The Brew Cabin

Hi.

I will be laying a new patio using concrete slabs which will be mortar pointed (i.e. not butted). I want to lay the slabs on a mortar bed (I've always found it easier to bed the slabs down to level on wet mortar) but am not sure which type of subbase to use. 100mm of Type 1 or similar would be easier/cheaper but I've always understood that you shouldn't mix a flexible subbase with an inflexible bedding course - is this right? If I'm right then I would presumably need to lay a concrete base, say 100mm thick?

I will also be laying the slabs on the driveway - this is currently tarmac. I haven't checked depths on this yet but was hoping to take up the wearing and base courses and possibly re-use the existing subbase. Then lay the slabs on a mortar bed as above. Is this OK or should I lay a concrete base for extra strength (the slabs are only about 30-40mm thick)?

Thanks for any advive you are able to give.

Laying flags/slabs on wet mortar is a bloody nightmare (for me anyway), and, in theory, you're right that a rigid bed on a flexible sub-base is less than ideal, but it is done on quite a lot of footways for reasons that I've never fully understood, despite having it explained to me dozens of times. The joints between the individual flags act as a sort of 'control joint', and so allow the rigid bedding to crack in a pre-determined spot, which is all well and good, but then why not just rely on a flexible bed that can accommodate slight settlement of the sub-base? In my experience, when rigidly-bedded flags start to rock, they continue to rock indefinitely, and the only remedy is a lift and relay. With flexible bedding, the flags have a chance of re-establishing a non-rocking equilibrium, noot that it always works that way!

On public streetworks, the 'mortar bedding' is often a lime-based mortar that remains slightly more flexible than a cement-based mortar. This is reckoned to accommodate slight movement more successfully than rigid bedding and it does seem to work. However, getting a lime-based bedding mortar for a patio is not really feasible, as the manufacturers like to deliver in cubic metres, not in barrowfuls!

If you use a weak mortar (or the 10:1 bedding mix that I advocate), then you should be ok for a patio. For the driveway, I'd elect to re-use the existing sub-base if at all possible and then choose the bedding according to the type of flags being laid. If they were BS pressed concrete flags or 50mm+ stone, then a sand bed or a weak bedding mix would be my choice, but for cast flags (patio stuff) or the Indian sandstone wafers that are very popular just now, then you really, really, really need a full concrete bed, 50-75mm thick, to give then the necessary strength.

If you must use a cement mortar bed, then use a full bed, not spot bedding, especially for the driveway work. Spot bedding causes more bloody problems in the long term than any other method of bedding!

Thanks for that.

I will use type 1 subbase and lay the slabs on a screeded 10:1 mix for the patio (i.e., all flexible); for the driveway I will lay a mass concrete subbase and lay the slabs on a full bed wet mortar (i.e., all rigid). Does this sound OK?

Do I need to use an edging at the edge of the patio where it meets the grass or will a concrete haunch be OK (I will cover this with topsoil)?

Your spec sounds OK, Pete, but have you considered laying the driveway flags directly onto a bed of semi-dry concrete? It's a lot lesss work than laying a concrete base and then laying the flags.

An edging for your patio is not essential - it's a decorative touch that might look nice, but it does nowt for the actual structure. :)