Block paving onto concrete - Drainage when block paving on concrete

[cucci1968]

dear Tony McC
i would like to block pave onto my existing concrete drive,(approx 60 m2, 3m width x 20m long ) which is solid because its been down for years and approx 150mm thick. ive read your advice about drainage and have opted to drill 50-70mm holes for drainage,rather than the other options. but i am not sure how good the drainage is of the sub base under the concrete.for extra measures, would it be a good idea to add some small gravel (maybe 10mm) to the sharp sand base as to improve the drainage of this layer, and /or add cement (maybe 6-1) to the sharp sand (laying it dry) to make it more solid in the hope to prevent this layer going mushy if too much water does get through, and lastly to seal the block pavers, to further stop any water penetration. i am worried that the drilled drainage holes will not be enough if the sub layer under the concrete is badly drained. is it a good idea to try all or one of these extra measures. very much appreciate your help on this. thankyou . regards tony

[Tony McC]

If the existing concrete drive isn't flooding, that suggests the sub-grade is coping fairly well with the run-off. So: when you pop your drainage holes, try to drive a crowbar or similar into the sub-grade, creating a hole as deep as poss and then fill this with clean gravel, coarse sand or grit. Also; when you form your edge courses, place short lengths of 12-25mm dia placky overflow pipe beneath the concrete bedding at 1 metres centres, thereby creating a series of escape routes for any excess water finding itself trapped in the laying course.

Adding grit/gravel to the laying course does nowt to improve the drainage, and adding cement will really bollix it up!

[cucci1968]

If the existing concrete drive isn't flooding, that suggests the sub-grade is coping fairly well with the run-off. So: when you pop your drainage holes, try to drive a crowbar or similar into the sub-grade, creating a hole as deep as poss and then fill this with clean gravel, coarse sand or grit. Also; when you form your edge courses, place short lengths of 12-25mm dia placky overflow pipe beneath the concrete bedding at 1 metres centres, thereby creating a series of escape routes for any excess water finding itself trapped in the laying course.

Adding grit/gravel to the laying course does nowt to improve the drainage, and adding cement will really bollix it up!

hi, firstly, thanx tony for your reply. when you say lay short lengths of overflow pipe, do you mean under the haunching?from the sharp sand into where the water is going to drain? (which will be the grassed area of the garden) again thanx.
regards tony

[Tony McC]

Yep. Summat like this....

[cucci1968]

Yep. Summat like this....

thanx tony that diagram is excellent very very helpful indeed.i have just one more question, if u dont mind. i have been getting quotes into hireing out a diamond tipped hole borer, and was shocked at how much its going to cost.( £100 plus vat per day,and£20 per half a mm of diamond ware) it ll be cheaper to rip up the bloomin concrete! would it be possible to do this job if i purchase a diamond tipped cutter myself and use my sds drill? again thankyou for your time. rgards tony.

[Tony McC]

A core cutter is all well and good if you're a contractor - you can buy a reasonable set of three for under 500 quid and they'll last you 10 years or so, but if you're only ever going to need them once in your life, it's just not worth it, so it's either hiring-in at 150 quid (by the time you've allowed for bit wear) or finding a cheaper alternative.

There'sno doubt that a 35mm core bit will chew through 100mm of concrete, with or withour rebar, in 3-5 minutes, and leave you with an ideal hole, but for a DIY installation, you could get away with using a Kango fitted with a 18-22mm bit, and pop holes @ 600mm centres rather than 35-50mm holes @ 1000m c/c.If getting hold of arotary hammer Kango is a bit of a problem, you could manage using a beefy 800W or bigger hammer drill fitted with a 12-15mm genuine quality tungsten razor bit @ 500mm c/c.

The ball-aching part of the job is the drilling and a decent bit at twice the cost of a budget yoke will chomp through the concrete in a quarter of the time, so it's definitely money well spent. A proper butch drill is essential, too. Owt less than 800W is likely to burn out with frustration.

Remember that the layout of the paving will determine just where the pop holes are needed. If youre using the edge course overflow yokes, then no great need to drill at 500mm centres within 1 metre of those edges - you can get away with one hole halfway between each outlet. However, you might need more holes at any low spots, just to make sure water doesn't stand any chance of getting trapped within the laying course. Don't forget to put a layer of permeable membrane over the holes to prevent laying course sand trickling away down all those holes.

[cucci1968]

A core cutter is all well and good if you're a contractor - you can buy a reasonable set of three for under 500 quid and they'll last you 10 years or so, but if you're only ever going to need them once in your life, it's just not worth it, so it's either hiring-in at 150 quid (by the time you've allowed for bit wear) or finding a cheaper alternative.

There'sno doubt that a 35mm core bit will chew through 100mm of concrete, with or withour rebar, in 3-5 minutes, and leave you with an ideal hole, but for a DIY installation, you could get away with using a Kango fitted with a 18-22mm bit, and pop holes @ 600mm centres rather than 35-50mm holes @ 1000m c/c.If getting hold of arotary hammer Kango is a bit of a problem, you could manage using a beefy 800W or bigger hammer drill fitted with a 12-15mm genuine quality tungsten razor bit @ 500mm c/c.

The ball-aching part of the job is the drilling and a decent bit at twice the cost of a budget yoke will chomp through the concrete in a quarter of the time, so it's definitely money well spent. A proper butch drill is essential, too. Owt less than 800W is likely to burn out with frustration.

Remember that the layout of the paving will determine just where the pop holes are needed. If youre using the edge course overflow yokes, then no great need to drill at 500mm centres within 1 metre of those edges - you can get away with one hole halfway between each outlet. However, you might need more holes at any low spots, just to make sure water doesn't stand any chance of getting trapped within the laying course. Don't forget to put a layer of permeable membrane over the holes to prevent laying course sand trickling away down all those holes.

thanx tony, that was very helpful, youre a top man. again ill post my pics to you when im finished, cheers, from tony