Patio foundation and dpc - Foundation depth in these circumstances?

[Robodigger]

Summary: How deep should a patio foundation be in my circumstances (see below)? All research says 150mm (6") but my builders are saying as little as 15-20mm (0.6" - 0.79") of sub base on top of existing foundation/ground because we're already starting on rocky clay. And is the 150mm below DPC rule unbreakable?

It's a 40sqm patio going around a semi detached house on 3 sides. At the front and side the patio is 1m wide and at the back larger at ~7m x 3m.

Detail: I had a patio laid by cowboys years ago and rain water then damaged the soft brickwork. Now after a lot (A LOT) of research I've found a decent builders in the area to replace it.

All the research online says the patio needs to have a foundation of 150mm (6") and sit 150mm below the DPC. I asked the builders about that and they said "no way" and said digging out 300mm (12") - because the current patio goes right up to the DPC - would be a mammoth task (there's also an old tarmac path underneath they'd have to dig through doing this) and instead what they want to do is dig out 15-20mm (0.6" - 0.79") just for the subbase sand etc then slabs on top.

They said that for foot traffic 15mm-20mm is fine. Is that right? I've dug 5 test holes and the ground underneath is either dense rocky clay at the front and side of the house or the red/black brick hardcore the cowboys put in at the back of the house. There is more ground to fill at the back as it drops away into the garden so they used a lot of this red crushed brick material to level it out. The patio at the front and side is walled in by a 300mm high terrace wall because the house sits at the bottom of a slope so you walk down two flat terraces from the drive way potentially adding more stability to the patio.

They also want to put the patio only 50-75mm (2") below the damp course but to compensate place it 100mm (4") away from the wall with gravel to drain water away - there are three drains around the house wall they could aim the water into.

Does all this sound right? After spending thousands repairing the brickwork I really don't want to go through that again.

If the patio absolutely must have a 150mm foundation and be 150mm below DPC then I’m thinking of hiring a cheaper day labourer just to dig it all out so the builders start with a hole 300mm below the DPC. The builders would charge me a lot more to do the digging themselves I think.

I phoned Wickes and they said 20mm subbase is fine if it's on hard ground already (see above re clay and red brick) and that's their experience from what customers have done.

I also asked my neighbour who is handy and he said when the council tarmac'd the front and side of his house decades ago they didn't leave 150 below DPC and he said it's fine and "they know what they're doing". If I were to have 150mm below DPC there'd be a drop of about 75mm from his tarmac path to my patio slabs. This drop would be under the wooden fence that separates our gardens.

The builders I'm using come recommended and not only did I do the usual checks you do when hiring builders (read reviews, look at photos etc) but I also got a list of 4 previous customers and actually went to their houses and they showed me the work and told me exactly which workmen did it and what they thought of them. All very happy. And it's these builders/landscapers that are saying they've been doing it for 30 years and 20mm is fine.

It took me 3 months to find builders that weren't dodgy and honestly these are the only ones I've found (I'm between Bristol and Bath).

[Tony McC]

15-20mm is NOT, abso-bloody-lutely NOT a sub-base. The max particle size for sub-base aggregate is 37.5mm, so how do you get just 15-20mm of that?

Unless you've misunderstood the dimensions and what you were told was actually 15-20 cm (centimetres) of sub-base, I would be marching this "builder" straight back to the livery stable to collect his horse and send him packing straight back to Dodge City!

The very minimum depth fora patio sub-base is 75mm and I'd much rather see 100mm, as would most, if not all *professional* paving installers. Whatever some jack-of-no-trade builder tells you, 75mm is the rock bottom, to pick an apposite phrase, and for these reasons....

Sub-base FAQ

You can choose whether to heed the nonsense spouted by some eejit "builder" or some no-skill pillock behind a counter in a DIY store, or maybe follow the advice provided by someone with over 509 years experience of paving and pavement construction.

[Bob_A]

I'm here to learn so can't give qualified advice but from what I'm reading the builder wants to make this a really easy job for themselves by simply laying paving on top of 15-20mm of aggregate with no regards to your DPC.
I've read it on here before good builders don't always make good landscapers, I would look for a reputable hard landscaper who'd use a mini digger to achieve a decent depth, won't be a cheap job though the cost of skips alone will cost a fortune which is probably why your builder doesn't want to do it.