flat roof strengthening

[TANK]

Bit of a strange one here. 

Never built anything before, but I'm now nearing completion of my block shed/workshop. As has been the case throughout the build, another rookie mistake has been made with the flat roof. Having decided to change from 400mm joist centres to 600 in a money saving exercise (I worked out from the tolerance tables found online that my 2.75m span would be OK with with 6x2 @600mm), I then proceeded to cover with 11mm osb decking... uh oh! Then realised it was a tad too thin, particularly since the noggins give zero support as the deck is now higher than them as it rests on the firrings later placed on the joists to give a slope (i know I know...).

I have marked lines on the deck so I can better see the joists and know where to put my feet when working up there, and the boards do support me but I do have to take care as there is some flex if I miss the joists with my feet.

I have strengthened the middle section by inserting another stretch of noggins higher up, which was a pain. 

 

So, am I over thinking this? I'm about to go ahead with putting the epdm down. Just worried that if I don't act now I might be in for trouble in the future. Will it hold a heavy snow fall? Will the boards start bowing, causing an unsightly sag? Have just acquired by good fortune a large amount of free 18mm osb (just over half the roof total space), the option I'm toying with is to place this across the top half of the roof, leaving a small 7mm 'step' down to the thinner portion of lower roof which will be strengthened later internally. Does this step sound viable with epdm on top? 

 

any help or advice or help before I take the plunge would be greatly appreciated. 

 

Thanks

 

Tank

[markc]

2.75m span will be fine with 6x2 timbers. 11mm OSB is no good for walking on. Snow loads are distributed so no problem.

[Russell griffiths]

If you have enough 18mm to do half then you only need half the amount to cover the whole thing in 18mm, no step. 

You cocked it up once by trying to skimp, don’t repeat this. 

Cover the lot in 18mm, then go underneath and add in as many noggins as you can pushing them up tight under the boards. 

 

If you dont , it will sag and you will get puddles. 

[Mr Punter]

I think you need 18mm all over.  I bought a couple of sheets from Wickes yesterday at £47 each, so I feel your pain.

[markc]

Just re read the post, so the OSB isn’t sitting straight onto the 6x2’s ?? And you are packing off the 6x2’s to create the slope? In this case you have created a hell of a lot of extra work and materials and my last post isn’t right, unsupported 11mm OSB will sag and fail

 

 

Edited November 5, 2021 by markc

[makie]

Flat roofs should always be done with 18mm thick wood.

If I ever turned up to a job and it wasn't done with 18mm I would walk away.

[TANK]

My decision was based on the fact I already had a fair amount of 11mm, and my research indicated that it would be OK. 

Having seen the 11mm on action (ie on the roof) I fully understand and agree with you all. It might be viable - although far from perfect - atop 400mm centres with a good placing of noggins, but they get very much found out atop 600mm centres. 

I'm gonna have to think and work quick on this now, as I was banking on using this weekend's weather window to get it finished ?. 

 

Thanks again all. 

 

ps the problem with doing something for the first time is that you are learning a multitude of new skills as you go - it's harder to look too far ahead as you're so focused on one thing at a time, and inevitably problems will arise. 

[Conor]

Would be easiest to fit another layer of 11mm OSB on top of the deck? No messing with noggins.

[TANK]

it would be easier perhaps, but more expensive as I already have a decent amount of 18mm. 

cheers