Composite panels on wooden purlin.

[jon9000]

For a potato shed I've been told that wooden purlins are better due to condensation. However at 20 ft bays do I need to worry about sagging

 

[Hotbed]

All the sheds I've put up with 20 foot wooden purlins have sagged and have caused gutter problems, if your putting composite panels on the roof go with z purlins, you won't have condensation with composite panels.

 

[Forever Fendt]

For a potato shed I've been told that wooden purlins are better due to condensation. However at 20 ft bays do I need to worry about sagging

Yes

 

[upnortheast]

A steel eaves rail on 20ft bays will prevent the wooden purlins sagging. Think ours is 9" x 5" rsj

 

[Forever Fendt]

A steel eaves rail on 20ft bays will prevent the wooden purlins sagging. Think ours is 9" x 5" rsj

What about the rest of the timbers further up the slope

 

[upnortheast]

What about the rest of the timbers further up the slope

The sheets transfer the load down the slope, if the eaves rail is rigid nothing else can go anywhere.

 

[Forever Fendt]

the op asked about the purlins sagging, you have come up with a solution for the eaves beams/wall plate ,20ft timbers with comp panels is not an ideal situation any movement in the timbers will pull on the fixings and mat cause leaks on the fixings

 

[upnortheast]

Just saying what has worked here for >10 years with no problems

 

[TripleSix]

I'd rather have timber purlins on a potato shed.

With Z purlins there is too much thermal transfer through the screws causing much dripping.

 

[zyklon]

Do steel purlins rust? Chatting a guy the other day who works for a local company and they never use timber now, unless it’s specifically asked for but I think some paper work is signed so they don’t come back crying looking for compensation if it sags etc.

Every Agriculture shed going out now is with steel purlins. I asked him about them rusting in an Agriculture environment and he replied with “they are galvanised” so shouldn’t and no one has complained about them rusting.

Isn’t that the whole point of galavinising or am I missing something?

 

[Hotbed]

Galvanized z purlins can rust in the wrong atmosphere, they are not hot dipped galvonised, if I put up a cattle building I would suggest to the customer that they have 15 foot bays on timber, when we are near the sea I also specify galvonised nuts and bolts and stainless steel roof fixings as salt air and z purlins don't mix.

 

[Forever Fendt]

Do steel purlins rust? Chatting a guy the other day who works for a local company and they never use timber now, unless it’s specifically asked for but I think some paper work is signed so they don’t come back crying looking for compensation if it sags etc.

Every Agriculture shed going out now is with steel purlins. I asked him about them rusting in an Agriculture environment and he replied with “they are galvanised” so shouldn’t and no one has complained about them rusting.

Isn’t that the whole point of galavinising or am I missing something?

The edges are not galvanised as the purlin when in flat form is slit from a wide coil so raw edges A well ventilated cattle shed with a high roof should not be a problem,in my opinion steel roof sheets and zeds are not the best combination as a lot of drilling swarf remains under the sheet on top of the zed and steel sheets tend to have a wide flat pan and trap moister in between the two most metal roofs that rust always start at the purlin line but not so with fibre cement