Flat roof

[gedduffy]

Dear Sir,

I need to raise the height of the flat roof covering the garage of my house. The roof is to be raised by approx 300mm. My question is regarding the rebuild of the roof. What size joists do I need for a span of ~3m? There is nothing to be built on top of the roof. Also, how much of a fall is required for rain to drain off?

Thanks in advance,
Gerard Duffy

 

[PowerTool]

Quick rule-of-thumb guide I read :-

For joist thickness - divide span in feet by 2,then add 2
i.e. 3 m = 10 ft
10 / 2 = 5
5 + 2 = 7,so use 7" x 2" joists

However,as you are raising an existing roof,I assume it has joists now,so what size are they ?
And the same goes for the slope - if you raise the whole wall structure by the same amount,you automatically have sufficient slope.
My garage is about 7metres long,built of brick,and the back is 3 courses of bricks higher than the front - have never measured the angle,but the water runs off