Building an apex roof

Cazilou

Ok so I am going to replace the roof of my sectional garage with a corrogated bitumen roof. The garage is approx 5.5m long with a 2.5m span.

The walls are quite low at only 207cm to the top of the wall plate so I want to keep as much height as I can inside. I have toyed with the idea of building the trusses as standalone trusses with OSB/plywood holding them together at the ridge and attached at the wall plate with a birdsmouth and an overhang. This would mean I could insulate and board the roof rafters and keep the apex inside to create more headspace.
Having researched it more I have found that I may need a collar tie to hold the trusses together. If I was to do this I would need to either leave the roof and rafters exposed, board around the collar ties so they are beams within the room or put in a false ceiling.

If I was to put in a ceiling, could I use the ceiling joists as collar ties and nail into the rafters at each end or will I need to use a separate tie further up as a support?

There seems to be so many different ways to make trusses I keep toing and froing about which way is best.

Otherwise is it better to have a ridge beam spanning the length of the garage and attach the rafters to that? 6m is a long span for the beam and I was unsure if having 2 pieces as a ridge beam would work. Another reason for thinking of doing separate trusses is so that I can build them on the ground to the right measurements etc and then when the old roof is removed I can put up the new roof without having to have no roof on the building for too long as I will need to store the contents indoors and I have zero space.

Please excuse my terminology I am a complete amateur and have been doing research online with no real idea of what I am doing! 

Please help me work out what is the best way to build the roof and how you would do it, even if its something I haven't mentioned, and also anything I might have overlooked

TIA

Heedtheadvice

Cannot answer you question, I am going through a similar process, but....

One of the most important parts of the design is to prevent the downward forces (of and on the roof) being translated to lateral forces on the walls as the wall probably will not resist such unintended forces from a pitched roof! Hence all the standard ways with beams, trussed roofs etc to prevent those sideways forces that are not significant with a 'flat' roof construction being mainly vertical. You might only be making a lightweight roof but need to consider wind and snow loading too!

I wish to maximise the use of the roof space so am considering a rectangular framework (bolted or welded steel joist or similar) to resist the lateral forces and leave the interior of the roof space to be used as desired, provided it meets structural requirements of the regs.          Just an idea!

Do make sure to have planning permission if needed. Ridge height above ground level is my downfall so I need approval.

DOH_a

You cannot just make trusses. They’re a specialist item of which are designed and supplied by a truss manufacturer who would also produce the accompanying details, calculations, profiles and layouts.

What’s wrong with a traditional cut and pitch roof of which the design would be dealt with by a structural engineer?

stuart45

It's not a big span. You can use ceiling joists or collars. Ceiling joists are actually better, as the higher up the tie  the weaker it gets. It does not take long to pitch a roof that size. You can get all the rafter details online. https://www.blocklayer.com/roof/rafter.aspx

Cazilou

Heedtheadvice said:

Cannot answer you question, I am going through a similar process, but....

One of the most important parts of the design is to prevent the downward forces (of and on the roof) being translated to lateral forces on the walls as the wall probably will not resist such unintended forces from a pitched roof! Hence all the standard ways with beams, trussed roofs etc to prevent those sideways forces that are not significant with a 'flat' roof construction being mainly vertical. You might only be making a lightweight roof but need to consider wind and snow loading too!

I wish to maximise the use of the roof space so am considering a rectangular framework (bolted or welded steel joist or similar) to resist the lateral forces and leave the interior of the roof space to be used as desired, provided it meets structural requirements of the regs.          Just an idea!

Do make sure to have planning permission if needed. Ridge height above ground level is my downfall so I need approval.

Yes we have to stay under the 2.5m height as it's near a boundary so quite a low pitch for the apex. Thinking I might have to change it and just go for a flat roof and raise it a little to increase the ceiling height inside. Luckily it's an outbuilding under 15m2 in area so doesn't need to meet building regs. It's only going to be used as a summerhouse come shed.

I might get some people in to quote for it and see if its worthwhile getting someone who knows what they are doing, I don't want it collapsing on me in the winter (not that we get snow here ever!)

Cazilou

stuart45 said:

It's not a big span. You can use ceiling joists or collars. Ceiling joists are actually better, as the higher up the tie  the weaker it gets. It does not take long to pitch a roof that size. You can get all the rafter details online. https://www.blocklayer.com/roof/rafter.aspx

Good to know I can use ceiling joists and that you think it would not take long. I am not sure if I am taking on more than I am capable of, I normally put my hand to anything, I enjoy doing to scale technical drawings so tend to design most things myself (I wish I had gone into structural engineering!). On this occasion though there is a lot of calculating and knowledge involved that I think it may be beyond my capabilities. 

stuart45

It's not that difficult. Key thing is to be really accurate with the cutting of the rafters. I've done a few roofs( I'm a bricklayer, not a chippie), so I do have to try and pay attention to detail. If the building is out of level and out of square or parallel it makes the job more tricky. Always gives me more satisfaction when I  can do something different from my trade. Much easier nowadays with the help of the net.

Heedtheadvice

I would still go for the pitched roof. Flat roofs can be a real pain in the derrier when they start leaking! Fibreglass covering or other specialist coverings can be ok.

You sound like you enjoy the design element and as Stuart write there is a lot of info out there ....so go on give it a go!