5m x 5m flat roof purlin joist size

[Northster1]

Hi guys (and girls) i cant seem to find much info from google so wanted to ask here for advise on what size joists or purlins id need to span 5 metres, id prefer to use metal purlins as non combusible as its right on a boundary
thanks lee

 

[RonnyRaygun]

What do you need, a joist size or a purlin size? And what type of roof is it?

 

[noseall]

A flat roof joist spanning 5m would need to be mahoosive unless you decide to fit a mid-span purlin.

 

[Northster1]

Hi the roof will be single ply membrane with insulation boards on either metsl decking sheets or plywood so not much weight
I would like to know size of metal purlins and centres
Would metal purlins sag as its quite a span ?
Cheers

 

[catlad]

Have you looked at using ibeams

 

[Northster1]

Looked at I beams but think they would be very expensive given the size and hiw msny id need but anything would be considered

 

[Paintguy]

Hi the roof will be single ply membrane with insulation boards on either metsl decking sheets or plywood so not much weight
Cheers

A few years ago my flat roof was covered in 12" of snow, which does weigh a lot.

I'm no roofer but I'm pretty sure your roof will need to be able to carry much more than the lightweight materials you'll be decking it with.

 

[RonnyRaygun]

Looked at I beams but think they would be very expensive given the size and hiw msny id need but anything would be considered

Something like 50x225 C24 at 400mm crs if using timber.

I take it you are talking about Metsec or Kingspan cold formed purlins to use as joists? Just give them a call and they will be able to tell you what you could use, but it'll be around the same depth as the joists, maybe 200mm...