Print Reading, Estimating, and Code Compliance
Reading the drawings
Start with the title block, scale, and north arrow, then read the roof plan for slope notation, because 6:12 means 6 in. of rise per 12 in. of run. Plan dimensions give plan area, not roof area. Sections and details show eave heights, overhangs, deck thickness, flashing, and ventilation, and the keynotes name the tile, underlayment, and fastener specifications. Cross-check the roof plan against the elevations before you order anything.
Estimating quantities
- Actual roof area equals plan area times the slope factor, where the factor is the square root of one plus rise over run squared: 4:12 is 1.054, 6:12 is 1.118, 8:12 is 1.202, 12:12 is 1.414.
- Squares equal roof area divided by 100.
- Tiles per square equal 14,400 divided by the covered area of one tile in square inches, using exposure times covered width.
- Waste runs about 5 to 10 percent for simple roofs and 10 to 15 percent for hips, valleys, and diagonal layouts.
- Price hip, ridge, starter, rake, valley metal, underlayment, and fasteners separately by the piece or linear foot.
Code compliance means the adopted IRC Chapter 9 tile provisions, manufacturer instructions, permits, and inspections, plus local amendments such as high-wind requirements.
Worked example. A 40 by 30 ft building with 2 ft overhangs gives a 44 by 34 ft plan, or 1,496 sq ft. At 6:12 the factor is 1.118, so the roof is 1,672 sq ft, or 16.7 squares. With 10 percent waste, order about 18.4 squares.
Traps. Bidding plan area as roof area; forgetting hip, ridge, and starter pieces; skipping the slope factor on steep roofs; ignoring manufacturer instructions in favor of habit.