I'm posting this as a single blog entry for a number of reasons. One as it's the biggest bit of work we've done without professional help. Two as sheet roofing seems to be a topic of interest on BH and three because this stuff was hard going, with minimal information available on fitting guidance, so hopefully this might help others.
We are using eternit profile 6 sheet roofing. It is fibre cement board and we initially chose it due to some perceived benefits over wriggly tin (principally acoustics, condensation management and durability). Cost wise it seems to come out roughly similar (from what I could tell).
One of the biggest differences is in weight. Each cement board sheet is in the order of 55kg. All 54 were manhandled on to the scaffolding, and the roof. Without doubt this is the biggest downside and I'd strongly recommend anyone considering using this stuff to be sure they have a way of moving it safely.
Another challenging aspect is the sheet thickness. At around 7mm thick, this means potentially 28mm thickness where 4 sheets overlap. To avoid this, the sheets are mitred, which is a pain to do accurately on a curved sheet. The profiled wavelength and amplitude is greater than on most metal sheeting which when combined with the sheet thickness makes neat fitting around velux quite hard, irrespective of the pre-planning we did. A profile 3 sheet is available which is more similar to traditional metal sheeting.
This probably sounds highly negative, but we're pretty pleased with how it looks. We've had a few downpours recently and it's looking quite solid. Still a couple of small bits to complete, but nearly done. As to whether I'd use it again, jury's still out...
While it's still fresh in my mind, the other things I'll note are:
-there is a right and wrong way to overlap sheets which is not obvious in the eternit guidance. It's not crucial for weatherproofness, but it can make the roof appear bumpy if not done correctly.
-eternit helpline was not hugely helpful, but the local reps were definitely worth speaking to and very useful.
Anyway, the photos:
Above shows the sheet overlaps, with the mitre clearly visible. When the top right sheet is added, this disappears.
Above shows the sheet overlap in section, but it also helps illustrates the sheet thickness.
Below are a couple of more general shots of the finished product.