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Posted: Thu May 24, 2012 10:44 pm
by London Stone Paving
Every now and again we will get a phone call from a customer saying that there new patio is full of damp patches.
In every case we find out that the patio was laid by a builder using a concrete base instead of MOT type 1. Its not my intention to have a pop at builders btw. Does anybody ever lay patios on top of a concrete base?
I'm assuming that the moisture from the bedding layer has nowhere to go apart from through the natural stone which results in ugly damp patches all over the patio.
It does always seem to happen with builders or an architect who has spec'd a concrete base. I've got no doubt the intention is good, but its just wrong
Posted: Thu May 24, 2012 10:56 pm
by lutonlagerlout
if the falls etc are all good why is it wrong?
its a waste of money but i cant see the badness in it
water should run off flags not through them or the pointing
cheers LLL
Posted: Fri May 25, 2012 7:16 am
by local patios and driveway
Thats true. Certainly in the case of block paving, laying on concrete leaves patchy wet areas on the driveway that take ages to dry out. But any pointed stone or concrete flags the water should be running off the area via the fall rather than soaking in. That said, a concrete base without a dpm would soak up water like a sponge and would cause dampness throughout
Posted: Fri May 25, 2012 9:01 pm
by London Stone Paving
I dont mean the surface water. I mean the moisture in the mortar mix
Posted: Thu May 31, 2012 6:30 am
by mickavalon
We've had this specified a few times, especially when using thinner Natural stone, leading out from an internal floor, more like floor tiles and I've read it on the spec from quite a few Stone suppliers. It tends to be where it's felt a MOT sub base will not be strong enough?? or it's connected to say an internal floor or balcony etc. I never like doing it, especially as they are usually laid with insufficient falls or no fall!! I suppose if the Concrete Slab is left to fully dry out, and the stone sealed, and a flooring adhesive used instead of Mortar, but mostly they do seem to trap water from all ways, regardless of falls.
Posted: Fri Jun 08, 2012 12:07 pm
by Dallas
all the flatwork in Dallas texas is laid onto a concrete base without any problems
Posted: Fri Jun 08, 2012 12:18 pm
by GB_Groundworks
Dallas is a lot warmer and drier than the uk
Sat here sheltering from the rain
If they've pointed with polymeric then they are permeable
Interested in this as a builder has asked us to lay a 60m2 slab for him to lay slate on
Posted: Fri Jun 08, 2012 5:13 pm
by DNgroundworks
Im laying on concrete at the minute, an ancient 200mm thick slab that has never moved, 4-1 soft sand with sbr, sbr + cement bondbridge, pointed with easi point - they aint going nowhere!
Posted: Mon Jun 11, 2012 1:22 pm
by Dallas
Dallas is seasonal it rains alot , don't know what that has to do with cement curing
Posted: Mon Jun 11, 2012 4:22 pm
by lutonlagerlout
Dallas averages 60 rainy days per year
Manchester averages 140 rainy days per year
thats quite a big difference
LLL
Posted: Tue Jun 12, 2012 7:24 pm
by cookiewales
DNgroundworks wrote:Im laying on concrete at the minute, an ancient 200mm thick slab that has never moved, 4-1 soft sand with sbr, sbr + cement bondbridge, pointed with easi point - they aint going nowhere!
dan i hope soft sand does not mean bricklaying sand its a no no sharp sand or grit sand or any stone dust is fine 
Posted: Tue Jun 12, 2012 8:12 pm
by DNgroundworks
Whynot cookie, millions of houses all over the world are held together with it?, i just thought i already had the strength from the concrete slab, so modify the mortar with sbr, cement+sbr bond bridge, it is absolutely solid?
Just finished gunning the easipoint in today 
I have my reservations with a semi dry or even a semi moist mix with sharp sand, in my experience there is zero bond with indian stone, the flags always sound hollow, even thought i always use sharp sand, except for this job :p
Posted: Tue Jun 12, 2012 8:13 pm
by DNgroundworks
Im actually going to say that in this case it feels more solid that when i have laid flags my usual way on sharp, tap the flags with a chisel or hammer and they ring like you are tapping steel
Posted: Tue Jun 12, 2012 8:28 pm
by lutonlagerlout
oh noes 
dan i knock up sharp sand like bricklaying mortar and lay on that,never had a problem,however laying on a concrete base is different as the concrete doesnt drag the water out of the mix like type 1 does
i find a wet mix with sbr in it eliminates the need for a bond bridge which is a PITA
this technique of yours may work over a concrete base but almost certainly would fail on type 1
cavaet emptor
LLL :;):
Posted: Tue Jun 12, 2012 8:41 pm
by cookiewales
its all about strenght and freeze thaw soft sand degrades very quickly on freeze thaw and even more on movement am sure t mc has some views on this ps the laying on semi dry should allways be soaked with lots of water at intervals throught the day if you do that with soft sand they will sink .you have to be carefull when using mersey grit sand dreged it turns to sqeulch when wet you darent breath on em