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QuestionShould I lay flag stones on a bed of mortar or just sand?

I have an existing concrete patio and am going to lay flagstones over the top. The stones will be around 450 mm square.

What are the pros and cons of using a mortar base or just sand?

10 Answers

Best Answer

always on a wet mix of 5 parts washed sharp sand to one part cement, they will not shift that way. Then to point them, use a three to one dry mix. wet the joints first with a hose, let flags dry. the pointing joints will still be damp brush dry mix into joints and watch the mortor soak the wet up. compact it into joints with side of finger with gloves on, or you will burn your hands or use a pointing trowel. then da dar you have your flag stones down and will last a life time.

Alan Watson 25th Apr, 2011

I always lay paving on mortar beds. Its easier to get them level, they wont rock if they are laid right and its a lot less time consuming. Sand works fine too, but the screed needs to be perfect, remember to wacker it too or you will have all kinds of sinking going on. I only use screeded sand on block paving.

My opinion would definitely be to lay it on a mortar bed.

R.Hughes Building and Conservation Ltd 25th Apr, 2011

must use a wet mortar mix ,dry sand will move underfoot ,lay on 4-1 mortar mix leave 10mm joints and point ,more time and effort but a solid long lasting job

substructure 25th Apr, 2011

sand can move or be affected by other things (ants etc).
I would recommend a wet cement mix, a mix of sand, sharp sand or stone dust and cement we find best, about 3:3:1 use pallas with the mix for more give and make sure that the edges are well packed. some prefer a full screed, some prefer the dot method, whichever suits you best, with the dot method make sure you support the middle as well as the corners and edges.
for pointing use sand and cement 3:1 with some pva adhesive in the mix, make semi dry - grab a handfull and squeeze, no moisture should come out but it should hold together in a ball, push into gaps and compress well, finish with a pointing tool, clean off excess asap.

Proline Fencing & Landscapes 25th Apr, 2011

Dont use sand on its own, they will forever move.
Best way is to lay on full bed of sand/cement mix and tapped down level, use a rubber mallet.
Use a coarse sand, ie floor screed sand, at 4 sand to 1 cement.


I would always lay flagstones on a mortar bed particularly if they are of varying thickness. In which case you will need a wet mix of sharp sand and cement to allow for the varying thickness of bed joint. You should initially prepare the area by removing the top layer of soil with any grass/roots etc. You should then lay 75/100mm hoggin or similar and compact this in layers prior to laying the flags. A weed block membrane is also a good precaution to lay beneath the hardcore layer.In your situation you may be able to lay the slabs on a dry screed mix of sharp sand and cement. If you have a good base to start with this will save a lot of work but if your patio is near the house be careful that you keep your finished level at least 150mm below d.p.c of any adjacent buildings as this could create damp penetration to the building if ground level is too high.

jakirkwoodbricklaying 25th Apr, 2011

flag stones should be layed in mortar base becouse it's solid when it goes off,
depand on the area you need a mixer, you can hire from hire shops,get the required building sand and cement, mixing ratio for laying is 4:1 and for pointing 3:1

if it has a solid base you can do it in sand and do pointing in same ratio mis 3:1 it will be difficult to bring in level in sand that's recomended to be lay in mortar base

am general builder 25th Apr, 2011

Hi Edwin.

If you have room on your existing concrete patio so that you do not come up to close to d.p.c. level you should always aim to be at least .150mm below to top of paving you would need to bed them a bed of mortar minimum thickness .025mm
subject to what existing concrete is like.
If you used sand alone this would just move.
Other tradesmen will have other views but this is what i would do also make sure
that the existing concrete is not to badly cracked as over a period of time this may show up in new paving.
Hope this helps

Cheers Southcoast Brickwork


Personally i would not lay your flagstones on top of the existing concrete patio. Ideally you should be lifting these up, laying 2 inches of hardcore (compacted) and then your flagstones on a semi dry mix of sharp sand and cement.

Pure Building and Plumbing 25th Apr, 2011

Either, I wouldn't use the 5 spot method it would have to be a full bed for me it's stronger. Screed is just as good if the screed is straight and has a solid Mot base that's been whackered down.

Nick Andrews Construction & Groundwork 17th Nov, 2015

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