Cut roof design doc for building control.

[epsilonGreedy]

Would building control need to see an engineering design for a cut roof? I understand that when engineered trusses are specified from an external source, BC accept the engineering competence of the truss supplier.

 

I am just thinking about the garage roof at this point which is 7.2m x 5.6m hipped, 30 degree pitch with a slate covering specified, so not a large weight from an engineering perspective.

[Carrerahill]

On my BW drawings there was just a written description of the roof's construction and a simple layout showing truss and purlin locations.

 

Roof Construction:

 

King Post Trusses: 200x50 C16 for joist span with 150x50 C16 rafters with king post made up of 2 No. 150x50 spiked together. 4 No. trusses in total spaced as drawing.

Purlins sat on trusses, trusses notched to seat purlin. Rafters 100x50 sat on ridge, purlin and wall plate. Rafters to be notched. Rafters 600mm on centres.

 

I have a truss above each gable end, then I have 2 trusses tied together with 6x2's 750mm apart, this centre truss arrangement is centred in the middle of the garage and the purlins run from the gable truss to the centre truss nearest it, the purlins run wild through the 750mm span where they just touch but are not joined in anyway. This creates a very robust roof with lots of open space as there are technically only 2 "joists" at wall plate height the rest is totally open. I have 420 concrete tiles up there and when I was up doing the ridge tiles I had a little jump and the whole thing is solid as a rock!

Edited February 6, 2019 by Carrerahill

[Onoff]

This imo is a great bit of software. Can only go up I think to 5m span on the demo version and you can't print (Snipping Tool etc  ) but good for outbuildings etc:

 

http://www.runet-software.com/WOODexpress.htm

[PeterW]

2 hours ago, epsilonGreedy said:

Would building control need to see an engineering design for a cut roof? I understand that when engineered trusses are specified from an external source, BC accept the engineering competence of the truss supplier.

 

I am just thinking about the garage roof at this point which is 7.2m x 5.6m hipped, 30 degree pitch with a slate covering specified, so not a large weight from an engineering perspective.

 

What is providing the lateral restraint there as that will try and push both walls out, worse with a hipped roof as the corner jack rafters put lateral loads on the points. 

 

Has this got buttress pillars down the sides ..?

[epsilonGreedy]

2 minutes ago, PeterW said:

What is providing the lateral restraint there as that will try and push both walls out, worse with a hipped roof as the corner jack rafters put lateral loads on the points. 

 

Has this got buttress pillars down the sides ..?

 

 

I lay awake at night contemplating lateral forces from my hip and common rafters.

 

By a stroke of luck the dividing wall (5m) between the garage parking area and the utility room of the garage sits right under one end of the ridge. This wall will be a cavity block wall and I intend to run a wood wall plate along this linked to the wall plates along the main walls. If I then add a couple of rafter support uprights supported from this wall roughly where an attic truss would have uprights then I hope this end of the roof will be suitable stiffened.

 

The ceiling of the utility room 5.0m x 2.4m will have joists fixed to the wall plates so that should deal with the corner rafter lateral loads 

 

The other end of the garage ridge has a brick lean-to shed 3.3m which will buttress the 5.6m end wall.

 

This leaves the lateral forces spreading out along the ridge 1.65m. I reckon the minimum starting point is a chunky beam to tie the opposing walls together an hence resist the lateral spreading.

 

 

[Oz07]

Dragon ties on hips

[Russell griffiths]

I take it you don’t want a ceiling in the garage

or is there another reason for not having trusses. 

[epsilonGreedy]

On 07/02/2019 at 17:34, Russell griffiths said:

I take it you don’t want a ceiling in the garage

or is there another reason for not having trusses.

 

 

One motivation is wanting easy access to the attic space above the internal utility room this is built into the garage, this will be accessible from the car parking side of the garage.

 

I was also impressed by a barn style garage built by a neighbour in my previous village, this had a cut roof and some useful storage had then been developed within the attic space. It seems counter intuitive but having looked at other garages I do not feel that a raft of closely spaced spindly trusses, actively promote useful accessible attic storage. I could be wrong and so would be interested to hear about garage attic space storage with trusses.

[epsilonGreedy]

On 07/02/2019 at 12:59, Oz07 said:

Dragon ties on hips

 

Yes indeed, simple agricultural and effective.

 

 

 

[bassanclan]

When i specced a cut roof I got a truss manufacturer to supply and provide engineering calcs, then liability was all in one place.

 

Probably worth having an idea of the design so you can cost up the loose timber independently anyway.