Flat roof joist strengthening for roof terrace

[Harry_W]

I'm in the process of re-roofing my flat roof and was thinking about the possibility of using it as a roof terrace in the future. I've stripped the roof and the current joists are 2 X 6 inches and are 2.45 m in length. The spacing of each joist is 20 inches. Would sistering with the same dimension joists suffice or would I need to move to 2 X 8 or 2 X 10? The total dimensions of the single storey roof are 2.45 m X 3.4 m.

 

[DAZB]

The joist size for a span as short as that would be fine in my opinion but you could also fit noggins of 6 X 2 mid section if there are none. Top it with an 18mm Marine ply and also consider doubling up with a row of noggins beneath the ply deck around any perimeter if you have one so you can have a solid point for fixing handrail supports etc....

 

[Harry_W]

Hi Dazb, thanks for your input. So do you think it would be ok to double up with 2 x 6 s. The missus will probably want a couple of planters on there so just want to make certain it's strong enough. I did think about buying a couple of 152 x 89 mm U Beams as belt and braces. What do you think?

Thanks for the advice about the noggins.

 

[DAZB]

According to the Span Tables you are within the limits for load bearing with just what you have now, i.e. single 6 X 2 joists.
Have a browse at the link :
http://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2010/Part6/section4/appendix.htm

Using C16 47mm X 147mm joists it shows that at 450mm spacings you can have 2.58m and at 600mm spacings it drops to 2.18m.
As you are spaced at round about 500mm it is a third of the difference (50mm of the 150mm gap in joist spacing) so taking a third of the difference in allowed span which is a third of 400mm, = 133mm. So taking 133mm from the allowed 2.58 at 600mm gives you an allowed 2.45 for the 500mm spacing you have. Without breaking it down to the mm I reckon you are all well and good with what you have.
If you have plans for a hot tub however then you will need to rethink.

 

[Harry_W]

Definitely not a hot tub Dazb but I can imagine pots with bamboo, a charcoal bbq and perhaps 8 people when we have a party etc. So U Beams would be overkill then? I might sister the joists just for peace of mind.

 

[freddiemercurystwin]

Will you be getting planning and building regs?

 

[Harry_W]

Yes but just wanted to know feasibility before I begin the process.

 

[freddiemercurystwin]

You might find this thread useful, irrespective of what you may use or do up there the deck and handrail and the connection between the two will need to be able to withstand whatever any future occupier does up there and an engineered solution will be expected by building control not something advised on a forum, planning permission aside of course! https://www.diynot.com/diy/threads/how-to-fix-decking-to-fibreglass-flat-roof.237647/#post-1712621

Anything's possible if you throw enough cash at it.

 

[Harry_W]

Thanks Freddie, I've been thinking about how the railings will be fixed to the flat roof.