Trade Topics
September 2009

Built Up Roofs and Re-roofing

Often mistakenly reffered to as flat roofs and/or tar and gravel roofs, the most common roof found on single story concrete tilt up buildings in the bay area is the "California Built Up Roof". The roofs are neither flat (they have a minimum slope of 1/4" per running foot) nor are are they "tar" ( instead their binding agent is the same asphalt that roads are paved with) and instead of having gravel these built up roofs have embedded granules much smaller than the 3/8" used in the traditional gravel surfaced roof.

Built up roofing - by Dick Tippett

Built-up roofing is constructed by installing alternate layers of roofing felt and heated asphalt on the roof. The asphalt both binds the felts together and acts as the waterproofing material in the assembly.

The strength and longevity, or working life, or the number of years that a built-up roof will last, is dependent on the number of layers of felt and asphalt put into the roof.

In the Sixties, Seventies and even the early 1980's in California, "two ply" roofing was very popular. This roof was promoted very similarly to "two-ply, four-ply rated" tires that were also popular in this time period and which performed equally poorly, but they were inexpensive for developers to build!

Regardless of the promotion, these roofs on average had a working life of 8 -11 years, which was (and is ) simply not acceptable.

Adding a third layer of asphalt and felt to a roof extends its working life on average to 12-15 years. In temperate climates such as the mid-California coast, three ply roofs can and do function in excess of twenty years. (NOTE: These statements are based on the proper construction of the roofs and base flashings. There are many construction errors that can shorten these expected working lives unless the roof is regularly repaired.) A fourth layer of felt and asphalt gives a roof an expected working life of longer than twenty years.

Surfacing
The most commonly used surfacing material is called "mineral surfaced cap sheet." These are thick roofing sheets made of glass felt heavily coated with asphalt and surfaced with ceramic granules. The sheets are set in hot asphalt on top of the roofing plies and protect the plies from weathering. Buildings that are subject to a change of tenancy every few years do well with this roof since, with respect to alterations of the rooftop equipment, it is highly repairable.

After fifteen or more years these sheets begin to lose their granule surfacing and at that time the roof should be given a reflective coating as a replacement for the granules.

Some economy built-up roof designs call for the felts to be given an asphalt emulsion surfacing and a reflective aluminum coating. Such a system requires frequent maintenance. The cost of this increased maintenance more than offsets the initial lower cost of these "smooth surfaced" built-up roofs. We do not recommend these smooth surfaced roofing for use on most commercial properties.

There are building code chapters that updates set parameters for roofing construction. Building codes published prior to 1994 addressed roofing in Chapters 17, 23, and 32. The 1994-1997 Uniform Building Code addresses roofing in Chapter 15. The new (2001) California Code of Regulations, also known as Title 24, incorporates the 1997 Uniform Building Code. Roofing contractors should be required to conform their work to the requirements of these chapters.

Dick Tippett is the founder and principal of ERTECH, Inc. They have been solving building water intrusion problems since 1979

The pricing given below is for a four ply roof. Three ply roofs are no longer seen as an economical alternative. These are rule of thumb costs - several factors will influence the final figures for a specific project, they are:

• Number of roof layers to be removed
• Access to the roof and working hours
• Amount of mechanical equipment on the roof
• Presence of asbestos in old roof(s)
• Height of building being roofed
• Working around sight screens

ADDERS (for both models)

• If the old roofing contains asbestos, ADD $1.10 per square foot to the removal cost
• Install rigid foam R-19 insulation to surface of roof prior to roofing, ADD $0.90 per square foot to roofing cost

SINGLE PLY THERMOPLASTIC MEMBRANE

The use of thermoplastic roofing membranes has grown dramatically over the past years. This has occurred for a number of reasons including the proven track record of some products, the move to light colored “cool” materials, and the variety of systems available.The Thermoplastic single ply welded sheet roof is priced as follows:

The customer will find that a roofing company's expertise will be in one or the other of these roofing systems. Their experience level with the poduct will be reflected in their pricing. In other words a company whose roofing volume is 90% Thermoplastic installation will generally offer a lower quote than one whose volume is 10%.

Trade Contributor: Above All Roofing, San Jose Ca