Fitting artificial grass on flat roof, how do I compensate for drainage fall?

2 Sep 2010
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United Kingdom
I'm working through the issues for a garden project, where the garden is several feet below the ground floor of the house.

The design involves decking/planters/walls around the boundary. With artificial grass in the centre. Decking and grass at the same level. The artificial grass will be placed on the block and beam roof of a storage shed underneath.

I need a fall on the storage area roof, but also want the artificial grass itself level. Roof is 3mx8m with drainage towards one of the 8m edges.

I've seen several examples online of people having achieved this, but no explanation of how it was achieved.

Project hasn't started yet, so would be grateful for any pointers/advice.
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I would just lay the block and beam to slope of 20mm/m . The grass can't be absolutely flat and you expect it to drain to one edge. So the question is how much of a slope can you put on it before it becomes noticable? and also not to flood during a cloudburst. It might make croquet a little awkward.
Thanks, I have been looking up recommended falls for roofs. The minimum recommended is 1:80 and advice is to design in 1:60 for flat roofs (1:40 for other roofs) to compensate for build tolerances and sagging (although shouldn't be any sagging in a block and beam roof). The grass and underlay (Enka Flex) should prevent any debris build up on the drainage layer of the roof.

Assuming a 1:60 fall I'm looking at a difference of 50mm or 1:80 give 37.5m. I'm planning to use EnkaFlex underlay under the grass, which gives a shock pad and drainage layer. I could try to pad the fall out with that a little at the lowest side. As the grass is only 27-37mm (depending on which I opt for) I'm worried it will be quite obvious where the decking and grass meet.

I was hoping there was some sort of permeable/drainage underlay with an equivalent taper to the fall that I could lay on top. Or another flat layer I could use with an internal fall.
Lay the decking at the same slope? I cannot think of any tapered substrate with this fine pitch other then a screed which would have to be done by an expert.
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Lay the decking at the same slope? I cannot think of any tapered substrate with this fine pitch other then a screed which would have to be done by an expert.

Laying the decking at the same slope is the other option, thanks for the suggestion. If I can't find another solution, I'll have to go this route as atleast it will mask the slope from the eye.

I can make the roof at any pitch (within reason for a flat block and beam roof). Screed wouldn't work as its not permeable. What i'd need is a permeable layer which is tapered to compensate for the fall. So for example if the block and beam roof is built with a fall of 1:40, I then lay a permeable layer (something like the enka-flex underlay which allows horizontal drainage) ontop in the opposite direction with a taper of 1:40. Ultimately giving me a flat surface on the top.
Had some advice from one of the Grass suppliers. Suggested as per Franks advice that it won't be noticeable and to run the decking to the same slope.

Or if I really want it flat use a GRP grating over the block and beam roof, but I'm looking at ~£1400 (incl adjustable feet) for the ~24 sqm :/

Debating wether its worth the cost, but thought I'd post the option of GRP grating for anyone with a similar problem that comes across this thread.
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Managed to find some "left over" GRP grating which is a lot cheaper (£500).

However, my Architect popped by to give me some advice on excavation for shed. He also suggested it wouldn't be noticeable and to just run the adjacent decking to same slope. Suggested it was waste of time/money compensating for a difference of 50-60mm over 3m.

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