Slabs on flat roof advice

[cwight]

Hi, I want to construct a flagged area on top of my garage roof. It's a new build property with a block and beam floor (i.e. roof), with a 100 concrete base on top. It's all being designed by the architect and the structural engineer with appropriate falls etc. I now intend to seal the roof with an EDPM membrane, and then want to flag on top of it.

Question is, should I use pads to sit the slabs on, or can I just put a bed of sand on top of the EDPM and then lay the slabs on top of that? I wanted to use a random pattern Indian sandstone flags, because I've got enough left over from doing the patio round the house. However, if I use pads I'm worried that the bigger flags will break since the pads only seem to go on the corners (i.e. the pads seem to be a bit like the 5 mortar dabs system of laying which is criticised on this site). Any advice, or should I ditch the Indians and go with concrete slabs instead.

[lutonlagerlout]

you need to bed the flags on to a full cementious bedding layer,or when it rains the sand will wash out
refer to the main site for full instructions,assuming that your block and beam roof is the subbase
ps dont try and wacker it!!!
cheers tony LLL

[cwight]

Hi Tony,

When you say a "full cementious bedding layer" do you mean a 10:1 dry mix that I used to lay the patio round the house, or a wet mortar full bed?

Cheers

[lutonlagerlout]

personally i prefer a wet mix,especially as i take it there will be no edgings on the roof?
you need a full bed though whichever way you do it
cheers LLL

[Stuarty]

It really depends on whether or not you require access to the roof, ie, below the slabs. Ive laid probably in excess of 1000m2 on the Caro systems - the pads and shims. They do leave a huge void in the middle of the slab, but ive never broken or had to replace a 50mm BS slab, only a few 35mm slabs, which I personally broke by tripping over a pallet and crashing onto the path. The end result when using this method depends on how true your roof is. Getting the slabs not to rock can be tricky using the shims, as they are all around 1mm thick but a handy little trick is cutting bits of DPM and using them to steady them. The water drains underneath the slabs, avoiding puddles on the surface, but if your roof has a slight hollow, the water will just sit below the slabs.

Choice of slabs depends on the laying method, Indian Sandstone will not be the easiest to do on these pads and shims. Plus they are not as strong as a 50mm BS slab. But they look more attractive dont they :p

The main thing to ask yourself is, 'will these ever need to come up for repairs?' Because the pads and shims will be easier to lift the slabs than when laid on a mortar bed.

The slabs ( BS slabs i mean ) are fairly sturdy when laid, mainly becuase of the weight on the pads.

But, in the end, its always up to you and what suits your needs best

[cwight]

Ok, thanks for all the replies. I def want to stick with the Indians, but some of the large ones are even under 35mm so, I'm not convinced that the pads are the best system for me. particularly since the differences in thicknesses of the flags would probably make getting it level with pads a nightmare. So this leaves me with a full bed. I can't envisage needing to lift them apart from if the membrane goes and I need to sort out a leak, but these EDPM systems come with a lengthy guarantee. However, just in case I do need to lift them I think I'll go with the dry 10:1 mix which is what I used round the house so I'm used to working with it. Final question is: given the possibility as noted in the first reply of rain washing the sand away, will a 10:1 give enough stability to the bed when set or should I up the cement content a bit?