Replies

1. 
   Kowboy | Nov 18, 2006 05:19am | #1

   I've only known granite to sit directly on level cabinets. Dab a little silicone and call it good.

   Kowboy

2. 
   Dave45 | Nov 18, 2006 05:25am | #2

   I just did three bathroom vanities and the granite guys wanted 1/2" plywood on top of the vanities.  These have top mount sinks and I made the cutout in the plywood when I installed the vanities.

1. 
   calvin | Nov 18, 2006 05:35am | #3

   Did you set it inside the face frame or on top and trim it out.  All the granite/solid surface I've seen around here is set directly on top of the cabs frame.

   A great place for Information, Comraderie, and a sucker punch.

   Remodeling Contractor just outside the Glass City.

   http://www.quittintime.com/

    

   Edited 11/17/2006 9:38 pm ET by calvin

1. 
   Dave45 | Nov 18, 2006 04:58pm | #9

   I usually just lay it on top.  Those tops had built up front edges with a bull nose detail that hid the edge of the plywood.

   If I'll be dealing with a 1/2" or 3/4" flat slab, I add a molding after the top is installed to hide the plywood edge.

3. 
   plumbbill | Nov 18, 2006 06:38am | #4

   Just finishing up 145 condo building.

   All tops are granite, marble, onyx, or cream of gravel pit (slab guys words not mine)

   Avg 6 undermount sinks per condo, KS are SS & lavs are china.

   All sub tops were 1/2" plywood min with most being 3/4".

   Most counters were 3/4" slab.

   I had them cut the sub tob so the sink would just barely fit.

   I used spacers that were the same thickness as the subtop & used the factory supplied clips to anchor the tops to the plywood subtop.

   I don't like using wedge anchors in natural stone --- too much risk of splitting the stone.

   “It so happens that everything that is stupid is not unconstitutional.” —Supreme Court Justice Antonin Scalia

4. 
   Karl | Nov 18, 2006 06:53am | #5

   I specify 5/8 " ply for 2cm granite with a laminated edge(4cm). The laminated strip of granite hides the ply. If you use 3/4" ply it sometimes shows under the granite edge. Just cut out a hole that allows the sink lip to hang on the plywood.

   For 3 cm granite no underlayment necessary. There is a trick way to slot the underside of the granite in a v cut with a diamond blade to catch flathead machine screws which hold the stainless steel sink clips in place.

   I learned the screw slot technique from a fabricators website someone on this forum posted a link to. It works great on 3 cm stone.

   Karl

5. 
   JDRHI | Nov 18, 2006 06:55am | #6

   That depends on the thickness of the granite.....a 1" or 1 1/4" slab shouldn't require any substrate....unless of course its all cut up....i.e. sink cutouts in close proximity to drop in range tops.

   For 3/4" stone, I typically install a 1/2" plywood substrate.

   Note too......an undermount sink is not actually "mounted" to the granite (regardless of stone thickness). Several sink manufacturers offer prefabbed bracing, but I prefer to build a wood frame inside the sink base to support the sink from below. Sink is set in place before counter top, and then shimmed and siliconed tight to the granite.

   FREE SPONGE BOB,SANCHO PANTS!

1. 
   jja28 | Nov 18, 2006 09:11am | #7

   Hey guys,Just out of curiosity. For those who don't use any plywood under the granite and just set the granite on the cabinets.... what kind of span do you feel safe with with regards to let's say...sitting on the countertop? I remember when we were kids... we always use to sit on the counters, both the kids and grown ups (when the kitchen became too crowded). The cabinets had plywood tops which were covered with mud and tile. I would think that the plywood supported the weight across the span between cabinet walls.I realize granite is one hard material, but is granite a strong enough surface to do that? If so, how far do you feel comfortable with that? For example, if you have a 30" base cabinet with 2-15 inch doors side by side. The granite spanning 30" by 24", can you sit on that? Would you go wider?Thanks,
   James

1. 
   Dave45 | Nov 18, 2006 04:53pm | #8

   I'm probably overly paranoid, but I always put additional support on top of my cabinets - either plywood or 1x boards spaced like skip sheathing.

   Materials used for counter tops (stone, tile, cement, or man-made) are usually strong but brittle and even a small amout of flexing may crack them.  I look at it as cheap insurance.  Adding some additional support is inexpensive and quick.

6. 
   woodguy99 | Nov 18, 2006 06:00pm | #10

   I've never seen a granite guy ask for a sub-top, but I can see where it could be a good idea.  SS sinks are always siliconed to the granite.  I like JDRH's idea of using the plywood to help support the sink, but in 50 or so tops I've been around at installation I've never once seen it done that way.  Bigger sinks get drilled for wedge anchors or the stone guys epoxy on a piece of wood. 

   For spans, I've seen 250# electricians stand on 1 1/4" granite on a 42" cabinet.  Not saying it's the best idea, but it's frequently done.  A painter I know stood too close to a cooktop cutout and broke the counter, but that was just stupid.  12" to 15" overhangs don't need support from what I've seen, but I wouldn't sit on it either.

1. 
   Karl | Nov 18, 2006 07:08pm | #11

   The distance granite will free span varies tremendously between stones. Blacks and dark greens tend to have a lot of tensile strength. Gold/brown colors tend to be very crumbly and prone to breakage.If you get countertops done, try and find a few offcuts from different colors and try breaking a few different colored stones to get a feel for how their strength varies.There is no reliable rule of thumb for how far you can free span. Not only does it depend on the stone itself but also the type of use it is subjected to. Countertops in a retired couples dream kitchen are one extreme compared to a household with multiple teenagers who like to throw parties while their parents are gone.Karl

7. 
   IdahoDon | Nov 20, 2006 04:16am | #12

   I've never had a stone installer not require a 5/8" or 3/4" ply top.  As heavy as stone is it's cheap insurance to hold the cabinets together and better distribute weight.  It seems that at least one of the stone counter top organizations also spells that out under their installation specs.

   There's so much flex in ply spanning across cabinets that I can't imagine that it is to protect the granite over a large span.  However, crossing over the cabinets does reduce the chance that a high point or a more ridgid portion of a cabinet will put too much of a strain in one spot, perhaps causing a crack.

   The ply top also makes shimming a great deal easier.

   Hmmm....   I can't think of a reason not to install a ply top.

   :-)

    

1. 
   woodguy99 | Nov 20, 2006 05:43am | #13

   Don, what happens at the edges of the plywood?  Do you add a trim piece to cover the exposed edge?

   This is a really interesting regional difference.

1. 
   IdahoDon | Nov 21, 2006 04:13am | #15

   The counter top is aproximately 3/4" thick where it goes over the cabinets and there's a strip added to the edge so it ends up looking like a 1-1/2" thick top.  The strip covers the plywood edge.  At least thats the only way I've ever seen granite around here.

   I have heard of a thin 3/4" thick granite top, but with prefinished full thickness tops costing as little as $100 for 8' I can't imagine there's much of a cost savings going with a single thickness.

   :-) 

   Beer was created so carpenters wouldn't rule the world.

1. 
   woodguy99 | Nov 21, 2006 06:15am | #16

   Ahh, that's the difference.  Around here 90% or more of the tops I see are 1 1/4" thick. 

1. 
   VermJr | Nov 21, 2006 07:04am | #17

   We  use 5/8" ACX under 2cm slab and nothing under 3cm slab, except sink bases that will recieve an undermount sink.

   Sink bases for 3cm get 3/4" ACX installed inside the cabinet so that the top of the sink will sit below the underside of the slab, with just enough clearance for the sealant. The last thing the installers do, is to drive some screws through the plywood and pinch the edge of the sink tight to the bottom of the slab.

2. 
   JDRHI | Nov 30, 2006 05:14am | #18

   Yeah.....I don't recall ever doing a kitchen install with a granite top of less than an inch in thickness. Most are 1 1/4".

   "But JD is dead right...."

                           Stephen Hazlett

8. 
   FastEddie | Nov 20, 2006 06:35am | #14

   My granite shop wanted 5/8" ply under 2 cm stone, and nothing under 3 cm.

    

   "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt