Metal sheet roofing or fibre-slates

[Joe Shmoe]

I'm almost at the stage of completing the framing on my new workshop build and now considering roofing.

I was intending on using Marley fibre slates, having already purchased the roofing battens, but having working out the costing of everything involved, it would be alot cheaper and easier to install 0.7m Plastisol covered metal sheeting (40yr life apparently). As the build is not visible to anyone, aesthetics are not an issue.

Q1. One thing I'm not too sure about is loft ventilation. I'm guessing the metal sheets should be installed over a solid decking (18mm OSB3) and as I intend to have a proper ceiling, the loft space would be fully enclosed. How do I deal with the ventilation? Does it need it? Would soffit vents be sufficient?

Q2. What is the correct membrane to use in between the sheets and the OSB3? Is a regular Tyvek breather membrane OK, or should it be something else?

Membrane spec and ventilation issues aside, any other reason that metal sheeting would be a bad choice over man-made slating?

Cheers guys.

 

[Woody2Shoes]

Hi - My 2d's-worth:

I think metal sheets are a good option. Things to watch out for include:

Handling large sheets on a roof can be more tricky - not something for a windy day - and cut edges can be extremely sharp.

Because metal conducts heat so much better than many other roofing materials, it is (more) susceptible to getting condensation forming on the underside (even if you've stopped warm, moisture-laden air from inside getting to it, using insulation and a vapour-check on the "warm" side of that). Condensation can and does form on the underside of other roofing materials, but less frequently, I believe, which is why they all need some kind of barrier and/or air gap.

If it were me, I'd be tempted to go for the galvanised sheet (which to a small degree is self-healing, with the zinc being a "sacrificial anode" which, to a limited extent, protects even exposed steel) and paint it with bituminous paint. Even galvanised is pretty inexpensive.

So - I'd have timber battens or metal purlins on counter-battens with a gap allowing air circulation up and down the fall of the roof, then a vapour-permeable membrane, then optionally OSB3 sarking boards (help strengthen the roof against racking and wind uplift, but add weight and smallish cost), then the rafters.

Something like the picture at the top of this page - but with metal not tiles obviously! http://www.dupont.co.uk/products-and-se ... brane.html

Ridge and eaves vents would be good.

Cheers, W2S