Garage Flat roof

[David Vieyra]

I am currently having a 75sq m garage built which will be 6.1m wide inside. Building control want the rafters to be max 3m at 400mm centres. The roof will be 18m OSB or similar with 3 layers of felt and hot bitchumen and stones to comply with BC. I will need 3 steels set on piers to hang the 3m rafters. I was proposing to use 203 x 102 x 23KG S355J0 Universal beam with a 175 x 50 beam bolted either side and the refters hung off joist hangers. Would this size steel be correct?

 

[^woody^]

Maybe.
Why wont building control tell you if they are telling you about the timber sizes? And why can't you use deeper timber at greater spans and do away with one or two beams?

And are you talking about rafters or joists?

 

[David Vieyra]

Sorry joists. Building control want 3m joists at 400m centers. I wanted 4m joists so would rater go with their requirements. Apart from wanting extra piers in each wall they have not asked for any other changes. I want to tell them what I propose so was looking for confirmation a beam of this size would be correct. I told them I proposed 175 x 47 timber but as we are not up to roof height they may want bigger timber.

 

[^woody^]

You won't get confirmation unless someone wants to do the calculation - and show you the calculation to prove it.

You can tell building control your proposal and see of they agree based on their experience or if they ask for the calculations to prove it.

I can't understand the 3/4m joist issue. If 4m joists are adequate then you can use them and building control can't say jack.

But in general terms you need a whole solution, not just one based on preconceived "3m joists and 3 beams". Bigger sections at wider spacings and fewer beams may well work out better. But you need a suitable person to design it.

 

[David Vieyra]

The 3m joist are because BC want piers every 3m as it is single 100mm blocks so it makes sense to put the steels on the beams. At 4m joists, it would still require 2 steels. I am also limited on height to 2.5m as the building is just inside the boundary. I was hoping for some guidance from someone who had used a 6.1m steel and the size they used.

 

[David Vieyra]

As no one has been able to help I thought I would post the solution to help others. I have used 152 x 152 23Kg UC at between 3m and 2.6m centres and the joists at 400mm centres. The steels are only 5.65m in length. This time I got the normal BC for my area who was more realistic than the first standin BC who was an ex-architect. This guy was an ex-builder and relatively speaking the roof is not heavy and using 147 x 47 joists it will be more than adequate. The only stipulation was to use fire rated felt so I do not need hot tar and stones.

 

[ivixor]

Why felt? It's an obsolete roofing material and an absolute pig to get on. EPDM is far far better for flat roofs.

 

[David Vieyra]

Cost and installer skills. The roof is an odd shape more a rectangle with a triangle. If anything an alternative would be a fibreglass roof. Also, I understand rubber can be punctured and so leak. Felt is not obsolete or it would not be so readily available.