Here is my dilemma,

The slab of my extension was intentionally poured 55m below the top of the floor boards of my existing stumped home to allow for 35mm battons and 19 mm flooring. The slab was pretty good when initially done, well so I thought however after running a laser level across it yesterday the slab has high 7.5mm spots in one corner ( the worst) and on the other side It is spot on then falls away to -10mm down on the edge of the bottom plate. The average packing required under the battens would range between 3.5mm-6.5mm but on the most it is spot on the occasional High and low spots. The average batton length (of which I have 28, spaced at 450mm centers and are 9 meters in length.
One batten requires 13mm packing right at the end of the slab and tapers down to 1mm over 5 meters. Today I packed every fourth batten with masonite packers every 1.5 meters (where required). The hardest part is setting the laser level (get a self leveler).
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Just remember some gaps under the battens just taper from large to small and others are kind of wavy. I obviously need support under the entire batten given the thickness of the batten(35mm) otherwise it would flex.
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Now I can do one of many things and here is where I need really good advice/experience or if none of that, ideas on how I can get the top of the battens flat for flooring. I am a little bit of a perfectionist but this task is beyond the joke and I do not mind being 1-2mm out over 3 meters. It is important to get it flat so when the kitchen goes over the top, there is no gap and given the length of the room and the hallway (25meters)I reckon it needs to be done right. Here we go:

1. Buy bigger battens and shave/mill down as required. I think this is near impossible.
2. shave the battens slightly where needed and make packers to fit the low spots. If this is your preferred method, please explain how you propose I do this. Preferably using hardwood of the same width (45mm)
3. I know some of you would suggest a self leveling compound but I have 2 issues with this method
a) I have the plaster on the walls with a 10 m gap from the floor for movement. How would you stop this gap from closing and the compound dribling off the slab
b) On 110 sq meters and some height of 7.5 or 10mm won’t this be too expensive (have not yet researched this.
4. (I tried a section on Sunday) placed the spacers every 1.5m, lifted the batten off, mixed 50/50 cement and sand, spread the mud where the batten would lay( between the spacers) and used the batten as the leveler and the spacers as the guide. I spread it an inch or so both sides of the batten so it would not crack. Well this worked a treat and the batten seems well supported, and perfect height too ( a little messy first time around but if this is the way to go, I think next time will be a little neater). The only thing with this way is, I have no idea if this is the best mud mix? for 1-10mm packing so it does not crack as I would need it to lasts forever and holds the weight of the flooring. I have not yet drilled through it to place a pin to hold down the batten as I want it to dry good and proper (and no, I will not forget the plastic)

I am happy for anyone to shoot holes in the above methods or suggest an alternative.

Thanks
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I'd just pack at intervals, and use a building adhesive under the remaining length of batten... It's worked for me...

Pack what you can, and fill the rest... Once the timber floor is on, it'll form a very solid platform...

What would you pack it with? I would imagine from what I know about building adhesive, it would not last that long and the floor might buckle after time. i know it would be super strong, but I rekon wonky and on floorboards with windows at the end, it will show.

I intend to stay in this house.

Villaboard, Masonite, and laminate for thin packing....

good one, I will try it on 1 row and see how it works.

any other ideas?

wouldn't use masonite packers for subfloor like that - any moisture and they lose their shape & you have a possibility of sag.

look at using cement sheet offcuts, the other suggestions above are good too.

frameles glass pool fence here was packed under using UV-stabilized packers that seemed specially made for that kind of thing, the guy doing it had a whole box full of different size (depth) ones, you could try asking a frameless fence / shower screen place if you wanted lots of them.

My suggestion is get real and don't worry about it. In the building and flooring trade the perfect job does not exist. On a 110sq. metre job you will not detect visually a rise or fall of a few millimetres, even 10mm. is very hard to see.
Lay all the battens, forget about laser levels, just rest a long straight edge on the fixed battens and plane off the high points if they worry you.
Don't worry about the kitchen sitting level, a good cabinet maker will scribe the kick- boards to the contour of the floor, so they sit tight. It is a good idea to rough sand the floor prior to fixing skirting boards and fiting the kitchen, this will also enable a close fit.
Once again don't get too worried. I've worked on hundreds (possibly thousands) of floors and differences in level of way over 10mm are virtually impossible to detect spread out over an area of this size.
Concrete slabs are never poured dead level, and timber battens are rarely of a regular thickness, plus there will always be timber shrinkage and concrete movement/settling in time to come. Relax, the floor is going to move of it's own accord anyway and your perfectly level floor won't be for very long.

ps. We are not equipped with lasers for eyes. The slab probably looked flat before you put the level across it. If the slab looks flat then so will the floor layed on top of it. By all means pack and/or plane the battens as near to level as realisticaly possible, but a 5-10 mm variance spread over 100sq. metres is quite acceptable. Bear in mind that with the kitchen fitted, furniture in place, curtains hung and possibly a few rugs spread over the floor you will never detect variations in hight of up to 10mm or even more.
Good idea using waterproof packers and maybe sealing the slab first, or else laying battens on a waterproof membrane. It is almost inevitable that moisture will be released from the concrete to the timber, or at some time water will find it's way in to the house. Rarely have I seen an old concrete slab that has not been subject to water ingress at some stage of it's life.

Have you considered using metal battens? Don't know if they are available in 35mm but would be worth having a look at the Rondo site or similar. These would still need to be packed but the spacing of the packing would not be so critical and there is a better chance they would remain 'true' over their length. Also, I would stick with blueboard or compressed cement sheeting for packing regardless.

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You say that you are using a laser level that is not self levelling which suggests to me that it may be of questionable quality as would be the readings that it gives. I may be wrong but I would think that unless your laser level is perfectly level then it may be the laser level that is out and not the slab or a combination of both. I suppose what I am trying to say is that unless you can be certain about the levels you are taking on the slab, then all your hard work could be wasted.

Don't use metal battens... They will forever be noisy.... And their expansion rate is vastly different to timber flooring!

I agree, not only this but how does one nail to steel

As for the laser being out...like I said i am not after a 100% job.
where the laser says the slab is down, you can confirm this by placing a 5 meter batten over the slab and you see a gap of exactly the same size. I must admit it is not the most expensive laser, however, I personally think it would be less than 1mm of tollerance out at 5 meters.

Tim, great to hear you cannot stuff it up even if you do nothing. It is a bit worrying, this end spending $5k-$10k on flooring to find out after you lay it that you really should have taken the time to get the subfloor level. I will take that on board though.

I am glad though it was mentioned that masonite packers are no good for this as this would be what most people would reccomend as a packer firstly.

Doesn't anyone like the sand/cement mix?

Have you contemplated a floor grinder for the high spots?

Can you plane the battens down in some areas and pack with CFC in others?

Some people dump a slab then screed then batten.

Keep in mind that 3mm over a metre maybe acceptable as deviation and not really noticed in a floor. Correct me if I'm wrong.

Battens to floor are relatively a new concept, 10-15 years.

Many products today require more precise construction. In the past certain construction methods were to bed things like wall plates in mortar. They were well aware that on site is not a classroom environment.

People often by a product thinking a. they purchased something at a good deal then eg: their porcelain tiles are so hard there is no instrument to cut them. They are slightly bent and the only answer is glue them to the space shuttle to straighten them on re-entry. There is other intensive labour requirements to install the product.

Building is not perfect. Houses are not precision machinery.

I digress, sorry.

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Masonite would be fine... If you have enough moisture in your slab to create issues with Masonite, you're going to have more problems than you know what to do with...

Yes, the ideal is Fibre cement, but can you get it 3mm thick?....

Hence also why you would use a construction adhesive/filler under the length of the battens... To have support over the entire length...

Check out the timber qld website, and they have specs on the level variations in floor systems allowable...

but can you get it 3mm thick?

You can split 6 but its far from accurate for that type of task.

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