which size rafter or purlin?

[tate]

hi all
im doing a low slope roof above decking at 5 degree and it going to be covered with 900mm x 4mts long polycarbonate sheets..i need to know what size joist i need to span 3.900mm without the use of purlins..the joist spacings will be 900mm with 4x2 noggins..also it will need to take the weight of snow etc.. or joist size with purlins and what size purlins..anyone could answer me on this?
many thanks

 

[noseall]

Probably 200mm x 47mm at 450mm centres.

900mm centres is too wide a spacing a does not even exist in any tables.

 

[Static]

You'll need something like 225x50 C16 members or 250x50s at 900 centers.. but you will need to check that your polycarb sheets can span that distance, normally they will span between crossing members at about max 1.5m spacings.. check with the sheeting manufacturers info..

 

[tate]

thanks for the replies
i didn't explain which way the polycarbonate sheets will be running as they will be fixed onto glazing bars which are fitted on top along the whole length of the rafters..the slope will be running away from the extension at 3.900 mts .
that why im doing 900mm spacings between the rafters..as it seems i need large depth joists on the replies so far which i dont have enough height due to the extension soffit is quite low when standing on the decking and getting the reguired fall to the other end of the rafter which be fix on a timber beam on joist hangers..i need to get much head height i can get . will it be ok to use smaller sizes like 150mm x 47mm and have a purlin going the opposite way under the rafters supported with 4x4 posts..if thats ok what size purlins should i use and how far the span should be before being supported by posts? hope this makes sense..
thanks

 

[catlad]

Why are you using timber when there are profiles in alloy that are made for the purpose and will probably not be as deep.

 

[tate]

to match the decking with timber handrails, spindles..don't wont to change the timber affects...