Factors Affecting Roof System Costs

by Kent Mattison, P.E.

When it comes to roof systems, determining the criteria in selecting the "best" roof can be very difficult. Every item that affects the quality of the roof system obviously affects the cost of the roof system, but to different degrees. The importance of each item is also dependent on the specific conditions in each situation. So when we are asked for budget estimates to replace a particular roof, many assumptions must be made if all the design factors have not been thoroughly addressed.

More often than not, only a few of these factors are considered when a roof system is chosen. Therefore, the cost can vary dramatically depending on which factors and how many are considered. Following are some examples of various factors that often have the greatest affect on the cost of a project.

Code and Insurance Requirements

Fire related membrane sheets can increase the membrane cost by 20%. (Remember the membrane cost may only be 10% - 20% of the total project cost, so the roof replacement cost increase in this case is only 2% - 4%).

Requiring a Factory Mutual 1-90 rating in lieu of a 1-60 rating can increase insulation fastening requirements up to 10%. But this cost increase typically is only an additional 1% of the total project cost.

Project Size, Height, Location and Access

These factors typically have some of the greatest affect on the cost of the project. The primary reason for the cost difference with the size of the project has to do with the setup costs required by the contractor. The contractor has basically the same setup or fixed costs whether the project is 1,000 square feet or 100,000 square feet, all other factors being equal. Therefore, the cost per square foot will be affected accordingly. (However, increasing the size from 50,000 square feet to 100,000 square feet will not significantly decrease the square foot price, because the fixed costs are already a smaller percentage of the initial 50,000 square feet).

If you were to compare a roof replacement project of the same size, one being a highrise in a downtown location where access is extremely difficult, and the other a single story building in an industrial park with easy access, the costs of the projects would be significantly different. It is not unusual for roofing costs to be 300% - 400% higher due solely to height, location and access factors.

Time of Year

When contractors are busy, they raise prices. When they are hungry for work, they lower prices. Basic supply and demand. The best pricing from contractors usually occurs when they bid work in the winter for early spring work. Prices are often 10% and up to 20%lower when this can be done. You will also usually get their most experienced people since they are the first to be put to work.

Insulation Requirements

The type of insulation used and the R-Value required are the two factors having the greatest influence on the cost of insulation. The R-Value per inch of insulation varies from 2.78/inch for fiberboard and perlite insulation to 6.5/inch for isocyanurate insulation. The cost per R-Value also varies widely, from approximately $0.03/sq. ft. for extruded polystyrene insulation to $0.20/sq. ft. for cellular glass insulation.

Factors other than price will often dictate the insulation used. However, the most often used insulation over the last several years has been isocyanurate due to its high R-Value, low cost per R-Value (slightly over $0.03/sq. ft.), and versatility with different types of systems and applications.

Amount of Rooftop Equipment and Penetrations

Another significant factor in the cost of a roof system is the amount of equipment on a roof. The largest portion of a roofing project's cost is labor. Anything that has a major impact on labor hours will have a major impact on the roof replacement costs. A heavily cluttered roof can cost twice as much as one with very few penetrations, due to the increase in labor hours.

Removal or Recovering of the Existing Roof System

Tear-off costs typically add $1.00 - $2.00/sq. ft., but may vary widely depending on several factors. In many cases, tear-off is required due to the existing roof's condition. But where recovering the existing roof is a sensible choice, reroofing costs can sometimes be reduced 20% - 50%.

Presence of Asbestos

When asbestos is present in the roofing felts and asbestos abatement is required, the reroofing costs can easily increase 10% or more. The dump sites the asbestos containing material can be taken to are restricted. In some areas the distance to the dump site and associated costs can be substantial.

Warranty Requirements

The cost of a manufacturer's warranty is dependent on its length and coverage, but usually will cost $.03-$.07/sq.ft.; not a major cost contributor to the roof system.

Drainage Issues

When replacing a roof having little or no slope, adding slope and/or additional drainage devices can add significantly to the project cost. If a fully tapered insulation system is to be installed, additional drains and raising of rooftop equipment, curbs and perimeter edge heights is usually required. The cost increase is so great that often times it's prohibitive. Because several roof system manufacturers will warrant their roof systems in ponding water, drainage is an issue that is sometimes ignored.

Substrate Components and their Condition

Costs that are often hidden at the time a project is awarded are replacement of deteriorated or defective decking, insulation, and nailers. These costs can end up being a significant portion of the total project cost. For example, wood or metal deck replacement typically costs $2.50-$6.00/sq.ft., and insulation will vary depending on the thickness, but is usually $1.00-$2.00/sq.ft., and wood nailers may add $2.50-$5.00/board foot.

The following table provides an overview of the roof replacement costs of 100 projects for which we recently wrote specifications. The range in pricing is obviously very large due in large part to the factors discussed above. But also note the rather small range of average costs of one type of system as compared to another. This indicates the type of roof membrane has a fairly small impact on the cost of the system relative to all the design factors.