polycarb roofs and snow loading?

[skyblob]

For a lean-to polycarb car-port type roof, (allowed by planners in my case), running with timber sizes from Trada span tables for snow loading (1KNM^2 in my case) ends up with enormous timbers because of the big spans required. - Much bigger for instance than one would "normally" extect to see in use for a typical polycarb roof. (eg 9x3 purlins required in my case).

Polycarb roof manufacturers all fudge this issue when asked (and I have asked). None have any clear answers to the building regs questions, other than to say their roofs don't fall down (which is of course perfectly true). It's pretty clear then, that snow loadings are not normally applied for polycarb car ports.

I have Building Inspector visits to come (for other parts of my house, still underway). Would we expect him to get upset about inadequate snow-load allowance for a polycarb lean-to if I build it now? - Do I even bother to ask him?

So has anyone had any problems with polycarb roofs and building regs, or shall I just ignore regs, go for what works, - like everyone else seems to get away with?

 

[Static]

Normally the poly carb is designed to span the centres of the roof joists and support just basic snow.. i think the manufactures will give guidance on thickness of sheeting required for various spans..
Or they run on a load sharing either way its down to them to supply the correct sheeting for the loading you have..

The joists should be designed for something like 0.75kN/m2 snow on 0-30deg roof and 0.6kN/m2 on pitched over 30deg..
That does not include the self weight of the sheeting or any services (lights etc)..

What are your spans?

 

[skyblob]

Hi Static.

I'm Surrey, at 350' ASL, on the edge of 1Kn/M^2 and I have used that figure for that for my other (proper) work.

Plastic roof will be lean-to, 3.5M top to bottom and 3M wide. Interpolating a little from Trada (table doesn't go beyong 600mm), and fudging things a little suggests something like 75x100 rafters at 1000 spacing, supported by 2 purlins 75x200 1M apart, and also an eaves beam 75x200.

It's the purlins I have a problem with because of the depth.

I could do it without purlins if I use rafters 75x200 at 1M spacing and eaves beam doubled up 75x200.

 

[Static]

I'm Surrey, at 350' ASL, on the edge of 1Kn/M^2 and I have used that figure for that for my other (proper) work.

Urm not sure where your getting this figure from, I use BS6399-Part3..
You designing it for snow drift load rather than just simple snow loading? Or are you over 100m from sea level??

The trada tables are more directed towards traditional roofing, where the roofing deck forms a load share system. Which doesnt really apply to poly carb.. or at least i wouldnt rely on the poly carb to form a membrane..

What kind of length is the car port? 5m?

 

[skyblob]

Hi Static, well yes I am above 100M (not by much) and just into the 1knM^2 area. Snow loadings are in Trada tables, not drift , just heavy snow. I thought that was standard stuff. (I don't know about BS6399-Part3 though!)

Anyway. structure is 3.5M gutter-wallplate and 3m wide. Intermediate supports are not acceptable, so I need a 3M eaves beam span. Am planning on 1000mm rafer spacing. New plan is eaves beam in 9x2 doubled up, and rafters in 8x3 without any purlin, but possibly central noggin to hold them square.

 

[Static]

8x3s are a little spongy but with the central noggin should be fine..

A couple of 8x2s bolted together should be fine for the 3m eaves beam.