The original roofs of the Victorian terraces were of slate, which was relatively light, waterproof and lasted a good many years, the evidence for which can still be seen in places, concrete tiles are now the popular choice for reason of economics, even though they are slightly porous and considerably heavier.
The timbers in the our attic supporting the roof covering are actually 2" x 3", a light structure presumable allowed by the relative lightness of slate, which meant in preparation for the heavier concrete covering (and to meet the inspector's approval ) a certain amount of reinforcement of the attic timbers had to be implemented. Beams were place under the joists running from fire wall to fire wall and stretchers and props positioned in various places. The new roof also benefited from a layer of under felt which was not present in the original structure.
While the roof was being renovated we took the opportunity to have a Velux window installed, even though a proper conversion of the attic to say a bedroom was not feasible due to lack of height, the window does make it much better when it comes to using the space for storage etc. Opening the window in summer particularly, helps to dispel the the stuffiness of the attic making it much pleasanter to work in.
The other thing to consider of course while working on the roof was the state of the chimneys. Our chimneys, thankfully were in a reasonable state with no more than a little re-pointing needed, in retrospect though it might have been wise to treat them with a waterproofing agent as since we have had a small ingress of water in that area which does not seem to arise from damaged tiles or poor flashing and a roofer has suggested to us that the problem might possibly be down to porous bricks in the chimney stacks - we will see.