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Help with Shed rafters

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spearson92

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Hi all,

I've turned to the forums for some advice on shed rafters.

I'm currently building my workshop/shed in a funny part of the garden. It's forced me to make a wedge shaped building, with the wider end being about 2.9m, and the narrow end about 1.4m. It's about 4.9m in length measured on the front of the shed. It's a pent roof style, and the pitch angle on the wider side is 7 degrees, with the narrow end being 13 degrees.

My question is: for these kind of shallow pitch angles, do the rafters need birdsmouth cuts in them, or can they be fastened to the top-plate straight off with hangers/brackets? I was planning to have one rafter at each end sat on the frame, but these would then sit slightly higher than those that have birdsmouth cuts unless I shave a bit off, is that correct? The frame by the way is 38x89mm c16 frame.

Any help and advice would be greatly appreciated! Already enough of a job having to measure each rafter angle separately :shock:

Cheers,

Steve
 

MikeG.

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Nightmare, Steve!!

Are you saying that the plates front and back are level (horizontal?). I which case, the pitch of your roof varies from one end to the other (ie it twists). What on earth are you planning on covering it with? I think I might have been tempted to have a sloped plate so that the roof was planar.

As to your questions....

At such low angles having a birdsmouth isn't absolutely necessary, but it would be ideal. With lightweight rooves, the biggest forces tend to be upwards during strong winds, so the most important thing is to hold the rafters down well, with straps or purpose made clips. One end of your rafter is only going to make contact at one point, I reckon, which isn't ideal. At the least, you might consider wedges fixed permanently in place to support the raised edges.
 

spearson92

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Hi MikeG,

Thanks for the reply! Yeah that's correct, the roof will twist slightly due to the changing pitch. Originally I thought of OSB'ing the roof and having felt shingles. They recommend 15 degrees pitch though and the shallow side is the one exposed to the elements, so I think instead I will OSB it and felt it, with a good membrane underneath. Might have to cut the OSB down though into thinner strips to make it work. I thought the felt might have just enough give to work with the twist.

I see what you're saying. I was originally going to cut a birdsmouth on them anyway, but I was actually going to cut 2 (one for each wall). I wasn't sure if this is standard though for monopitch rafters, but I'm guessing not :D

As for the rafters on the ends, which are sat on the frame, I wouldn't be able to cut a birdsmouth. In this case would it be a problem if they are then sat slightly higher than the rafters that do have a birdsmouth?

Cheers

Steve
 

MikeG.

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All the rafters have to sit at the same level. You can't have the outer two higher than the others.

I'd recommend EPDM instead of felt, and I'd also say good luck getting the OSB to sit flat. That'll be a pig. You might have to do it in 2 layers of 9mm rather than one of 18. Even so, it won't want to cooperate. Honestly, if you aren't committed, I'd have a little think about flattening that roof off ......removing the twist. You could do that fairly easily by setting up a temporary false plate parallel to the higher one, but at the lower level, setting your end two rafters in place, and marking up an infill piece (which will be a long wedge) to sit on top of the lower plate. It shouldn't be too difficult, and will save you some grief.
 

Hornbeam

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Enjoy all the variable angles. It would make things much simpler to have a monopitch roof all at teh same angle and slope the wall to accommodate
 

spearson92

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Thanks for the advice guys, much appreciated!

Sorry for my ignorance MikeG, could you explain a bit more your idea about the parallel plate please? Would this cause the roof to stick out further behind the shed?
 

MikeG.

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No, the roof ridge and eaves would stick out exactly as far as they do now, but the top of the roof would be planar (ie not twisted), and one of the walls would have a sloped top.
 

RobinBHM

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As Mike says, the far easier and most attractive solution is to make the wall plate slope.

It will create a sloping gutter though.....

I think I wouldve gone for a flat roof :D
 

spearson92

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OK guys. Thanks for the info, much appreciated!

I have a few days before at work now so will have a think about the best way forward.


Cheers, Steve
 

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