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Basic info on roof cladding on C purlins for cold climates

Basic info on roof cladding on C purlins for cold climates

Basic info on roof cladding on C purlins for cold climates

I usually provide C purlins on top of rafters in steel buildings and wait for architect to specify cladding on top.

When it comes to cold climates there seems to be several variations, usually BUILT UP cladding apparently to satisfy insulation and moisture requirements

I would like some basic ancyclopaedic info on the subject especially where trapezoidal sheets and rockwool insulation are a part of the BUILT UP sandwich.

RE: Basic info on roof cladding on C purlins for cold climates

IJR...one of the best sources of Cold Regions roofing information comes from the Cold Regions Research and Engineering Laboratory (CRREL), one of the U.S. Army Corps of Engineers divisions.  The roofing expert there is Wayne Tobiasson, who has written extensively on the subject.

If you are looking at a panelized roofing concept, you might want to check out the panel suppliers for cold storage applications (just with a reverse thermal drive!).  It sounds as though you are looking at something that might create lots of joints/seams.  If this is true, it is a nightmare for roofing, particularly when subjected to significant movement, such as for a steel frame building.  Beware that you might get tagged with designing a system that has extraordinary maintenance costs.  Guess who gets the blame!!

By far (this is an opinion based on doing over 20 years of roof system failure investigations), the best roof systems are still the built-up roofs and hybrid built-up roofs (typically modified bitumens or combinations of the two).  These offer great history of performance, ease of construction, redundancy, and ease of maintenance and repair.  Further, they force you to design a more substantial decking system for them that enhances the structure as well.

Good luck.

RE: Basic info on roof cladding on C purlins for cold climates


Here again you have proved your standing. Thanks thousands for clarity and millions for all cause thats kind of what I wanted to hear.

Your supporting built up roofs makes me more relaxed. Currently we are working on a metal building with roofing as follows:
               0) Very low slope(3 degrees)
               1) C-joists on rafters
               2) Trapezoidal sheets
               3) Vapour barrier on top of sheets
               4) A thick high density rockwool panel
               6) PVC roof sheeting(sort of a membrane)

The thickness of rockwool panel is the min. specified by local codes.

If you find time please comment on this arrangement.  


RE: Basic info on roof cladding on C purlins for cold climates

IJR...what is the gauge of the deck (trapezoidal sheets) and what is its span?  Can you handle 3 to 5 psf on top of the rockwool?


RE: Basic info on roof cladding on C purlins for cold climates

That is the point,

Maybe I should not interfere with the architect, but I had that worry from the beginning.

 In the area I am roofs are supposed to carry snow level of 75kg/m2. I guess that  comes to about less then 2psf. And my C purlin spacing is about 4.5ft.

As for gauge of the trapezoidal sheeting. I have checked that, using available manufacturers data.

But there is no data showing stiffness of the rockwool panel.

I am in a position to make an opinion. Are you not comfortable with stiffness of the rockwool panel (4"" thick high density)?

If you have an alternative, please comment so that I can force it in my other projects.


RE: Basic info on roof cladding on C purlins for cold climates

IJR...I think I'm beginning to see your point.

Don't include the rockwool insulation in any of your structural calculations.  It offers nothing to the structural composition for several reasons....

(1)  For it to provide any diaphragm action, it would have to interact rigidly with the decking.  It is installed on top and any lateral interaction comes only through the fasteners, which are too sporadic and small to provide composite interaction.  For trapezoidal decks, you must either provide a plastic fill (cellular concrete or lightweight insulating concrete), or a thin sheet of gypsum board with many, many fasteners to achieve lateral compressive interaction (diaphragm).

(2)  All joints in the insulation would have to be completely in contact with each other, which will cause localized crushing and/or buckling of the boards in the event of excessive bending and lateral load.

(3)  If the roof leaks and the rockwool gets wet, it will lose most of its integrity, therefore you do not want to depend on it for structural adequacy.

In the future, when the PVC membrane fails (which it will, and probably prematurely), someone will likely remove the insulation (it will be wet) and replace it with something else.  You would not want the structural reliance to be dependent upon whatever some roofing contractor might want to fill the void!!!

Design the structure...the let someone waterproof it.  Don't mix the two.


RE: Basic info on roof cladding on C purlins for cold climates

Very sound advice Ron



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