for 3cm. (based on memory). It sounded like he was basing this on the Marble Institute standards, and mentioned ASTM standards-- I wish I had the link or remembered where I read this. Other guys on the forum were saying the max. was 12" when he cut in with specifics.

[charlieb]

Originally posted by Brandon Whitmore

Charlie,

While researching this, I remember some granite expert writing that the max. is 8" for 3cm. (based on memory). It sounded like he was basing this on the Marble Institute standards, and mentioned ASTM standards-- I wish I had the link or remembered where I read this. Other guys on the forum were saying the max. was 12" when he cut in with specifics.

I hunted and read quite a bit a year or so ago but had nothing to hang my comments on. I did a McManson 2 weeks ago with granite. There was a crack in the island top that was perpendicular to the overhang. The unsupported depth was 14".

It was a real piece of work. I spent an additional 5 hours on the report once I left the site. Wind and water entry. The roof was a tear off and the siding should have been but I got the feeling the buyer wasn't going to push things that far. Mr. & Mrs. ATTY closed and moved in about 6 weeks before it was "complete" and the builder lived next door. "But our kids play together". This is their dream home. Provisions were in place for an elevator. They intend to die in the house.

Image Insert:

27.94 KB

Image Insert:

38.72 KB

Image Insert:

20.33 KB

Ignore the siding. Look at the EDPM

Image Insert:

30.14 KB

Any one ever seen a smoke chamber like this?

Image Insert:

21.73 KB

Image Insert:

33.63 KB

Image Insert:

19.09 KB