Commercial roofing estimating, CSI Division 07 thermal and moisture protection, is the business of pricing the roof assembly that keeps water out of a building. You work from the roof plans, the details, the specifications, and the deck type shown on the structural drawings. Every square foot of membrane, every linear foot of flashing and edge metal, every drain, every penetration, and every insulation layer has to be counted, measured, priced, and labored into a bid that holds up on a flat roof where the seams, the flashings, and the penetrations are where the leaks start.
What You Are Estimating
Division 07 covers low slope and steep slope roofing, plus the insulation, the vapor barriers, the cover boards, the flashings, the edge metal, the drains, and the penetrations. The major cost groups are the membrane, the insulation and cover board, the flashings and edge metal, the penetrations, and the accessories. The membrane is the wear surface and the waterproofing, and on a commercial flat roof it is most often TPO, EPDM, modified bitumen, or built up roofing with a ballast or a coating on top. The insulation is polyiso or XPS laid in layers, with a cover board of high density gypsum or wood fiber over it.
On a typical commercial job the roof area comes off the roof plan, the deck type comes off the structural drawings, the insulation thickness comes off the specification, the flashing details come off the detail sheets, and the drains and the scuppers come off the plumbing and the roof plans. You also carry the parapet walls, the curbs for the equipment, the roof hatches, the skylights, the expansion joints, and the safety fall protection, because every one of those is a line item on a roofing estimate.
Units and Workflow
Roofing quantities are squares, linear feet, and counts. A square is 100 square feet of roof area, and the membrane, the insulation, the cover board, and the base sheet are all priced by the square. Flashings, edge metal, drip edges, and coping are measured in linear feet. Drains, scuppers, curbs, penetrations, roof hatches, and expansion joints are counted. The labor is by the square for the field and by the foot or the count for the detail work.
The workflow runs deck up. You start with the deck because the deck type drives the fastening and the attachment of the insulation. You take the vapor barrier where the spec calls for one. You take the insulation by the layer and the thickness. You take the cover board. You take the membrane. Then you run the details: the parapet flashings, the curb flashings, the penetration flashings, the edge metal, the drains, and the expansion joints. Each layer has its own labor unit and its own material cost, and the details carry more labor per foot than the field carries per square.
Step by Step Estimate
First, set up the estimate with your labor rate and your labor units. Roofing labor comes from your shop average by system, by deck type, and by detail. A fully adhered TPO system carries a different labor unit than a mechanically fastened one. A self adhered modified bitumen system carries a different labor unit than a torched down one. The labor rate comes from your shop average roofer rate plus burden.
Second, do the field takeoff. Measure the roof area in square feet from the roof plan and convert to squares. Tag the area by deck type and by membrane system. Third, do the insulation takeoff. Multiply the area by the number of layers and the thickness, and price the board by the square. Fourth, do the detail takeoff. Measure the perimeter edge, the parapet flashing, the curb flashing, the penetration flashing, and the expansion joint flashing in linear feet. Count the drains, the scuppers, the roof hatches, and the skylights.
Fifth, apply the labor. Multiply the field area by the field labor unit and the detail lengths by the detail labor unit. Sixth, price the material. Seventh, load the spreads: labor burden, tear off and disposal on a recover or a re roof, the crane and the hoisting, the safety fall protection, the permits, and your overhead and profit. Tear off is a real cost and a real labor line on a re roof, and forgetting it cuts the bid in a way that cannot be made up in the field.
Where the Money Goes
On a commercial roofing package the membrane and the insulation together run 50 to 65 percent of the installed cost. The membrane is the higher of the two on a high end system like a fully adhered TPO or a KEE, and the insulation is the higher of the two on a job that calls for a thick polyiso stack to meet the energy code. The flashings and the edge metal run 15 to 25 percent, and this is where the takeoff matters most because the detail work carries the labor and the leaks.
Penetrations and accessories run 8 to 15 percent, with the drains, the scuppers, and the expansion joints carrying real cost in small quantities. Labor overall runs 25 to 35 percent of the installed cost on new construction and 35 to 45 percent on a re roof because the tear off and the disposal and the protection of the building below are labor heavy. Hoisting is a smaller line item that can blow the budget when the roof is on a high building and the crane has to sit for the duration.
Common Mistakes
The first mistake is undercounting the details. The field area is easy to measure off the roof plan, but the parapet flashings, the curb flashings, and the penetration flashings hide in the detail sheets, and on a roof with a lot of mechanical curbs the detail linear feet can equal the perimeter. The second mistake is forgetting the edge metal. The drip edge, the gravel stop, and the coping are line items, and forgetting them cuts the bid by a real number.
The third mistake is underpricing the tear off on a re roof. The tear off is labor and disposal, and the disposal is a dumpster, a hauling fee, and a tipping fee. The fourth mistake is ignoring the insulation thickness. The energy code in most jurisdictions sets a minimum R value, and the polyiso stack to meet it is a real cost that has to be in the bid. The fifth mistake is forgetting the safety fall protection. The perimeter warning line, the safety monitors, and the tie off points are line items, and on a roof with no parapet they are required. The sixth mistake is pricing the membrane from a stale quote. TPO and EPDM pricing moves with the oil market, and a job bought six months after the bid can carry a different price.
Putting It Together
A clean roofing estimate is built deck up. You take the roof plan for the area, the structural drawings for the deck, the specification for the insulation thickness and the membrane system, the detail sheets for the flashings, and the plumbing and roof plans for the drains. You measure the field, count the details, measure the edge, apply the labor, price the material, and load the spreads. When the bid number lands you compare it to the last similar job by the cost per square of roof area and the cost per linear foot of detail. If the number is out of line you go back to the detail takeoff and the insulation thickness before you submit. Roofing is a trade where the details hide the money and the takeoff is where most roofing subs make or lose the job.