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Landscape Gardening

Why is it that my paving slabs have become uneven through time?

I have an upper patio area that is laid with Paving Slabs. However, through time, the slabs have become uneven, especially at the edges, and therefore, is very much a safety hazzard. Why does this happen? And, would it be a case of just putting some 'Sharp Sand' underneath the slabs to even it out again? Or would it be a case of redigging the whole thing out and re-lay?

Thanks in advance for your kind help


P.S. Hi all again, and thanks for the responses. How much do you reckon it will cost for me to do such work to make sure it wont be uneven again? My area of patio is 4.5 x 8m. Looking forward to your replies again.


8 Answers from MyBuilder Landscape Gardeners

Best Answer

Hi.The first mistake is laying the paving on sand.Over time the sand can be washed out and allow the paving to become uneven.The correct way is to dig out around 6 inches,then place a membrane on to the dug gound,cover this with 2-3 inches of hardcore,consolidate the hardcore with a vibrating plate.You are now ready to lay the paving.The paving should be layed on 5 spots of cement mix 5-1 ratio(sharp or building sand)1 spot in each corner and 1 in the middle,and gently tapped down to the required level.Finally point the paving with a wet mix of 3-4:1.Finito.Now go and choose your tables and chairs. regards Darren


Answered 6th May 2011

Hello. Plain and simple answer. The whole lot wants coming out. Slabs can be saved and relayed. The patio is laid on sand putting more sand under will only improve short term. If its sharp sand sometimes that is worth saving to use with cement/water, but needs to be clean sand.You will probably need more sand. I would lay a steel reinforcing matt in 100 mm of stiff concrete and lay slabs on a bed of sharp/ cement on top.This is done if on a hill or the ground is loose. Holds every thing together. They want cement pointing with damp proofer and then sealed when dry. Best of luck Trev


Answered 8th May 2011

Well we have an intresting debate 5 spots or a full bed or should it be layed on a lime mortar mix defintly not just sand.Although I lay most concrete slabs on a full bed
of sand and cement some of the cheaper slabs are recomended to go on spots as this allows the water to soakaway that can blow the face of the slab if it freezes, and although I would not recommend these slabs in this case a cross of sand and cement would seem sensible.It is always advisable to check with the supplier if you are unsure Pete.


Answered 13th May 2011

yet again we have a cowboy in our midst....laying slabs on 5 spots of mortar is the old fashioned and out dated and cowboy way of doing things. take a tip from the good guys such as B J D...lay on a full bed of sand/cement or on a compacted bed of sand dependant on area and type of slab.


Answered 11th May 2011

Michael. In response to your querry regarding the lifting of the patio area. Without viewing the area I would presume the bed for the patio is insufficient for the sub-soil conditions hence their is ground/frost heave having taken place, Subsequently would suggest having to re-lay the area.
Taking into consideration drainage requirements if required, excavating material to correct depth and placing a geotextile membrane such as terram, subsequently frost certified resistant aggregate and a sand and cement (sulphate resistant) mix bed for the slabs to rest on.
James Anthony


Answered 5th May 2011

i said to everyone any patio or driveway must be sourounded by edging set in concrete and the sub lay underneth must be mixture of sharp sand and cement if is a patio,another cowboy job among us
even my builder must take action on the traders who they claim they are experts in every aspect of building and landscaping(so wrong),cheap prices cheap quality
thaks peter


Answered 12th May 2011

Lift all the slabs, check you have a good base before relaying the slabs on a full bed of sand/cement mortar.


Answered 6th May 2011

Most major paving manufacturers will always suggest you excavate the area to begin with and then lay a bed of at least 100mm MOT Type 1 sub-base (or equivalent) which must then be compacted down. MOT Type 1 is a product that has been tested by the Ministry of Transport for stability in extremes of temperature. It can vary across the country according to what is available locally, but ranges from crushed limestone to granite or even crushed concrete. Upon this, the paving should be laid on a full wet mortar bed - this is imperative for natural stone products and also good practice for concrete paving as it ensures the paving is fully supported. Without the proper preparations and installation, the paving will move and even in simple cases where it is not so apparent, it may be that the pointing splits away from the paving and weeds develop in the cracks.

Have a look at paving options on as there is a huge range of products with home delivered / VAT inclusive prices - also lots of manufacturers installation instructions. Also have a look at for sub-base / mortar calculators. If you have a contractor quote on the job, test them on their installation technique up front now you're an expert on the process! Alternatively, you could have a go yourself.


Answered 10th Jun 2011

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