Cut roof method

[deuce22]

Hi.

 

I've just started framing my roof with a carpenter friend. The way that he frames roofs with steel ridges is to bolt timbers into the web, flush cut the rafters, tie the rafters together with bolts into a cross section and then nails long legged joist hangers under the cross section below the steel. He believes this is stronger and it is more cost effective than angled rafter hangers. His preferred method is to cut the rafters to sit on top of the steels, but as I'm doing a warm roof and I don't want to add any extra height above the ridge, this is the other method.

Is this something people have seen before?

[Simplysimon]

10 hours ago, deuce22 said:

Hi.

 

I've just started framing my roof with a carpenter friend. The way that he frames roofs with steel ridges is to bolt timbers into the web, flush cut the rafters, tie the rafters together with bolts into a cross section and then nails long legged joist hangers under the cross section below the steel. He believes this is stronger and it is more cost effective than angled rafter hangers. His preferred method is to cut the rafters to sit on top of the steels, but as I'm doing a warm roof and I don't want to add any extra height above the ridge, this is the other method.

Is this something people have seen before?

nope, but sitting the rafters on the top of the steel will keep the steel warm and inside the envelope. more secure fixing and strap fixed to each other across the top.

[deuce22]

The reason we're not sitting them on top is because the person I had dig the foundations, didn't do them to the correct level (they were almost 500mm to high). I was told that this would be within the tolerance that planning would be ok with. If I sat the rafters on top, it would add, close to another 500mm.

As the chippie told me not to worry as he can flush fix them, I wasn't concerned. However, I am now wondering if this will be strong enough, once its loaded with weight.

I mentioned about using the rafter hangers and he said he prefers the way that he does it (he believes 2x 20mm rod, bolted each side, with the horizontal tie supported on joist hangers is stronger).

The area of the roof is around 140m2, the weight of the tiles is around 6 tonne and I've allowed another 2 tonne for the other materials. This works out at 57kilos of weight per m2, so with a rafter length of 4m @ 400mm centres, one rafter is supporting just over 90kilos. If these calculations are correct, it doesn't seem like a large amount of weight per rafter.

I'm just trying to find the best solution to this issue before we get too far ahead.

 

Thanks.

[Thorfun]

20 minutes ago, deuce22 said:

The reason we're not sitting them on top is because the person I had dig the foundations, didn't do them to the correct level (they were almost 500mm to high). I was told that this would be within the tolerance that planning would be ok with. If I sat the rafters on top, it would add, close to another 500mm.

As the chippie told me not to worry as he can flush fix them, I wasn't concerned. However, I am now wondering if this will be strong enough, once its loaded with weight.

I mentioned about using the rafter hangers and he said he prefers the way that he does it (he believes 2x 20mm rod, bolted each side, with the horizontal tie supported on joist hangers is stronger).

The area of the roof is around 140m2, the weight of the tiles is around 6 tonne and I've allowed another 2 tonne for the other materials. This works out at 57kilos of weight per m2, so with a rafter length of 4m @ 400mm centres, one rafter is supporting just over 90kilos. If these calculations are correct, it doesn't seem like a large amount of weight per rafter.

I'm just trying to find the best solution to this issue before we get too far ahead.

 

Thanks.

if you're worried then I would consult a structural engineer to perform the calculations for you. even if it costs £500 surely that's worth the peace of mind?

[markc]

As above, and to add, if I’m picturing the detail correctly then then his proposal would be more than adequate to take the loads but as it’s unconventional the BCO will likely want a SE sign off

[JohnMo]

Can't understand why you would build and not have a structural engineered design and then follow it.  I like the Scottish rules, no structural design certificate, no warrant, no build.  Keeps everybody right.

[George]

Could you post a picture or a sketch of the proposal?

 

For vaulted roofs the default answer is to use joist hangers (don't need angled ones, just birdsmouth the rafter), but not necessarily for the vertical load. There are more unusual forces at the apex of a vaulted roof than in a traditional cut roof or truss roof.

 

The connection in vertical load might not be overstressed, but when combined with lateral racking and uplift etc, the combined forces may cause some movement. And movement in a vaulted roof is not a good thing.

[deuce22]

22 minutes ago, JohnMo said:

Can't understand why you would build and not have a structural engineered design and then follow it.  I like the Scottish rules, no structural design certificate, no warrant, no build.  Keeps everybody right.

I agree. I don't think its my job to have to question things, but the carpenter thinks I'm over reacting. The SE is a family friend and the carpenter is also a friend of his. He's obviously just assumed that the carpenter knows what he's doing and hasn't given me any calculations.

The angled hanger that is advised for this has a SWL of 520kg, but a normal hanger is stronger. He has said he will do it with normal hangers as he prefers these to the angled type. He will birds mouth the rafters at the top, just like George has mentioned, similar to the image. He will then attach the horizontal tie and bolt into the rafters.