New patio on sharp sand and dot & dab

[RussellF]

Hi, About to lay first patio - 20 B&Q large (600x600x35) textured paving slabs, so roughly 3m x 2.4m - with limited traffic as we'll just put low chairs and a garden coffee table on it.
I appear to have dug v deep as I ended up putting in 70 large bags (c25kg each) of sub-base, twice more than B&Q suggested and originally delivered. I've hired a wacker twice, it's super solid and now don't want to undo that with a poor bed.
As many people who've advised me to use a mortar bed, have told me to go for dot & dab given the large size of slabs and fact that I'm a novice. The older guy at B&Q, who argued with his pal (!) said I'd struggle, pulling the slabs up and down, to get them level on a pre-levelled mortar bed. He recommended 2in sharp sand bed, then compacted, then 5 "low" dot & dab at 1:3 ratio.
My only concern is it will take me 3in of only sharp sand (1st row laid as pictured below), rather than his recommended 2in, to get to a level which will see the top of the slabs above the lawn. That presumes most though not all the height of the dot & dab will bed in below the surface of the sharp sand. Will this be ok, given the solid hardcore base, if the sharp sand is compacted enough and I add two gravel boards at the front and right, to fully enclose the patio?
Many thanks,
RF

Sharp sand in patio bed

• RussellF
• 19 Sep 2016

Paving slab

• RussellF
• 28 Aug 2016

B&Q Textured Paving Slab 600x600x35mm

 

[r896neo]

Honestly your going about this a bit all over the place. If you dug out too deep then just bite the bullet and add more hardcore, and yes that means compacting it again unfortunately.

Trying to correct your mistake by using a very deep bedding will be more work as you will need to mix much more mortar and the cost of extra cement etc. It also will affect the long yet stability of your paving.

The moron from bandq and this layer of compacted sharp sand and dabs on top idea will be a huge failure. Never trust workers in stores like band q because their pride means they cant bring themselves to say I don't know and so they tell you whatever crap they heard on a diy show 20 years ago.

As an experienced paving professional I can assure you that dot and dab will bring severely premature problems to your paving in particular the pointing. Water gets into the voids and freeze thaws damaging the bedding and pointing. It may last in good condition for 4/5 years or it may Las for 10 but a properly laid patio on a full mortar bed will last 25years +

You should use a solid bed of 6:1 grit sand:cement 35-50mm thick and it should be wet so that as you tap down the flags it spreads under them. Those flags will break if you batter them down too hard.

There are lots of techniques for laying heavy flags like 2x2s but my preference is to get a piece of brick just thicker than the bedding mortar and balance the flag on this and pivot it down to its rough position then move it into its exact spot by lifting it level and placing gently on the bedding. Watch some videos for a few tips.

If you absolutely cant bring in more hard-core then bulk up your bedding mix with small stones 6-10mm as this fine concrete can be laid as thick as you like with no adverse effect. Laying a very deep 75mm sand and cement only bedding could cause problems. Make it 4 sand 2 gravel 1 cement.

Get yourself onto paving expert.com for detailed advice and how to guides

 

[RussellF]

Many thanks. Have avoided the sand only, dot & dab method and started as you've advised (option two) mixing small stones into 6:1 sharp sand and cement bed, first column of four slabs looking okay with some tentative weight on them (though I guess the real test will be over time...) thanks again.