Garage roof replacement, steel to timber.

[rash_decision]

Hi all. I'm currently looking at replacing my concrete sectional garage roof. It's currently a corrugated, galvanized steel roof, but it's never been fitted correctly. It leaks, and water runs back under the front, and along far enough to drip into the garage. The run is currently back to front. The condensation is also quite bad, hence the preference to change to timber.

I plan on fitting a timber, pitched roof over the next couple of weeks. My mate has a load of 4.8m length of 9" x 2" that are likely to be getting disposed of in his work, and his boss has said he can have them to use as trusses. My garage is 9' 6" in width. I have suggested to my mate that he finds the midpoint, and cuts from the middle to each end, tapering down to 2", giving us a run from 9" to 2", and a slope of 7" over 2.4m.

I've suggested 10 of these, as there are 10 sections along each side of the garage. I know this may be overkill, but the timber is going to waste anyway, and the stronger the better I suppose!

Does this run of 7" be enough to shed water? The roof itself will be 19mm ply, with a covering of felt.

[Etretat]

Not a structural engineer but what about the weight? The trusses are going to be a lot heavier than the existing presumably metal structure.
We are talking about a concrete sectional garage here aren't we? Not just the roof that's concrete?

I have no idea what weight your wall structure will support but think you should find out!!!

Edited by Etretat on Wednesday 2nd December 08:17

[TA14]

1 in 100 will shed water fine that's a two inch fall. 1 in 50 would be better and give a four inch fall. The way I read it you plan to alter the pitch to a duo pitch front to rear. It would be easiest to do this by putting one or two 9" x 2" beams across the width in the centre of the garage and then spanning the un-furred (if that's a word, if not, uncut) front to rear with noggins. I'd also put in intermediate supports at the quarter points.

[TA14]

Etretat said:

Not a structural engineer but what about the weight? The trusses are going to be a lot heavier than the existing presumably metal structure.
We are talking about a concrete sectional garage here aren't we? Not just the roof that's concrete?

I have no idea what weight your wall structure will support but think you should find out!!!

The direct vertical load isn't a problem but the effect of the extra weight on instability (the building collapsing like dominoes as opposed to crumbling under the weight) is a potential problem. To overcome this you need bracing; the ply on the roof will mean that the roof is well braced so you need to ensure substantial and frequent fixings from the roof to the walls.

[rash_decision]

Thanks for the replies guys.

To clarify, the garage is a concrete sectional, braced across the width with steel reinforced concrete lintels, which bolt onto the top of every second section. The existing steel roof is flat across the width, but runs from back to front. I want to fit a pitched roof with the peak running front to back and sloping down to each side.

I had considered the strength, but if I have 10 trusses, and 5 of then can be sat on the existing lintels, I reckon this will be strong enough, even taking into consideration a couple of inches of snow.

It was more the slope for shedding water I was concerned with, but in reading your reply TA14 this should be fine.