But, before that you need to find the measurements of the roof as accurately as possible.
There is no need to be accurate to the last decimal point, but it sure needs fair bit of accuracy so, that you can order the required raw materials in the right quantities. Either too little or too much is a lot of trouble particularly if you are hiring workers to do the job and you find that you are short of shingles or nails. You will end up paying more in labor what you save in materials costs.
There are simple methods to calculate roofing area. If your roof is already stripped and you can see the plywood then counting the number of sheets and multiplying the area of the sheet into the number of sheets will give you the area.
Tip: You can add up the half sheets to make up as one sheet, any sheet that is 3/4 size or more are treated as one full sheet.
Ordering extra roof shingles is not a problem. It is always possible to return the extra materials to the store. Just mention that you will return the unused shingles and they will take it back or keep a few for repairs later.
Ordering less shingles than needed for the job unnecessarily holds up the work which is left unfinished, it causes you to make extra trips to the store and if the store has run out of stock of the particular shingles you bought then you are in real trouble as you might have to wait longer for them to place order from the manufacturer.
There will be some amount of wastage, breakage and damage to shingles when workers are working on your roof. So, that needs to be factored into the ordering. A roofing calculator usually factors in wastage in its calculations.
Certain sections of the roof will need extra shingles. For example, the ridges and valleys, the roof edges, the eaves are places where you use the shingles more densely than on the plain surfaces. This is done to prevent water seepage in vulnerable areas like projections and depressions.
A roof shingles calculator usually requires – the roof area, the number of ridges, valleys, complexity of roof, roof pitch or slope, the type of shingles used and your local zip code(where materials is bought). The tool is per-loaded with data about the shingles prices according to zip code. If your roof is simple with not many openings like chimneys, skylights, dormer windows or other vents. Then you can key in simple roof. If there are many of these on the roof, then the roof is complex. The roof that slopes gently is low pitch or moderate roof. If it is steep gradient where the worker can’t stand on the roof comfortably then it is a steep roof.
Key in the above details in to your roof shingles calculator and you will get the amount of shingles to order.