Roofing Drawings, Specifications, and Layout
This domain tests your ability to read what the designer has drawn and specified, convert that into quantities and geometry on the roof, and catch the conflicts before they become change orders.
Reading the set
Know the difference between the roof plan, elevations, building sections, wall sections and enlarged detail sections. Confirm the scale before scaling anything, check the north arrow and grid lines, and read slope notation as a ratio (4:12, or 1:50) rather than degrees. Specifications are organized by MasterFormat divisions, with membrane roofing generally in Division 07 50 00; the specification controls fastener types, membrane thickness, warranty term and submittal requirements. Where drawings and specification conflict, the specification usually governs unless the drawing is more stringent. Raise RFIs, submit shop drawings and product data, and build mock-ups where specified.
Layout and takeoff
Square a roof deck with the 3-4-5 method before setting the first course. Roof surfaces are always larger than the plan footprint, so apply the slope factor: for a 6:12 slope the factor is 13.416 ÷ 12 = 1.118. Quantities are ordered in squares, where one square equals 100 ft² or 9.29 m².
Worked example
A roof plan measures 12 m by 9 m at a 6:12 slope. Footprint area is 12 × 9 = 108 m². True roof area is 108 × 1.118 = 120.7 m². In squares: 120.7 ÷ 9.29 = 13.0 squares. Adding 10 percent waste gives 14.3 squares, so order 15 squares with accessory trim sized from the linear measurements of hips, ridges, eaves and valleys taken off the plan, not estimated.
Traps that cost marks
Measuring plan dimensions and ordering material as if the roof were flat. Misreading 6:12 as 6 degrees; 6:12 is about 26.6 degrees. Forgetting that hips and ridges consume extra material and extra fasteners. Missing a detail sheet that requires a cant strip, cover board or cricket that is not on the plan. Quoting from the drawing when the specification names a different membrane or warranty.