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6 Fundamentals of Good Roof Design


By Wiss, Janney, Elstner Associates Inc.

More items for Building Envelope

Roofs fail prematurely or fail to reach their rated life of approximately 20 years for one of four reasons: (1) design; (2) materials; (3) workmanship or installation; or (4) the owner’s failure to maintain the roof properly. This article focuses on the first of those, proper design.

The six fundamentals of good roof design are the following:

  1. Sound substrate
  2. Proper slope and drainage
  3. Sound mechanical attachment or adhesion
  4. Expansion joints
  5. Proper flashing of penetrations
  6. Separation of dissimilar materials

Each fundamental is discussed separately.

Sound Substrate

A sound substrate may be concrete, steel, wood, cementitious fiber, or some other material, but it must be engineered to accommodate not only the weight of the roof but equipment loads and wind uplift forces. The substrate or deck must be dry and in good repair with all elements securely fastened to the structure. Metal deck sections must nest tightly and be welded or secured at each structural member. No roof is better than the substrate over which it is applied.

Proper Slope and Drainage

All roofs should slope. There should be no such thing as a “flat” roof. The industry recommendation is 1/4 inch per foot, or 2 percent. Good drainage can be achieved by sloping the structure or by using tapered roof insulation or a lightweight insulating concrete system. Water weighs 5 pounds per square foot per inch deep, and standing water damages the roof membrane, promotes water infiltration, and may cause structural collapse.

Standing water on roof indicates poor drainage
Photos: Wiss, Janney, Elstner Associates Inc.

Fig. 1: Standing water on roof indicates poor drainage.

Sound Mechanical Attachment or Adhesion

Roofs must withstand wind uplift forces from 60 mph to 160 mph in some coastal areas. This consideration has become much more important in recent years with changes to building code requirements. Material manufacturers have tested assemblies to meet these criteria, but the designer must be aware of local geographical requirements. The roof perimeter must receive particular attention, because most roof blow offs start at the perimeter. Industry standards require increasing the fastening pattern for roof insulation by 50 percent more than the field of the roof at the perimeter and by 100 percent in the corners.

Roof covering blown off

Fig. 2: Roof covering blown off starting at the edge or perimeter of the building.

Expansion Joints

A roof covering or membrane is not intended to accommodate structural building movement despite claims of 100 percent to 400 percent elongation by some roofing material manufacturers. All roof warranties exclude repair of moisture infiltration due to structural movement of the building. Roof expansion joints should be installed on all “L”, “T”, or “U” building shape roof plans, as well as at a change in structural or deck direction or a change in deck material type.

Proper Flashing of Penetrations

Most roofs do not leak in the open field of the roof. Moisture infiltration tends to occur at building discontinuities, such as changes in elevation, walls and penetrations. Penetrations and interruption of the roof membrane occur at mechanical, electrical, and plumbing penetrations, and unless properly flashed, moisture infiltration is likely to occur. Mechanical equipment should be mounted on curbs or platforms approximately 12 inches above the plane of the finished roof. Although elastomeric chemicals are promoted as a filler for “pitch pans,” the proven tried and true method for flashing conduit and piping is still a curb with a hooded metal cover with the conduit and piping exiting the hood at a 90 degree angle.

Mechanical equipment is mounted 12 inches above the plane of the finished roof

Fig. 3: Mechanical equipment is mounted 12 inches above the plane of the finished roof.

Separation of Dissimilar Materials

Materials with different coefficients of expansion should be separated. For example, sky-facing sealant is not a satisfactory waterproofing material for a sheet metal joint. As the sealant weathers and the metal expands and contracts thermally, the joint is likely to open and admit moisture. Likewise, wood, metal and bituminous or chemical sheets all have different coefficients of expansion, and the connection between the materials must be designed to accommodate the differential movement, or a moisture entry point is likely.

In summary, the six fundamentals of good roofing apply to all types of roofs whether metal, single-ply, multi-ply bituminous, shingle, tile, or any other material. Buildings move, water runs downhill, wind blows hard and materials weather and age. A durable roof design takes all of these considerations and events into account.

About the Author

Edis Oliver

Edis T. Oliver, PE, is a principal with Wiss, Janney, Elstner Associates, Inc. He has more than 40 years of experience in the roofing industry, both as a contractor and a consultant. Edis has specialized in developing roof management programs for large-scale commercial, industrial, and institutional facility owners. He also has been instrumental in designing over 2,000 major facility roofs for schools, colleges, municipalities, and commercial building owners. You can reach Edis at eoliver@wje.com.


Wiss, Janney, Elstner Associates Inc.

“Solving for Why” is written by professionals at Wiss, Janney, Elstner Associates Inc. Since 1956, WJE’s primary goal has been to provide the best solutions for its clients’ new and existing construction-related problems. The firm’s highly qualified engineers, architects, and materials scientists possess a collective knowledge gained from solving, as well as helping clients avoid, thousands of problems. Author information is available at the bottom of each blog entry.



Tagged categories: Architects; Building envelope; Consultants; Contractors; Designers; Engineers; flashing; Humidity and moisture; Moisture detection; Roof coatings; Roofing contractors; Roofing materials; Waterproofing; Waterproofing membranes

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