The most common roof style is the pitch roof and the building is usually
divided at the center which is the apex.
The pitch roof is the easiest to calculate though I won’t bore you with
details, but you can ask your contractor for the size of each roofing
sheet except you are using longspan aluminium roofing sheet which can be cut to exact lenght of the roof.
To make fast calculation, measure the length of the side of building let
say ‘x’, measure the breadth ‘y’. To get an approximate surface area we
will add an arbitrary value for the difference in length due to the pitch
or depth of roof.
So lets say for a house of total length of the front is 1.8m or 60ft
and side which is the longer part is 27m or 90ft.
Area =length of front of house x side of house=18 x 27=486m2
Add allowance for the pitch or depth 20% of Area=20% + 486m =583.2m2
Now find out the length and breadth of each roofing sheet and also
multiply length by breadth.
Assuming the ceilling sheet is 3.6m in length and 1.8m in breadth
Then area of roofing sheet =3.6 x 1.8 =6.48m2
Therefore total numbers of roofing sheets needed approximately is
(Area of building + Allowance)/area of one roofing sheet =583.2/6.48m
Please note that this is not a standard way of approximating but this is
derived based on my experience for your enjoyment. In any case there is
never a standard way.