Best mortar/concrete base for pedestal supported patio - thin slab

Hi

I'm looking for advice on the recipe to use for concrete.

I'm about to lay the concrete slab on which I'll fix my 30mm thick, large, rectified stone slabs using pedestals. The pedestals are needed to overcome multiple problems of levels and drainage.

It will be laid on top of block and beam (which bounces a little). I need a 1:100 slope.
Height restrictions mean the slab will range in height from 55mm to 20mm.
It's thin but I'm really tight on height thanks to idiot builder.

Fibre reinforcement has been suggested, in addition to rebar.
The pedestals will sit on top of it. So it will get wet as well as being exposed to cold

Please can anyone advise on what mix to use? Should I use fibre and if so what type? I'll be mixing it on site.

Also, would you use an isolating membrane (would add height which I am reluctant to do)

Do I need anything to bond the screed to the block and beam?

Sorry for mixed use of terms concrete, screed and mortar but after a lot of reading I'm confused as to what to say when.

thanks in advance!

Rebar isn’t going to work with 20mm concrete.

Also if there’s flex in the existing structure it’s just going to crack the concrete you lay as it’s not going on thick enough.

You need to rethink those thick slabs and get much thinner ones (porcelain tiles for example) if you want maintain the max height as you suggest...but still I’d say it’s sketchy if the floor had bounce.

Have you considered a flexible surface such as decking? Would seem more suitable in the circumstances.

Hi -- thanks - yep, figured out the rebar won't work.

Half way through the project so it's hard to change - slabs already bought (8K). Decking difficult to build in this context and I don't like it either.
The builder (long gone) made a lot of mistakes - the block and beam is simply too high. I've had much of the extension rebuilt to make the DPC 1.5 courses higher which was the most possible.

Given I have to have a thin base - how thick is the minimum would you say?

The pedestals are going to concentrate the weight and cause further issues with the bound and the weak/thin concrete.

Do you need the pedestals? I assume the slabs are large in size as well as thick. Could you not just lay them on a deep bed directly onto the beam and block?

@kingandy2nd -- you're right - the slabs are from 50 x 80 to 50 x 40, beautifully milled limestone so very smooth

I have considered what you say; but we need to create a slope of at least 1 in 100. So we'd still need a thin screed to lay the slabs on. If we just mortar on the block and beam trying to create the slope, I think the grout would disintegrate and we would have mortar collapse problems as well. The beams are at most 3.5m long so they don't flex terribly. There are three foundation walls beneath the patio area, so some parts hardly flex at all.

There are too many complications to go into- already 4 builders and 2 surveyors have given up on finding a solution, which leaves me to DIY. Building control spent several hours discussing it with us and proposed knocking down the whole patio and extension, just because of the patio height problems. To be practical they agreed something like we are proposing would be our best bet.

The pedestals overcome 75% of other problems with the site - I'm reluctant to give up that advantage tho i have considered it.

I've had a think about design and if we use pedestals, I can create slopes in the screed in different directions. It's an irregular S-shaped patio so I can cover it in intersecting 1:100 slopes such that no individual slope needs to be more than 2.5m long or lower than 30mm. There would be creases in the screed but you wouldn't see them because of the pedestals.

With polypropylene reinforced, what do you think? It's an awful solution until you consider the alternatives...
Do I need primer between the blocks and the screed?

I’m no pro, so I can’t offer you the answer/assurance you need, sorry.

I cant see you achieving a complicated selection of scree slopes any easier than creating a slope in a mortar bed to be honest.

Back to the mortar bed slope - Just work out how out of level your spirit level needs to be for your fall, and then start at the highest point with the slabs (assume next to the house) and then work outwards using the spirit level to give your fall.

If you go with a thin scree, your going to just end up with another bodge/failure.

Perhaps if you post up pictures, others might be able to suggest more suitable suggestions.

I'll get some pictures up tomorrow. I am only reckoning on 4 planes in total - it isn't that insanely complex!
Will think some more. I have yet another landscaper coming tomorrow. They usually spend hours pondering and then say they can't fix it...

I'm looking to do a scree for my kitchen floor, and by all accounts 50mm is the minimum and apparently is a pain of a job to do and get flat level. Not sure of the read across to what you're attempting, but sounds even more challenging - especially where the planes meet.

Here's my shuttering plan -- the square kitchen patio already has a 1 in 100 slab laid, it's just the curvy bit we need to build. All slopes will be such that the raft thickness ranges from 55mm to about 30mm.

Can anyone suggest ways to minimise risk? And should I prime the block and beam before laying?

As I've said, there are so many problems with levels and drainage that this bad option seems the best available.

Can’t you put a support under the block and beam to stop the bounce?

It worked - with the above plan and some special fibres for ultra thin screed... beautiful

Hi @mashie, starting to consider options for ripping out our 20 year old rotten timber decking, where we have a similar site challenge to yours. Can I ask what fibre you used in the thin concrete?

Hi @Mudscience The fibres were from Adfil -- but you can't buy small quantities. I was very lucky that they were kind and gave me a sample (my patio wasn't tiny - 100 sqm). I can't remember which specific type. I think the fibres you typically get for DIY are not really effective (from the research I did at the time). To be honest, although the builder said the block and beam would flex, I am not at all sure it does!