Shed roof insulation

[rosinante]

Hi all , I am framing a shed from 4 x 2 and 18 mm ply throughout, I plant to insulate the roof on the outside with 50mm polystyrene covering it with 1000 x 34 box profile steel roofing sheeting, will that be ok or will I have problems, the walls will be framed and lined with 18 mm ply and insulated with rock wool, the inaccessible sides will be clad with the same, will that be ok ? The visible sides will be timber clad, please let me know if I am building in a future problem,
Thanks
Chris

 

[Hornbeam]

Hi Chris. Just a couple of thoughts.
I would use a PIR insulation board such as Cellotex or Kingspan rather than polystyrene as it has much lower fire risk. How are you securing the trapezoidal sheeting to the roof structure. Generally you can span up to about 2 metres between fixings but you need something solid to fasten in to and fastening through insulation board is not really rigid enough. Use valley fixing stainless fasteners with EPDM washers. Dont crown fix. Also think about any moisture and how it might move through the structure inside to out (shouldnt really be an issue if your shed is dry inside.
Ian

 

[rosinante]

Hi thanks for reply, I will fix in the valley as you suggest, I was hoping to fix at the top and at the bottom of the sheeting as the roof will dual pitch with a total span of a nominal 3 metres so each side will be less than 2m , I have never built a shed before so some of it will be designed as I go, budget restrictions apply so the polystyrene wil have to suffice, it's less than half the price ,I plan to start with the floor setting out the roof trusses on a la' American style , their sheds look to be the size of my house, ! framing the sides and ends , I will be using reclaimed windows if I can find some, also considering a dormer style roof light on the south facing side, we shall see,
thanks

 

[Hornbeam]

I work in metal cladding and roofing and really suggest you reconsider the polystyrene. The total cost difference is probably less than £100 but the fire implications are massive. None of the modern panels use polystyrene and none of teh insurers will cover it.

 

[rosinante]

Ok , I will bite the bullet and insulate the roof with Celotex or similar, will under the floor be ok with polystyrene, The space won't be used as a workshop and f it did it would be for woodworking , no welding or other hot work, in principle is the method of construction ok, ie timber frame rockwool insulation, steel cladding , 18mm ply lining ? Wiring etc run thru framing, Thanks
Chris

 

[morfa]

Sounds ok.

Just make sure you ventilate it well. That's been my mistake with my shed, it actually 'rains' inside.