QUESTION
I want to lay a patio and currently there is a concreted area.
There is enough room to lay slabs on top and still be 150mm below the damp course.
Would you recommend breaking it up or laying the slabs directly on top?
Should I add a layer of sand on top of the concrete or should I fix the slabs directly to the concrete?
Where the patio goes further than the concrete base I'll have to dig up the lawn should I lay scalpings to the level of the original concrete base and then add a layer of sand over both areas?
ANSWER
Thank you for your enquiry regarding a patio.
It is fine to lay slabs on top of concrete if the concrete is physically sound i.e. there are no cracks or sunken areas. However if you have any reason to doubt the firmness of the concrete then it should be removed and a suitable hardcore base installed. The recommended way to lay paving slabs is as follows:
• Dig out the soil approximately 300mm deep.
•
Lay scalpings approximately 200mm and compact with a vibrating plate compactor.
•
Then lay slabs on a wet sand and cement or stone dust and cement mix at a ratio
of 5:1 to bring up to the finished level.
•
Joint the slabs with a damp mix of 3:1 sand and cement.
Good luck with your gardening project!