Garden room Flat roof Joist sizing

[James99001]

Hi all, I've read many posts on the subject but thought it was time to post my own query. The ultimate question - will a 50x150 roof joist be sufficient?

Background - I'm building a single leaf (100mm) concrete block garden building with a flat roof design, keeping below the 2.5m rule for permitted developments. The building will be Class 6 under the building regulation exemptions and therefore does not need to comply with building regulations, however I believe the regs are there for a reason and have still reviewed Approved Document A and trying to follow it or follow closely.

There will be no permanent access to the flat roof and we live in the South of the UK, so snow is not a big concern. Biggest live load will likely be the neighbours' cats.

The building is a slightly unusual shape due to the shape of the garden. The front wall (which has 2 doors and a window) is 4.8m long with the end walls square, however the rear is tapered. Essentially the left hand wall (and hence the roof) will be 5.7m long but the right hand wall is only 2.6m long.

Whilst ideally I would choose 75x220 joists, this takes me over the 2.5m.

On top of the concrete blocks will be a wall plate which will be strapped to the walls at 2m centres (at least), firrings about 100mm high at the high end (albeit I could take down to 72mm to meet the 1:80 drop if necessary), topped with 18mm OSB and then EPDM. Cold roof insulation above plasterboard finish.

So my options or any other suggestions from you lovely people:
a. 50x150 joists at 400mm c/c will be fine - stop worrying - plenty of tolerance in the span tables.
b. 50x150 joists - but do 200mm or 300mm centres on the left hand side (hidden by plasterboard and soffits/facias so won't be seen)
c. trust the span tables - go with 75x220 and redesign something else

Thank you in advance for any advice or suggestions.

James

 

[Dereekoo]

a) no way
b) at 200 centres and assuming the dead load is not greater than 2.5kn/m2 and snow loading does not exceed 0.4kn/m2 as opposed to standard 0.75kn/m2 it just comes within acceptable deflection limit of 17mm
c)the span tables are the span tables

 

[James99001]

Thanks for your input. Part of the reason I looked at 50x150 was that I was able to find lengths of 5.4m+ but was having difficulty finding longer lengths for the 75mm width from my usual suppliers. Had another search after your comments and found a supplier that will do longer lengths for 75mmx150mm, so I'll use that width until I get below 3.3m on my span. Will still do at 200mm centres until I get to 4m span.

 

[Dereekoo]

a) no way
b) at 200 centres and assuming the dead load is not greater than 2.5kn/m2 and snow loading does not exceed 0.4kn/m2 as opposed to standard 0.75kn/m2 it just comes within acceptable deflection limit of 17mm
c)the span tables are the span tables

Typo that dead load should read 0.25kn/m2

 

[Dereekoo]

Thanks for your input. Part of the reason I looked at 50x150 was that I was able to find lengths of 5.4m+ but was having difficulty finding longer lengths for the 75mm width from my usual suppliers. Had another search after your comments and found a supplier that will do longer lengths for 75mmx150mm, so I'll use that width until I get below 3.3m on my span. Will still do at 200mm centres until I get to 4m span.

75 x 150 will enable you to have snow loading of 0.75kn/m2 ,dead load of 0.5kn/m2 and have deflection of 17mm which is 3mm above recommended max of 14mm but its got to be quite a large snow fall for that and down south snow loading can be reduced slightly

 

[cdbe]

Use 12 or 9mm OSB and assume that 1mm of rubber and a bit of glue weights less than 18mm OSB plus 3 layers of felt plus a few sacks of gravel. Cutting joists to a taper instead of firrings will help a bit as well.

 

[James99001]

Thanks cdbe. I was saving the 18mm to 11mm osb board change to compensate for my lack of skill in keeping to a 10mm bed between courses!

In terms of the firrings, cutting the joists was my original plan until I realised that using a 220mm joist and doing an 85mm drop would take it to only 135mm - lower and less structurally sound than the 150mm joist minimum discussed above.

One idea I have been thinking about is changing the direction of the joists so it is a maximum of 4.8m span width wise. The firrings would then be perpendicular. They would need to go to a minimum of 38mm (rather than 0mm) but I would lose the (57mm) wall plate as there would be no openings on the joists wall. I'll probably decide when I get there.




Ps. As an aside - really not enjoying working with Aircrete blocks. Sucking all the moisture out of the mortar with this hot weather exacerbating the problem. Highlights even more my lack of skill in doing a decent mortar bed with no workability once the block is placed.

 

[James99001]

Sooo - I'm having great difficulty in sourcing the 75mmx150mm c24 joists in 5m+ lengths. I plan to do the sideways span which is 4.8m plus some overhang. I will then do the firrings perpendicular.

Instead of using 75mmx150mm can I bolt together 2 47mmx150mm with say 4 bolts to double them up? As this would effective 94mm, can you use that width in reality when looking at loads and spacing?

Thanks

James