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.
Subbase?
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84-1093879891
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!
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!
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petehughes
- Posts: 2
- Joined: Mon Apr 28, 2003 8:49 pm
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)?
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)?
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84-1093879891
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. :)
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. :)