So when you try to estimate how much roofing material you will need (by calculating the roof surface), remember to add the requirements for the actual roofing shingles, including hip and ridge shingles. If you are roofing with tiles you will also need rake shingles. Then just add flashing for the drip edges, roofing felt and nails and you should pretty much be ready to go. This is how you can estimate your requirements.
When it comes to estimating the amount of shingles and felt that you need, I would recommend adding about 10% more than you actual need to cover the surface. This allows for any waste, double coverage at the eaves and any future repairs that might be needed.
Ridge Shingles Guidelines
In regards to estimating the amount of hip and ridge shingles that you need, measure the length of hips and ridges and divide the total by the exposure recommended for shingles and voila! This is the number of shingles you need. If you are using asphalt shingles on the other hand, you can normally make 100 feet of hip and ridge shingles per square of roofing shingles.
In order to estimate how much flashings you need, measure the total length of all eaves and rakes for drip edge flashings and the total length of any valleys for valley flashings. If you require vent pipe flashing, it is sold by the unit. When it comes to chimney flashing, measure the outside dimensions of any chimneys on your roof and then decide on the chimney flashing pattern.
What a lot of people forget to take into account when they plan to lay a new roof is to include the cost of nails. This might seem like a minor thing but when you start calculating the number of nails needed in a project like this, you will quickly realise that the cost of nails are not a minor thing to forget about. So, the number and type of nails that you will need depends on the size of your roof but also on what roofing material you plan to use. You will need approximately 2 pounds of nails per square of wood shingles or shakes and approximately 2,5 pounds of nails per square of asphalt shingles. On top of this you will need roughly another 2 pounds of nails for the starter course, ridge and hip shingles. So, make sure that you sit down do your math. Once you have done that, you should be ready to order your roofing materials.