under granite plywood

skippy_2008

15 years ago

My fabricator showed up with the plywood to put on my countertops for my granite install.The problem is he showed up with what looks like cheap particleboard plywood.

it looks like compressed wood.My question is, is this ok or should I say something?is it speacial treated or something or is he just cutting corners.Never have been thru a granite install before so I don't know if this is standard or not.Please let me know what you think about this.He is coming back Wed to do this.

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Comments (10)

Circus Peanut
15 years ago
Honestly, I'm not sure most granite counters have anything under them at all between them and the cabinets. How thick is your granite, and is this over cabinets, or overhang on an island?

If you do need particle board, try to see if it's the water-resistant kind. This is the same stuff they use under most formica counters etc, and there's nothing wrong with that per se, but I'm curious about why there's anything under there at all.
raehelen
15 years ago
Is there such a thing as particle board plywood? Or is it particle board?

Can you take a picture?

The Stoneadvice.com forum is another good place to ask this question.
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The fabricator's plywood subtop viewpoint seems to me to be valid only if cabinet wood is significantly stiffer than the plywood. Otherwise, their argument would apply equally well to the supporting cabinets themselves. Exterior plywood has pretty stiff resin between layers, so its flexibility where weighted by the granite should be low. On the other hand, how well plywood would support an overhang is questionable. It would seem to me that a box channel beam with section height large enough to provide good stiffness would be large enough to weaken the granite if a slot that deep were to be cut into the granite to hold it. A shallow slot for locating the beam would be ok, but all beams and slots must be precise so support is equalized. If, as implied by cloud_swift's photos above, the plywood is mainly a means of retention of the beams, which do all the supporting, then the fabricator's plywood flexibility argument is seriously, er, weakened. I would imagine that the following would work as a conceptual approach. Adhere beams to the granite as required for support. Adhere plywood or other wood in between the beams that is barely lower in height. Use the wood as the point of attachment for whatever covering trim is desired. For a bar overhang without using typical corbels, I think I could use polished or brushed stainless steel beams (thickness TBD) set into the supporting half-wall framing. A scheme to keep the lifting ends tied down and of uniform height would be needed, as would a means to equalize the pivot side heights and keep the beams from compressing the wood there. More simply constructed, if flatness is ensured, is the steel plate method used by sherilynn shown near the top of this thread. I might opt for a thicker piece, depending on some analysis. Other approaches could be equally valid. kas
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What is the nature of the failure on the plywood you're referring to? I've probably laid hundreds of thousands of square feet of plywood over concrete and can't think of one failure I could attribute to the installation method (i.e. the failures have all been caused by other factors, like a flood). Granted we go a little overboard in that we flat trowel cold bond adhesive on the concrete at 50'/gallon, lay in heavy poly sheeting, trowel another layer of mastic over that at 32'/gallon, lay in the plywood, shoot it down with 1-1/2" hardened coil nails and fasten any loose areas with tapcons. Before sheet vinyl fell out of favor we used it under plywood all the time in conjunction with adhesive. I've had a few jobs recently where the client wanted something better that visqueen so we used a membrane like Traxxshield99 with seam tape. The plywood subfloors I have seen fail due to installation method, are almost always laid dry over a single layer of black felt paper (which is not a suitable moisture barrier), then shot or screwed down. I don't think there's any need to build up a subfloor like that and affect the relationship to doors and steps, but if the advantages were deemed to be beneficial, I'd recommend a moisture barrier, 1x4s fastened to the concrete 12" on center and a layer of 3/4" plywood over that, with the joints laid on the 1x4s. This can be a little complicated because it would be nice to have p.t. lumber for the 1x4s but they're generally soaking wet upon purchase and there may be an issue using steel fasteners.
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If the sink is installed properly and not leaking, installing with or without plywood is irrelevant. I wouldn't want plywood supporting a sink as it can't push a sink upward. The plywood should be cut completely away from the bottom of the countertop and the sink strapped in place with a Hercules Universal Sink Harness or a Sink Strap or two.
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igloochic
15 years ago
Some granite (especially most of it in California) is only 2cm thick Circus, which is why they need a plywood substrait (is that the name? anyhoo)

My contractor is building the counters for our stainless steel and he is using plywood, which was specified by the manufacturer. They won't install on particle board. I took the onus to take that one step up and use marine grade plywood, which is overkill, but definately provides peace of mind.

Now how often do they really see water...probably never but to me, it's in a water area....so I want my counters to be well protected just in case.
pharaoh
15 years ago
we built our cabinets with solid tops ie. 3/4" plywood on the top. so the granite (2cm) sits directly on the cabinet (no extra plywood needed).
if your cabs are hollow on the top, follow igloochic's advice. particle board doesn't cut it...
bill_vincent
15 years ago
My question is, is it really particle board, or is it OSB (oriented strand board) or Advantec? I don'[t do slab countertops as a trade, but where the particle board might be a question mark, the others, I'm pretty sure wouldn't be.
pbrisjar
15 years ago
Here's what our plywood looks like:

That's a pic of the underside of the overhang.

But yeah, it could be OSB. To me it looks a lot like particle board.
skippy_2008
Original Author
15 years ago
ok so yes i checked and it is osb board.
and i do live in so cal.so is this ok?
thanks for all the replys by the way.
bill_vincent
15 years ago
Skippy, you're fine.
skippy_2008
Original Author
15 years ago
thanks bill,
as you can tell we are new to all this and just don't want to be taken because we are getting high end granite and want it to be done right.
Diana Powell
5 years ago
last modified: 5 years ago
I asked a floor guy put Plywood and Screw it down in the Bathroom of a Rental Property. When I came to pay I saw it was OSB in the Bathroom we were remodeling. I asked why they nailed vs. screwing it down they said "Thant's the way it's Done!". I wasn't happy but paid them & didn't know what OSB was and thought it looked strange...Then My Tile Guy got upset that they put OSB vs Plywood in as well as Glued & Nailed it down since the Plumbing wasnt done and he could have lifted it to put the drain in etc. I let Carlos the Floor Guy Know that OSB should not have been installed in the Bathroom & wondered why he didn't buy the Plywood & screws as specified yet instead of acknoleging the fact that he didn't do the 2/3 things i asked him to do (1) Install a Plywood floor and 3) screw it down...As I also told him I needed extra screws for the deck etc,. yet he just got beligerant telling me my tile guy was stupid & should put redex over it. That is an extra step that could be avoided if Carlos the Floor Guy would have applied the plywood as specified. Also they didn't put blocks on the ends/edges so it sags. Then My Granite Guy put OSB even when Plywood was Specied. So I asked him to change it out for plywood...He got upset yet agreed to Change it as I had ot pay/as he charged extra for it etc. even though I contracted him ot put plywood down...I paid for the materials and labor, yet don't know why the guys I hired did't listen & picked up and charged the OSB on my CC when Ply was specified...They all say it is the industry standard yet I don't see where they are getting this information from? Prior to the floor or this year I didn't know what OSB was before. I guess I learned the Hard way since I already paid them. But just won't hire the floor guys again /recommend them...

under granite plywood

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