Roof for new shed/cabin

[roach]

Hi Folks,

Can anyone give me some advice on the best way to put a shallow pitch roof on my new shed build?

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• roach
• 7 May 2014
• 2

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It's 3m x 4m and I don't want to go up more than 500mm from the wall plate.

I'm wanting a 600mm overhang at the front gable end and 300mm at the back.

I'm thinking of having a 4.9m ridge beam with rafters coming off that 600mm centres, 18mm osb, underlay and shingles. Or maybe fibre cement slates?

 

[ree]

I hope that you dont mind me saying but your arrangements for supporting the floor carcass are unsafe, and with exposure to weather will become even more fragile. Perhaps, google up some pics and explanations of deck collapses in the USA.

As for the roof - set your rafters at 400mm centres, they will have to carry a lengthy ridge (16ft) with coverings weight.
Maybe use asphalt shingles?

Tied into your top plate and rafters you will require joists.
The rafters are cut as per normal for any cut roof, extended to overhang at the side walls.

Mark out a full scale section of the building span, top plates, and ridge rise on the cabin floor - this will enable you to get the correct lengths, and angles for the ridge and bird's mouths.
Dont cut the rafter tails until all the rafters are set.

 

[roach]

I hope that you dont mind me saying but your arrangements for supporting the floor carcass are unsafe, and with exposure to weather will become even more fragile. Perhaps, google up some pics and explanations of deck collapses in the USA.

How come? Because they are concreted in to the ground?

 

[ree]

The posts will rot in the earth.

The double joists that are acting as beams, or girders, are not fixed in an approved manner. The posts should be double notched at the top to take the weight of the beam joists.
Over time wet decay will cause sagging, and the nails to pull out - a common cause of deck failure.

The RH side of the cabin is perched on various non-Regs packings that in time will move and sink.

 

[freddiemercurystwin]

You've royally ****ed on his day there ree!

 

[roach]

According to the UK Timber Decking Association posts concreted in the ground have a lifespan of 30 years.

The intermediate beam is supported that way on the 4" posts but the outer 6" posts are notched in the correct way so the beam is sitting on the post.

 

[ree]

I didn't pick up on the notched 6" post but i'm now sure that what you have in the way of posts is adequate. I was wrong.


Fair enough that the lifespan is possibly 30 years - i'm wrong there too, and i apologise for giving you duff information.

That leaves the RH side?

 

[roach]

That's fine mate. Good to have the experts check it over!

I was surprised at the 30 years myself, but I thought if I need to reinforce it at a later then I will.

What do you think I should do with the RH side then? Add to the supports with some blocks on concrete?

 

[ree]

Perhaps leave it as it is - any more digging out and disturbing the soil on that steep slope wont help.
Just make sure that you screw Simpson tie connectors at each existing post and beam joists position. I can see some ties on the front posts.

 

[roach]

Can you advise on if any ventilation will be required if I insulate between the rafters?

Since the rafters will be 100mm thick and say I use 50mm celotex, do I need to have gaps over/through the ridge board to allow air to go from one side to the other?

 

[ree]

No, dont hole/vent the ridge board. Perhaps fit eaves venting of some kind but perhaps even thats not required.

I'm not the best guy to ask about venting such a small structure. Maybe contact Kingspan Insulation tech advice and explain your situation?

What you want to avoid are any roof penetrations, they are a constant source of leaks.