Why have my paving slabs become uneven?

Question

In my garden I have some paving slabs, laid a few years ago, which have become increasingly uneven since then. The paving itself seems fine apart from some wearing around the edges. Does anyone know why this happens or how it could be fixed easily? Or even, if anyone knows how I can prevent this happening again in future? I have been told that adding ‘sharp sand’ beneath the slabs to fill beneath can help even it – or would it be best to re-lay the slabs with some new sand underneath? 
Thanks, Mo

Tracey · Accepted Answer

Hi Mo, Sorry to hear about the trouble you’re having with your paving. The first issue here is that your paving has been laid on just sand. Sand will wash away over time, which is why your paving can become uneven. There are a few options that this could be done, but none will just be sand as the base for the paving like you have now. Most would agree that first, the area would be dug out about 6 inches deep. From here, you could add a geotextile membrane into the area dug out, before adding a full-bed of cement to lay the paving on. This ensures that the paving will be fully supported.
 Some may choose to do this part differently, possibly having a bed of cement and sand before adding the pacing or using the 5 spots of cement technique.  These days it would be recommended to go down the route of having the membrane (like Terram) included – particularly as you want to prevent any further issues in future. This isn’t necessarily the most difficult fix, but I would recommend hiring a professional even if it means paying a bit more to make sure it’s done right this time.

Michael · Answer

This usually happens for a few reasons. 1st the sub base is not good enough or there isn't one. 2nd it was put down on just soil or sand. 3rd they didn't use a sand & cement mix and bed them down properly. 
I'm sorry to say either way it all needs to come up and be done properly.

Michael · Answer

Hi, I would recommend they be relayed with the correct bed first. They may have moved due to poor installation or ground conditions. If your drains run under your slabs make sure that your drains are in good condition. This is due to leaks, which can cause a void under the slabs.

Grant · Answer

When the ground underneath the slabs settles, the slabs can become uneven and the material under the slabs will need to be built back up to level of the slabs.

Lianne · Answer

It sounds like they may not have been laid correctly, do you know if they were laid with a full bed or using the dabbing method? or the mix may have been too wet, the more water that is used the weaker the bond. I would recommend re laying. If they are uneven, putting sand is just a sticking plaster.

Florin · Answer

Poor ground preparation!
Always slabs laying on sand and cement!

Jack · Answer

Hi Mo, 

Paving slabs usually become uneven due to a lack of sub base or a poorly compacted sub base. To prevent this the sub base should be at least 100mm but we recommend 150mm if the slabs get a lot of foot traffic. It should be bedded on a full bed of mortar to fully support each slab.

Hope this helps,

Jack 

AJH Building

Wayne · Answer

More than likely spotted when layed this doesn’t last its time I’m afraid 
They need picking up and relaying

Daniel · Answer

Usually the paving base has failed, normally caused by weight - vehicles driving on and off for example.

Colin · Answer

It's often roots from trees and bushes growing underneath that will cause them to move over time.Depending on which slabs have raised or lowered, redistribute the sharp sand every so often, and then use a rubber mallet - bashing it on top of the slabs to compact the soil and sand underneath.

Why have my paving slabs become uneven?

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