Best glue for tension?

[James Baker SD]

I may have the terminology wrong above, but I need to know the best type of glue to use for a project. I have an existing kitchen island whose top in laminate on MDF (or something similar). It has two cutouts that use to contain glass cutting boards/trivets on either side of the cooktop. The glass was supported by aluminum frames that both held the glass and overlapped the cutout to support the surface. The frames were horrible dirt traps and always looked ugly. Plus any spills or boil overs always leaked into the drawers below them.

The wife decided we wanted granite surfaces mounted flush with the laminate so that cleanup would be easy. We had granite "cutting boards" made that are about 22"x16". I have routed one opening to the size of the granite. It should slip into the new cutout with a very snug fit. The granite surfaces weigh about 10 to 15 pounds each.

My plan is to install a sheet of 1/2" baltic birch ply under the cutouts and let the granite rest on the plywood (a little more complicated, but that is the general idea). I want to glue the plywood to the underside of the counter top which is raw MDF. The force of the weight of the granite will be perpendicular to the glue surface and will try to separate the ply from the MDF by pushing it straight down. There are drawers below the island top so support from the floor is not possible. I will try to also glue some supports on the vertical dividers (also MDF which define the drawer size) near the edge and for these the force will be parallel to the glue surface. I am oversizing the plywood piece making it quite a bit larger than the cutout to maximize the glue surface. I know the screw holding ability of MDF is pretty poor, so I think I need to rely on the glue.

So the question is what type of glue will best hold up to the forces trying to separate the plywood from the MDF? A different glue type for the two different force directions, that is one glue type for the plywood to underside of countertop and a different glue type for the supports attached to the verticals?

Any of approaches that might work which I have not dreamed up yet?

Thanks, James

[Jon Holmes]

I don't think you should obsess over the glue choice too much. Basic yellow wood glue (e.g. Titebond) will be strong enough that if you ever load this surface to the breaking point, it won't be the glue that fails.

You're right that MDF doesn't hold screws well, but that doesn't mean they're totally pointless. In addition to the glue, I'd use several short, fat screws through the ply and into the MDF. 1/4" sheet metal screws, or something in that ballpark, would work. Just as the large overlap between plywood and MDF gives you a lot of glue area, larger diameter screws have more thread area to spread their loads over. Drill clearance holes through the ply, pilot hole the MDF, and be careful not to overtorque during assembly. Besides the strength they add, the screws will serve as clamps for the glue joint, in an area where other methods of clamping would be difficult.

I think added supports on the cabinet verticals are probably unnecessary unless there are areas where the routed opening is very close to a cabinet side and thus gives you little room for glue.

Besides the vertical load of the granite and whatever you place on top of it, I would be concerned about spilled and splashed liquids getting into the joint and soaking into the MDF. I'm thinking that you might want to put a couple of coats of polyurethane on that routed edge, and also caulk the joint after the granite is in place.

One more idea: drill some large holes through the plywood, near the corners, before dropping the granite in place. That way you'll be able to push the granite up and out of the hole from below if you ever need to, so you don't have to turn the island upside-down and shake it.

[Myk Rian]

You do realize that anything dripping down the sides of the granite will soak into the ply.
I would replace the whole thing with granite or corian.

[Rich Enders]

A further on Jon's well thought out suggestions would be installing threaded inserts in the MDF, and then bolting through the plywood into the insert.. Probably the holding power data of inserts in MDF is published, but it seems like they would provide a stronger hold.

I also think that your granite is not so heavy, but a thanksgiving turkey, or worse someone standing on them to paint the ceiling is not impossible.

[Phil Thien]

I'd make the piece of plywood as large as you can possibly make it, and still fit it under the MDF. Otherwise the MDF is going to want to sag and the granite will sag with it.

To fasten the plywood to the MDF, I'd spread a generous layer of yellow glue on the plywood (avoiding the area of the cutout) and then staple or screw it from underneath.

Myk is right about liquids getting in the seam, you're going to have to use some silicone around the perimeter or something.

[scott vroom]

I think you're asking for trouble. Any moisture that gets in the seam between the granite and MDF is going to cause problems for both the MDF and baltic birch. I agree with Myk's suggestion of replacing the entire counter with a granite or corian slab. If that's not an option, then I would lose the 1/2" baltic birch and replace it with a piece of 3/4" exterior glue plywood with no side less than grade C. I'd attach the ply with a quality polyurathane glue, such as PL Premium, and screws (just to hold the ply in place while the adhesive is curing). To minimize potential moisture damage I'd paint 3 coats of Redgard on the plywood and MDF edge and seal the final seam with 100% silicon caulk.

[Judson Green]

Yeah definitely use some silicone around the seem. Even if the spills don't effect the ply supporting the granite, liquids might cause the MDF/particle board substrate to swell.

[James Baker SD]

Thanks for the ideas. Unfortunately replacing the whole island top is not an option at this time. (kitchen has matching laminate counters along the walls and a matching built in desk--too much area in total for the budget)

I had planned on trying to seal the MDF and the ply to protect against moisture and had definitely planned on a line of silicon around the edges. We always wipe the spills up quickly, but even a few seconds is enough for the current glass panels to leak; I was hoping that with a good tight fit and silicon, a quick wipe up would get the liquid before it had time to sink in.

I know they will have extra weight on them and we need to be careful. I have already pushed one of the glass panels completely out of its frame and into the drawer below when I was painting the ceiling last Spring. Luckily that first drawer is only about 6" below the surface so nothing broke (glass nor my leg). That was probably the final straw that prompted replacing them.

I plan on having threaded T-nuts in the ply and nylon tipped set screws in them that I can adjust with the drawers removed. This would allow me to level the granite to the laminate as closely as possible. I could remove the setscrews and replace them with long screws if I needed to remove the granite.

Behind the cutoff I have a lot of space and the overlap of the ply is about 6", the sides will only have about 2" and the front about 3/4" so I think I will want the side supports at the front at least.

Right now I am enlarging the cutouts with a template made by the granite cutter that fits the granite pieces perfectly. I am using a straight bit and squaring the corners with chisels. I had planned on trying to force the silicon down into an almost invisibly thin gap with a putty knife. Would it be better to use a chamfer bit on the laminate at the final stage to make a slight channel for the silicon?

Again thanks to all.

James

[Phil Thien]

I plan on having threaded T-nuts in the ply and nylon tipped set screws in them that I can adjust with the drawers removed. This would allow me to level the granite to the laminate as closely as possible. I could remove the setscrews and replace them with long screws if I needed to remove the granite.

A bead of silicone is going to make removing the granite a challenge. If there is a chance you'll need to remove the granite, I'd add a few larger holes in the plywood through which you can push from the bottom.

[Andy Pratt]

This seems like an issue that is likely to cause heartache in the future. This is a water/debris trap and no one wants that sunk into their countertop. Is there a way to fix the fundamental issue here, maybe span over the hole with a rabbeted insert cutting board that you caulk in? This could be as thin as 1/4-1/2" above the countertop depending on how you did it.

If you leave a vertical gap that water/debris/grease will find you are only going to have unpleasant issues in the future when things invariably find their way into the crack. Caulk it and now you have an eyesore on your counter, you're in a bad situation if you go this route, I think.

To answer your original question: if you clamp properly (which is really hard in this case unless you carefully use screws as your clamp force into the mdf through the underside of the plywood) you won't have any issues with glue strength, if you give yourself 3-4 inches of glue space around all sides of the perimeter and do a high-quality clamp job (key), it will hold the granite weight plus any shock you put on it. Get the glue joint wet/warm&wet and now it will probably fail on you, so keep that in mind if you go this route.

[Jerry Miner]

I don't think glue strength is the issue here. Will the mdf have enough integrity to not split apart next time there is a large weight (like a painter's foot! ) placed on the granite? I would cover my bets with some support on the sides---maybe some light metal angles screwed to the cabinet verticals, or wood cleats even.

The T-nuts are a good idea, but I would add a mastic of some kind so the granite is not resting entirely on four bolts. Bolts for leveling, mastic for long-term support (the granite guys use a polyester resin or epoxy for this, but you could use construction adhesive or silicone or ...)(you can use some plastic sheeting as a "release" if you want to make removal easier)

Seal the edges, seal the joint, go for it. What's the worst case? You step on it again, destroy the laminate top, and replace it with something better!

[James Baker SD]

In my defense on stepping through the glass panel, I did have it covered with plywood to prevent that, but I kicked the plywood while looking up without realizing it enough to allow my foot on the glass.

I did get my answer though, it seems the choice of glue is the least of my worries. Plan is to give it a try and if it is a miserable failure, find the budget to replace all the laminate surfaces. The wife picked out the piece of granite and she loves it, so I have to give that a try first. After all, she approves the tool budget.

Again, thanks to all.

[Jon Holmes]

Would it be better to use a chamfer bit on the laminate at the final stage to make a slight channel for the silicon?

Yes, this is a good idea. The chamfer would do two things for you: it would increase the surface area to which the silicone would be stuck, and it would increase the size of the silicone bead, making it better able to tolerate any movement between the granite and MDF.

I'd be tempted to raise the granite panels a bit above the surrounding laminate, maybe somewhere in the neighborhood of 3/32", with the granite edge just slightly rounded over. This would forgive any irregularities between the very flat granite and maybe only sorta-flat laminate, and it wouldn't be a problem to set down a hot pan with its bottom overhanging the edges of the granite because the pot wouldn't actually contact the laminate.

With the edges of the MDF sealed and the joint caulked, I can see this solution working quite well.

[Curt Harms]

Yeah definitely use some silicone around the seem. Even if the spills don't effect the ply supporting the granite, liquids might cause the MDF/particle board substrate to swell.

No 'might' about it. That's the primary reason we replaced our laminate countertops - the seal between the sink and countertop failed and water got to the particle board. I like Jon's idea of slightly raising the granite slab as long as it wouldn't look strange. It seems like it'd be easier to get a liquid resistant seal than with a flush install.