Estimating Roof Size

       Roofing materials are sold in squares. A square is the amount of material needed to cover 100 square feet on the roof when installed per the manufacturer's instructions. Since roof coverings come in different styles and sizes and are installed differently (overlap, layers, space between pieces, etc), the manufacturer designates the number of pieces needed to cover 100 square feet of roof surface and refers to that as a square of material.


       The best way to know how much roofing material is needed is to refer to the building plans and/or contractor or builder quotes or invoices.  When that is not possible, the next best is for someone to get on the roof and measure each roof surface. This should only be done, however, by someone experienced with working on the roof i.e. a professional roofing contractor, insurance adjuster or claims agent, or roofing consultant.


       If an approximate number is good enough, however, a rough estimate can be made from the ground by measuring the roof's outside dimensions including roof overhang and multiplying by a factor to take into account the slope of the roof. If the house shape and roof is complex, this may require calculating each wing or room separately. At this point, you need to know the slope of the roof (rise over run). The roof slope can be estimated using the method shown in the drawing below.


       Multiply the total square feet from above by the appropriate slope factor from the following chart. 

 Slope  Multiply By
    2 in 12 1.02
    3 in 12  1.03
    4 in 12 1.06
    5 in 12


    6 in 12  1.12
    7 in 12  1.16
    8 in 12 


    9 in 12


   10 in 12  1.30
   11 in 12 1.36
   12 in 12 1.41

       There is always product waste during installation (cuts for valleys, edge trimming, handling damage, etc) all of which vary by product, installation method, and installer care and skill. Normal waste factor is around 10% but fragile materials (concrete and clay tile especially) may require considerably higher factors and tough, durable materials less. A call to the material manufacturer (if you know what material you're estimating) may help if the job is large and/or the waste factor important to the total cost.


       Theoretically, you should now deduct for areas that will not require roof covering such as skylights and chimneys but they are usually small enough compared to the total that they don't seriously impact the total squares required. Dormer windows will also add slightly to your total but your waste factor probably will cover that need unless your house has an overabundance of such features.


       The total from the above work should give you an accurate enough number that you can begin to estimate the cost of the roof covering material. Your project, however, will most likely also require hip and/or ridge cap pieces, starter pieces, underlayment and flashing. These require a little more specific roofing expertise but for purposes of this rough estimate, just calculate or estimate how many linear feet of eaves, hips, and/or ridge you have.


       Armed with the above data you can have an effective discussion with any roofing professional and push your project further along toward understanding and completion.


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