Effects of the listed changes on roof life follow:
- Add flood coat and gravel surfacing – 1.4 years of extended life (equaling an expected life of 13.5 years).
- Add fourth ply with gravel – 6.5 years of extended life (equaling an expected life of 18.6 years).
- Use coal-tar glass felt pitch/gravel – 6.8 years of extended life (equaling an expected life of 18.9 years).
- Use BUR/protected membrane roof (PMR) – 8.1 years of extended life (equaling an expected life of 20.2 years).
- Use coal-tar with organic felts/gravel – 8.9 years of extended life (equaling an expected life of 21.0 years).
Expected life for modified-bitumen roof systems (MBs):
- Standard Glass Base, APP cap - 10 years.
- Standard Base, SBS cap - 10.9 years.
- 2-ply glass BUR, SBS cap - 13.7 years.
- SBS base/SBS cap - 14.6 years.
- Any MB in PMR configuration - 15.4 years.
Expected life for elastomerics and thermoplastics:
- Black EPDM/45 mil - 10.4 years.
- 60 mil reinforced mechanically attached/taped laps - 14.2 years.
- PMR configuration - 19 years.
- PVC membranes - 12.5 years.
- (TPO was not available in 1992.)
While these are undocumented opinions from a limited survey, certain opportunities are revealed.
With single-ply systems:
- There is reduced labor.
- More of the membrane is produced in the factory.
- There is a greater potential for recycling at end of the membrane’s life.
- Buyers can select from a choice of highly reflective colors.
- Users may not need to recoat.
- You can expect a lower carbon footprint since they have lower petroleum content.
With PMR systems:
- The membrane is protected against hail, mechanical abuse, and UV exposure.
- Both the ballast/pavers and extruded polystyrene thermal insulation may be reused at end of membrane life.
Use of overlay boards (i.e., gypsum boards):
- Increases thermal mass and resistance to hail, wind (depending upon attachment method), fire, and mold.
With vegetated roofs:
- All the additional components (vegetation, drainage materials, etc.) make potential savings ambiguous at this time.
Converting to Steep-Sloped Roofs
In the Construction Engineering Research Laboratory (CERL) Report M-85/05, dated December 1984, the advantages of converting low-slope roof systems to steep roofs were discussed. While life-cycle advantages are present, aesthetics are also improved and roofing problems are greatly reduced. Cost estimates were also provided in the summary, as follows:
Converting flat-roofed buildings to sloped roofs proved to be a viable alternative to reroofing. This method was cost effective when considering the life-cycle costs, particularly for smaller buildings. Sloped-roof conversions are being done in most areas of the country, regardless of geographic area or climatic region. Finally, this type of conversion often had the benefit of enhancing the appearance of the buildings and solving other problems associated with flat-roofed buildings.
For many Bauhaus-style buildings (i.e. box-like buildings with very flat roofs), converting to sloped roofs suitable for architectural metal, shingles, or tile has the potential to greatly increase life while improving appearance, providing a vented attic, and reducing roof maintenance to a negligible level
n the same September 1997 issue of Interface magazine mentioned earlier, RCI experts were asked to estimate the reduction in roof life if the roof system drained poorly and suffered from areas of ponded water. The estimated reduction in roof life ranged from 1 to 3 years or 3 to 5 years for most systems. (Only coal-tar pitch suffered less – a 0- to 1-year reduction in life.) That might not sound like much, but if roof life is typically 15 years, then a 3-year improvement equals 20-percent longer life.
The effect of maintenance on service life was similar, with a hybrid BUR/SBS cap sheet system benefitting the most, with life projected to increase by 1 to 5 years with proper maintenance.
With photovoltaics coming into their own, a steeply sloped roof enhances performance through improved solar orientation of the panels. Elevation of the panels above the shingles/tiles may be easier to accomplish on a sloped platform, and the attic space provided by sloped roof systems may help reduce the effect of increased heat load generated by the panels themselves. Access to the electrical components of the panels may also be facilitated by placement in the attic space.
One last thought: On a recent trip to Southern California, I stopped at an outlet mall in Pismo Beach; air temperature was in excess of 105 degrees F. As we approached one of the upscale department stores, we noticed that all four doors to the store were wide open. You could feel the air-conditioned air pouring out of the building from 30 paces away. Increased R-value in the roof won’t help much if we forget the basics.