Installing laminate floor on unlevel slab

[rling]

Hi all, first post, go easy... :-)

I was planning to install a laminate floor over a concrete slab, but now I found the slab is not level... in fact it resembles the surf off Hawaii during a tropical cyclone. In the main 5x8m room, the floor bulges up about 3cm in the middle, and drops away about 3cm below average along one wall. In some places it drops 1cm over about 50cm.

I can think of two solutions:
-level the slab itself, either by filling the valleys, grinding down the hills, or a combination of both; or
-build a subfloor, using sheets of wood on battens, level with the top of the bulge.

On the first option, the slab is a first floor in a block of units and another post here suggests grinding down slabs is dangerous and expensive. But to raise it (up to 6cm in some areas), will need LOTS of self-leveller. (Did I mention that a 3x4m room adjoining the main room is about 3cm lower? :-)) I'd be interested to hear ideas on this including ballpark estimates of cost.

On the second option, what should I use for the subfloor (thick particle board? plywood?) How thick must it be, and how far apart the battens, to support normal loads like heavy furniture? Would a typical floating floor (15mm wood veneer) directly on battens be strong enough?

Are there other options I havent thought of?

Cheers, and thanks in advance for any help...

rling

[glock40sw]

G'day.
For a first post, you really picked a dooozie :D .

It may be practical to have the slab "topped".

Have it done buy a pro. then install the floor over it.

Hooroo.
Regards, Trevor
Grafton.
The only true home of Australian Hardwood Timber flooring... G'day Bruce :D

[rling]

Hello all,
I had a little visit from a levelling specialist in Sydney.

First off we established the extent of the problem: the height difference from highest to lowest point is about 45mm. These points are only 4m apart!

Next, our first guess at the damage: 125 bags of self-leveller to raise 50m2 an average of 30mm, at $70 a 20kg bag (laid) is $8750! This option was quickly ruled out because the floor would flood various important things like doorframes, the dishwasher would no longer fit under the bench, etc etc.

The next plan was to slope the floor off the highest point at a more gradual angle, within the tolerance for the flooring, and finishing flush with existing doorframes etc. Damage: 70 bags and $4900.

Self-levelling compound is apparently the only option for filler: sand and cement is too fragile, and concrete would take a couple of months to cure sufficiently to receive a hard floor.

If I want to try lowering part of the slab, a 240v scabbling machine (which may or may not even make a dent) plus operator is $1800 per day. A bigger 3-phase machine, on a truck, with generator, etc... well, the sky's the limit. That's without considering the structural danger of thinning the slab.

So I really need to price the option of building up an elevated subfloor on battens. I am happy to DIY that option if I know what materials will make it strong enough.

Thanks for any info...

[alexeib]

just keep in mind that for floor needs to be level not horizontal.
How about a combination of grinding and levelling?
You could also lay and fasten sheets of ply, that will span some imperfections.