Hi Al When I renewed the front of my loft ,I covered the external frame with a 12mm ply then vapour paper I use on the houses. Then nailed the plastic cladding to the uprights at 400 centres,It can also be screwed using plastic caps It is classed as soffit board and is 5 mtrs long x 300 approx cost for white £10 per length There is one of a higher speck that I have just got for a friends loft approx £2 per length more The proper cladding which is 150 x 5mtrs is about £30 per length The coloured boarding is approx 3 times the cost of white Would never fit timber again personally Hope this helps Mick