Steel sheet roof to new large garage

[Pantera2075]

Hi all, Noob here, and i'm hoping to get some advice on the roof of my new garage if you would be so kind?

Garage is 9m wide by 6.5 deep being built under pd but due to the footprint building control are involved. We have dug into a slight incline so we are looking at the 2.5m pd plus about 600mm gained by the excavation.

Currently the raft base is part poured and shuttered for the final pour which should be complete in the next few days

It will be brick inner and outer skin with a cavity as I can get extra cheap reject bricks

2x garage doors and a pedestrian door in the long side which is also the gable.

It will be a very shallow pitch side to side around 12 degrees (Can't remember exactly but its something like that).

 

Now, i'm a mechanical engineer but not a builder so I might ask some really stupid questions in an attempt to square it with my mechanical mind.

 

With the price of building materials these days i'm looking to make it the most cost effective - not necessarily the cheapest but that will be a major factor.

 

Roof will very likely be steel sheet, dark grey but of as yet undecided design. Roof will be effective 1m high above the 2.1m eaves. I want to leave the roof inside as open as possible so I dont want multiple traditional trusses.

 

So, what are my options for the structure of the roof? I can see that an 8" zed purlin will do 23feet in a single span which is long enough to do the 6.5m.

Is it ok to screw steel purlins to a wooden wall plate? I'm assuming current steel prices mean its best to stay away from universal beams if possible?

What do I do at the gable ends where the purlins meet? Would these need to be steel? Would it be an idea to put in steel uprights within the walls to bolt the superstructure of the roof to? Is it an option just to have a truss, either steel or wood on the top of the inner leaves of brick?Could it jut be a wall plate angled up the inner leaf?? Obviously tied down to the walls. Would it need a substantial wooden ridge board to hold the ends apart and transfer load to the gables?

Is there a more widely used more sensible design for this?

 

Thanks in advance

Phil

 

 

 

 

[gaz_moose]

seems a shame to use steel sheet for the roof if you have cavity walls.

[markc]

So many possibilities here. 
Z or E purlins need to be secured on the vertical faces (portal frames have flat or pressed steel uprights to bolt the purlins to. 
what are you going to use the building for? A steel roof may be cheap but it’s going to be cold, draughty and lots of condensation unless well constructed and insulated.

yes steel is expensive but a UB ridge beam with timber rafters is a clean and cost effective answer again depending on what you want.

you don’t say which way around the pitch or double pitch will be.

if you can be more specific then I’m sure we can give better suggestions

[joe90]

My workshop roof is steel sheet but on pre made wood roof trusses. I was worried about condensation but used breathable roofing felt below battens and had no condensation issues at all. Yes it will be cold so why the cavity?, mine is single block (rendered) with pillars. Mine is also under PD. (I think 12’ might not work with roofing felt 🤔)