Roof slope for rain run off

[Tim]

after much umming and errming, I think I have come up with a suitable design for my DIY obsy.

I would welcome any thoughts on the slope required for decent rain run away. I know that even 6mm drop over 3m is plenty for gutters etc.

At the moment I am thinking of using around a 2.5 deg slope towards the gutter, but may prefer less of a slope for aesthetics.

I had originally intended the front of the obs to be higher and drain to the back, thus hiding the roof entirely from the house, but I dont think that is going to work.

Thanks

TJ

[tooth_dr]

I am currently building an obs, and I decided on a drop of about 7" over a distance of 6'. My roof is covered in mineral felt, which isn't smooth, so the guttering theory wouldn't apply. It absolutely emptied all day here, and the shed is dry inside so I know it works, whether or not it is too much, I'd imagine it is, but cosmetically it still looks ok.

[tinvek]

take a look at what run off angle commercial flat roof sheds use, you can be fairly sure that they have it as shallow as they can get away with to save on the materials that any extra height / roof length would mean

[barkis]

Mine slopes down, away from the direction of opening. It slides to the North. The North side, and the East and West side, have a 45 degree angled rail nailed to the edges. I cut my own by slicing a

lengths of 2"x2" diagonally through the bench saw.

The mineral felt covers the whole roof, and overlaps those rails, in effect making the roof a 3 lipped dish. Coupled with the slope, this ensured I only need to catch the run off via one short length of guttering attached to the bargeboard, The gutter needed a 90 degree turn to another short piece of gutter, the down a pipe into a water butt.

Make sure the roof slope is sufficient though, to allow rainwater to completely exit the roof, otherwise, should any remain, however little, the momentum as you open the roof, will propel any puddles down and into your obs.. And yes, it happened to me, and I had a mod to do..

Ron.

[Merlin66]

The gutter slope is based on a no-sag criteria.

A 1:20 is seen as a minimum.

[George]

What Ron said plus standing water will freeze in winter and crack your mineral felt covering.

[Tim]

Thanks.

At the moment I am looking at a building 4400mm in length, 2200 width, oriented roughly east west. It will be divided into an obs and warm room. The obs side will be 2100mm wide, on the east side.

The height of the west side, which will be the lowest, will be 1900mm, plus 18mm for OSB and 70mm planed arris rail around the edge (same as Ron has)

This felted roof side will be 2300mm wide, and the height of the right hand wall of it, is planned at 2000mm, plus 18mm OSB, plus arris. This gives a slope of 2.5 degrees, not sure what that is as a ratio?

The sliding roof will be on the same angle as the felted roof (I have some galvanised decorative sheet metal roof panels that may be used on the sliding roof), but raised above it. I plan to allow 10mm clearance above the felt roof with an overhang of 150mm. The roof itself will slide on rails mounted to the side of the building at a height of 1800mm.

The projected height of the east end of the sliding roof, (the highest bit) is about 2250mm IIRC, and I really dont want to go much higher than that, hence the roof slope question

Hope all that makes sense.

I'd be very keen to see pics anybody may have of obs with a roof that slides back over the warm room.

Another question I have taken the height of my mount as that of the pivot point around which everything rotates. This is 1500mm. The wall of the obs will be 1100mm away from that at a height of 1900mm. I have very poor horizons anyway with houses and lights etc, but am I right in thinking that theoretically, that would give me a shot at anything above 15 degrees?

PS, I hate maths with a passion, so pls go easy with numbers

Thanks again

TJ

[Wobbly Bob]

2 x 2 = Oooops sorry Tim , The point about standing water freezing is a good one. One of my sheds has suffered this very issue so needs refelting. I will check the angle on the good shed and come back to you.

[barkis]

I'm sure you have lots of room TJ, but in a neat environment, the pier should be offset towards the south to account for the Polar angle.

Ron.

[Tim]

Oh, thats interesting Ron.

The inside bit of the obs bit will be 2m x 2m between the walls.

What position would be best for the pier then? Good job you said, I am going to set the block for it tonight

[George]

My observatory is oriented due solar north/south, I used a couple of sticks to cast a shadow at solar midday (the shadow cast points exactly celestial north).

You can find the exact solar midday for your location here