Building garage roof trusses

Can someone give me a bit of advice about timber sizes for making roof trusses?

Looking at building them with a 15 pitch (as thats all i can do within the 8ft hight limit of permitted developement).
Garage is 8ft 6" wide and will be roofed with anti-condensation box profile steel sheeting so not very heavy.
Gonna be this sort of truss. King post i think they are called.


Will 2x4s be OK or will i have to got with 2x6s?

 

Would you not be better to get the trusses maufactured for you? Are you looking at 8 ft max due to boundary proximety?

 

Yep, it'll be about 1/2 metre from boundary. If it was 2m away i could build 3m (10ft ish) i think.

Can't afford to get them manufactured.

 

Fair enough. You dont fancy a 2.5 high flat roof then!

 

yeah why pitched and not flat..

 

whs ^^^^ I went through the same thought process when planning a car port but have now found somewhere locally that's much better for storage. If you are using profiles sheets they come in long lengths and could be laid with a very minimal fall.
So the cheapest and easiest option would be to run them end to end, joists/purlins at 1200 or even 1500 centres, metal trims and one length of 8ft 6" length of gutter. Only disadvantage could be lack of storage in the roof space but you would be able to maximise the ceiling height to get your van in.

 

with such a relatively small span (8'6") king post trusses will be overkill IMO, especially since you're planning to use box profile sheet. The only real advantage of using king post is that you will be able to store long timbers in the roof space...(I take it you're not intending to put a false ceiling in)
Personally, I'd specify raised Tie trusses, as it's surprising how much extra internal height you can achieve....they will be more than up to the job. Measure the total surface area of the roof, claculate how many sheets will be required and go to the manufacturers website...they should specify the weight per sheet. Timber size and number of trusses required is determined by both span and load. Any good roofing company will be able to work this out for you Free of charge, using a truss calculator.

Simply answer is 2 x 4 (finished size) will be adequate....pitched apex is almost always a better option to maximize internal height, particularly if you're limited to 2.4 m eaves height. Pent roofs can be max elevation 3m, but you lose internal height where you most need it in order to achieve a decent pitch (absloute min 5 degrees for box profile)this is a hybrid raised tie with small kingpost....best of both worlds, plus you can fit 8' fluorescents 'side on' without losing headroom

Try here for a calculator

 

i used 8in by 2in beams in mine

 

4 x 2 raised Ties here....

 

I would flat roof it and use the firestone rubber roof covering comes with a 15 or 25 year guarantee, I've used it on our garage and it's nice and easy to lay. Not too expensive either.

 

EPDM....

 

just guess , calcs are for structural engineers and their safety factors are mad. 4x2 is plenty fine , its more about spreading the load and how many you put up ,im sure your common sense could build one just fine .

 

The roofing sheets i'm looking at using can be fixed at a minumum of 7 degrees so if i had a single run from front to back it would need to be 28" higher at the front which is over a foot over the permitted height.

The box profile sheets according to the manufacturer weigh 140g/m2 so pretty light. Works out at about £180 including the ridge flashings.
I looked into EPDM. Worked out at about £150 for the rubber plus another £100 ish for OSB for under it.

We're only 1/2 mile from the coast so snow is pretty rare. Had less than 6" of it in 2010 when most of the rest of the country had double (or more) than that.

Vanorak, its only a maximum eaves height of 2.5 metres (maximum of 4m) if its 2m away from boundary. Its 2.5 metres maximum within two metres of a boundary.
Wall height is 7ft which doesn't give much height for storage in roof. Not that i've got anything to store up there anyway but it'd be nice to have the choice.

 

Good thread but dont think were quite answering the question. Just a thought though. I think the apex hight is 2.5 metres if your within 2 metres of the boundary if your building under PD. Just slightly taller than 8ft but I would think you would get more space with a flat roof.

 

Tell me about it

spent hours coming up with a spec for mine, that was within regs....that's why i suggested raised ties, as you can claw back some internal height, but keep total height within PD

 

are you right next to the house? Lean to regs are slightly different....but I reckon you're right to go with an apex personally
If you've got a Devon or Dormi...i think the absolute minimum height you'll get away with is 2.02m....ie no room for error....but don't quote me on that

 

How many purlins do you need for fixing the roofing sheets, your example picture just shows ridge purlins. How long is the garage? Your saying 7' to wall plate and only allowed 8' to ridge height?



I'm thinking 4" ceiling joist, plus 4" rafter, plus 4" purlin sitting on top of rafter. There's your 12" gone already and you don't really have a king post truss at all. Am I reading this right

 

^^^not many for box profile....(that's a technical answer btw)
if it's a short span, the sheets are only going to be short, assuming he goes with an apex
Call it 9' total span....15 degree pitch, sheets will come out around 5' (plus or minus with overhang)....I reckon you'd get away with one 4 x 2 purlin per side, two to make sure....
depends on how may trusses you decide to put in...

 

Just added a bit to post above.