Rafter sizing

[duj666]

Hi could I have some free advice please.
rear extension 9 metres wide by 4.5 metres with 15 degree pitch back towards house drawing says 200 * 50 rafters at 400mm centres.
at front 2.4 metres wide by 6 metres drawing has 100 * 50 rafters supported by 2 purlins does this all sound right. any advice greatly appreciated. the rafters are quite long 4.5 and 6 metres long.and wanted to make sure we have right size. thank you.

 

[theoldun]

All depends on dead load of roof covering.
What you using, slates, interlocking concrete tiles, plain clay tiles or concrete plain tiles.
Let us know and we will advise.
Regards oldun

 

[duj666]

HI thank you. were using concrete interlocking tiles @ 49 kg/m²

 

[theoldun]

On a half run of 4500 then C16 47x200 do not comply and you require C24 47x200, how ever am guessing that you have given external measurement and not the measure from outside of wall plate to face of top pole plate. If you have a 300mm cavity wall, then you will have a reduction in half run of 250mm which will make half run 4250. and length of rafter 4400 from top plumb cut to heel cut of birds mouth plus what ever overhang you may require. If this is the case then C16 47x200 will be okay..
Front extension C16 47x100 will span 2520 so with two purlins at third span and two thirds span C16 47x100 will be okay. Purlins need to be C16 63x275 approx 2000 centres and max span of 3800.
When ordering timbers do not forget your overhang for facia and soffit or your overlaps on purlins.
Anything you do not understand, then come back and ask.
Regards oldun