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Calculating Underlayment for Roofing

Based on the roof shown below, estimate the amount of 30-pound underlayment required. Assume single coverage, a crew-error waste of 4 percent, and a slope of 6 in 12.

”Underlayment_Chart_60ft

  • Roof Plan area = (30’ x 60’) + (30’ x 15’) = 2,250 sf
  • Actual Roof area = Roof Plan Area times factor for 6 in 12 slope-roof

    Note: the factor comes from a table like the one shown below

    ”Pitch_of_roof_1118

    Actual Roof Area = 2,250 sf x 1.118 = 2,515.50 sf = 25.16 Roofing Squares

  • Total Ridge length, according to the plan is 40’ + 15’ = 55’
  • Total Hip and Valley. There are 6 hips and 2 valleys. To calculate their actual length multiply 15’ (their run) times 1.50 = 22.5

    Note: 1.50 is the update by which you multiply the run to get the actual area.

    ”Pitch_of_Roof_150

    22.5’ x 8 (the number of hips and valleys) is 180’

  • Total Hip – Ridge – Valley lineal feet is 55’ + 180’ = 235’
  • Total Hip – Ridge – Valley needs 1 square foot of material per lineal foot. Since it has 171 lineal feet, it needs 235 square feet or 2.35 roofing squares.
  • Total gross Roofing material = Area + Hip – Ridge – Valley = 25.16 squares + 2.35 squares
  • Account for waste factor of = 27.51 squares + 4% = 28.61 squares
  • Number of #15 underlayment Rolls needed.

Find a table which gives you the coverage for the different types of underlayment.

”Underlayment_Coverage_Amount

From the table, you can see that 1 roll of # 15 underlayment covers 4 squares, so

28.61 squares/2 squares per roll = 14.305 rolls is the answer.

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