levelling cement to flatten table top?

[Bob Riefer]

I received two free things recently that I thought might be able to be paired together.

1) levelling cement

2) corian counter


My thought is that a cheap plywood table could have its top nicely levelled by adding a temporary lip, pouring properly mixed levelling cement, and then topping with a known flat/stable material (such as MDF or corian). Since the levelling cement is "self levelling" it should theoretically be a very nice/true surface. And once covered and protected, it would make for a nice heavy table with good reference surfaces.

Seeing as all components of this project are free to me, I'm willing to give it a try, but would be curious to get any feedback.

[John Nesmith]

Interesting.

I have occasionally pondered something similar using that thick bar-top epoxy stuff. I too am interested in replies.

[David Cefai]

I will get some first hand knowledge of self-levelling cement in about 10 months time - it will be part of a floor I have specified for a project. So my contribution is theoretical.
You will need to seal the plywood, otherwise it will absorb a lot of water and bend.
However, why bother with the cement? Isn't the corlan rigid enough to bridge over any imperfections in the plywood?

[Nick Laeder]

I'm in commercial/industrial construction management. We use self leveling grout to repair floors all the time. You'd need to protect the plywood, however, I'd use something more than sealer. You'd need to puddle the mix pretty good to acheive perfectly level results, but it sounds doable to me. I'd wait to put the MDF on until the grout has cured fully. Keep the grout wet throughout the whole curing process, it'll get stronger and harder that way.

[Chip Lindley]

I have to wonder about the flex in a countertop and it's effect on the thin layer of leveling cement poured over plywood. Levelling cement is usually reserved for making concrete slabs flat; not above ground structures. Keeping the grout lines in ceramic tile on upper level floors from cracking is an iffy proposition. Latex additives must be added to tile grout laid over cement board; not a wood product like plywood or underlayment.

The water in your levelling cement may cause more problems than the cement solves. A search gleaned the following info about Corian which seems generally accepted in it's installation:

DuPont more than discourages the use of any substrate under 1/2" or 3/4" material and does not allow 1/4" Corian to be used for tops. Reasons are several. 1st: Corian does not need a subtop. Corian is strong enough. 2nd: Corian expands and contracts with heat; wood with humidity. The movement may be small, but the forces can be great. Third, a subtop would trap localized heat, like from a hot pot, increasing possibility of failure. A cross support every 20 to 24 inches is all that is required.

But, it's an experiment. And, materials are free. Not like it's an expensive kitchen remodel!

[ian maybury]

Some of this may or may not translate into your situation Bob, but here goes. My experience (in Ireland) is in the context of leveling a timber T&G (tongue and grooved) planked floor (over joists in an upstairs bathroom) before sheeting with 3/4 in WBP (waterproof) ply and tiling. This is standard practice over here, it's only very recently that cement based backer boards have started to appear.

In this case the practice is to make sure the T&G nailing is tight adding screws as needed, level it with the finest (highly fluid) compound, and after it has hardened prime the ply on both sides and then screw it down with countersunk screws on about a 4in grid spacing. It's important they reckon that both the leveling compound and the tile cement contain a lot of polymer for flexibility (and that the grout gaps are pretty wide) so they can handle whatever flexing both traffic/spring in joists and seasonal movement induce in the floor.

Use materials from the same system throughout to ensure compatability, and be careful that the one you use is top quality (e.g. PCI/BASF in Germany do really good stuff in my experience) - there are some very poor quality ones about over here. My floor is nine years old and subject to heavy traffic, is over not the most solid of T&G floors and there is no sign of cracking anywhere. PCI had good manuals detailing the process. Be wary of technical advice from sales people which in my experience is often suspect - work from the maker's manual.

I'd be cautious of differential expansion causing warping (ply over T&G is probably not bad in this regard, and the screwing down of the ply probably leaves it free to move a bit - especially if the holes are a little oversize. I'd want to go very carefully if using the Corian - in that it could depending on whether its coefficient of expansion is different introduce issues. Adhesion might be another problem if for any reason you needed to bond it. Plus it may be a bit slippery as a work surface - some testing might be wise.

I'd also want to be sure the base platform I was using the compound to level was strong and weighty enough not to deflect when hammered on, and stable enough to more or less retain whatever contour/flatness it started with as either could easily break free the leveling compound. This could rule out using plain sawn timber unless it was laminated from strips on edge a la traditional bench construction.

It'd be preferable not to end up with countersunk screws all around the top surface (it'd make locating the dog holes interesting - it'd have to be planned from the start), but if this looked like a good idea (it'd at least leave it all capable of being stripped and re-done in the event of problems) then maybe you could sheet the top using hard board (some say 1/4 in MDF may be better/more inclined to lie flat) put down using a (low strength?) contact adhesive.

ian

[Mike Cutler]

Bob

Are you more concerned about flat, or level?
The leveling compound will seek level, regardless of the orientation of the top. If the top is not level the compound will flow over the sides.
You will need lath and the primer/sealer for the leveling compound. It doesn't go straight on top of plywood. The plywood is primed and sealed, the metal diamond lath affixed, then the leveling compound is poured.
You will need a very heavy 1/2" drive corded drill, and a paddle mixer for SLC's.
Personally, a medium modified thinset, screeded to flat, would accomplish the same objective.

[Matt Mackinnon]

I have always wondered about how flat a table top is. I have a table saw that has cast iron wings that bolt on. When I got them, I used a straight edge to check how flat they were. There was a definite bow in both of them, and I was all set to send them back. My father was over to help with the lifting part to assemble the saw and he said to me 'what are you worrying about, just bend them flat'. I didn't believe him. but with assembly, we actually flexed the bow out of the wings and they are perfectly flat on the table saw.

The part to this story is that is really doesn't matter how flat the table is, if the floor you put it on isn't. The table itself will also flex under weight, and move your perfectly flat table to another spot on the floor and it won't be perfectly flat any more. Perhaps it might be more advantageous to use the self leveling concrete to flatten some floor so that your bench will stay consistent when or if it gets moved.

Matt.

[Steve Schoene]

It's not all that hard to make a flat table and set the Corian on it, holding it in place with silicone adhesive. I don't see that the leveling compound adds all that much.

[george wilson]

I think you will end up with a mess that you will have to break up and toss.

[Bob Riefer]

damn, just lost my reply.. Here goes again..

Mike asked if I was aiming for flat or level.. My brainstorm (err... brain freeze? ha ha) was that levelling cement, by seeking its own level (like water), would by definition create a flat surface. That flat surface would then be topped to create a nice reference surface as a larger alternative to my table saw surface which is the best that I ahve going for me right now.

But, from the replies, I think I'm better off building a different style table and using the corian in the rec room to top the bar etc. on the 2nd story of the barn.

Thanks for the replies!

[ian maybury]

Self levelling is only approximate, even with the thinnest compounds - it needs some minimal screeding. I'd be inclined to agree that it sounds like a bit of a long way around job...