'Green roof' fall question

[Rick O Shea]

I wonder if anyone could advise on the correct fall for a green roof, the span of the roof is 5m or there abouts.  I (think I) know that the fall should be less than that for a flat roof but getting an answer seems tricky.

Any Advice?

And what  is the recommened membrane to cover the roof sheets , I am tempted to used pond liner?

many thanks   

[clockmanFR]

Its not necessarily the fall you should be worried about, its the weight of everything that rest on your walls. And don't forget to allow in your calculations for a good meter of snow sitting on top as well.

Green roofs look good in principle but boy have you got to beef everything up. 

[biff]

ànd then there is the weight of the ride on mower,,
                                                                      biff

[AlanM]

Biff, Its a lot less bovver with an hover...........

Depends on what you intend putting on the roof, turf or sedum, sedum will be alot lighter and doesn't require a deep (heavy ) growing medium. Turf on the other hand does need more depth. So the heavier turf roof will need a bit more slope to drain, sedum could be flatter, but doesn't need to be.

Alan

[wookey]

Actually, the weight of roof structure turns out to be almost irrelevant for normal domestic structures. Beam sizes are determined entirely by the snow loading and the point loading for maintenance. For green roofs you do need to do the sums, but the structure has to get pretty heavy before you need much bigger timbers than you already need for worst-case snow and maintenance allowance.

I have a spreadsheet for doing all the sums in EuroCode5 because it's not trivial.

[Rick O Shea]

For a 5m span I was gong to use 50x 225 or 9 x 2 timbers at  450mm centres, which I hope will be adequate.
The roof will be a seedum type so I hope will not be too heavy.

its the fall I am concerned about, 50mm over 5m?   

[rondurrans]

Rick - do not rely upon HOPE! 

[rondurrans]

The following may -

http://www.greenroofs.org/
http://www.safeguardeurope.com/
http://www.greenroof.se/
http://www.flag-soprema.co.uk/web/products/roofing-products/living-roofs.html
http://www.optigreen.co.uk/

[stannn]

Rick
Go on Google and write 'slope on green roofs', if you haven't already. There is lots.
Stan

[wookey]

OK, I put your numbers into my sheet. (5m span, 47x225 C24 timbers, 450mm spacing. I found a loading for the roof from here: http://www.thegreenroofcentre.co.uk/Library/Default/Documents/Green%20Roof%20Pocket%20Guide%20V3.pdf at 60-150Kg/m2 for a sedum roof.

As is typical, the design is deflection-controlled (i.e it's nowhere near breaking, but the sag in the roof gets out of spec), and that timber is borderline. It comes out OK up to 130Kg/m2 but there is too much sag by 140Kg/m2. 63x225, or 47x250 is OK at 150Kg/m2 (1.5kN/m2)

Note that I'm not a structural engineer (although I can do the EC5 sums for straighforward cases like this), and I haven't checked this at all carefully, so it's still using the snow-load from my house in cambridgeshire (1.03kN/m2). If you look at EC5 you'll see that the extreme (drifting) snow-loading depends on the surrounding building shpaes/sizes as well as the area of the country, so that is very likely to be wrong.

So, if you don't want to do the sums properly, then I suggest you stick it up to 47x250 (or use those fancy modern OSB-rib rafters), and/or keep the weight down to the lower end of the range.

[EccentricAnomaly]

Or reduce to, say, 300 mm centres?

[Rick O Shea]

Very many thanks for that information.  I have enough personal sag to want any in the workshop roof................ I did look up the roof beam sizes  and used an online calculation for the roof beams and while the beams size came close, it all seemed to be within spec. I might have to go up a size.  I wonder what the margin for safety is on those calcs?

I am going to go for a fall of about 100 mm over 5m? 
 
Best wishes
Mike
 

[solar_cambridge]

I am going to go for a fall of about 100 mm over 5m? 

I'd go for a little more personally. Also use nailed 'X' herringbone struts across every joist every 1.5metre9(e.g toolstation 77677+96841). That really stops any twisting and any deflection. You can feel the whole structure tighten up when you fix them.

What are you using for waterproofing layer? I went for GRP.

[solar_cambridge]

btw, 47x195x4.8 are bloomin heavy! If your shifting 63x225x5.2 don't underestimate having two strong men to assist and get your overdraft ready when you order! 

[clockmanFR]

Wookey,
As part of my original training i did structural engineering, you know what they say "a little knowledge is a dangerous thing", so its great out here in Normandy and i can put it to use, as a French builder remarked to me over my hanging/cantilevered staircase design, "you English you like your metal".
Unfortunately, I can not use any of my skills in the UK, so my earlyer comment "its the weight of everything that rest on your walls" was my serious penny's worth. Beams maybe good, but the wallplate and beam pads especially with flat roofs. ??