The Brew Cabin

Hi I am looking to lay a drive made up of randomly laid flags, with a variety of joint thicknesses, surrounded by gravel.

Looking through the previous posts it looks like I would need to lay stone flags or 50mm BS pressed concrete flags in a weak bedding mix over a 100mm mot class 1 subase or alternatively if I use patio flags I would need to lay them directly onto a 75mm full concrete bed.

Can you please confirm that my assumptions are correct and advise me on how to incorporate the gravel. Do I just pour it on to fill the gaps? if I leave the joints too wide will this cause the flags to lift when a car drives over them?

Any advice would be gratefully received, thanks in anticipation, Gardendesigner.

If you're having unnaturally wide joints, then it's best to lay the flags on a full mortar/concrete bed. I'd go for 100mm of C7.5 or equivalent concrete, and bed the flags directly onto that, but, if you're working more slowly, then use a concrete base, 100mm thick, then bed the flags onto a Class IV mortar over the top of it.
You could use a flexible sub-base (100mm DTp1) if you prefer, bt the full concrete base is best for patio flags, which have very little flexural strength.

Leaving big wide gaps can, and probably will, cause problem with flags being 'flicked' upwards when trafficked, which is why I'm so keen to push you towards a full concrete bed, that will hold the flags firmly in place. The concrete would, ideally, come yup the side of the flags, to within 25mm of the surface, and then fill the gap with your chosen gravel, pushing it into the still-plastic concrete before it sets completely.

A weak bedding mix (10:1) won't be strong enough to restrain the flags in this sort of design. It's fine for patios and flags laid with 'normal' joints, but for this type of project, it's best to think of it as a series of stepping stones, rather than as a composite pavement.

Hi Tony, thanks for the prompt reply. If I go for the slow method you describe and bed my flags into a full bed of class IV mortar on top of a concrete base, what depth of mortar should I use? Thanks Gardendesigner.

20-40mm will be plenty.