School Chooses Thermoplastic Polyolefin (TPO) Roof for Durability, Energy Efficiency
Investigation determined that the original built up membrane had rotted away with age, resulting in "alligatoring" cracks that allowed water penetration. According to Baker Roofing Co., Norfolk, VA, a roofing contractor, this is common with a BUR. It is exacerbated by thermal contraction and expansion and the effect of ultra violet light on the exposed layers of asphalt.
The contractor recommended a new thermoplastic polyolefin (TPO) roof from Stevens Roofing Systems, Holyoke, MA. The new roof would be composed of a 45 mil TPO white roofing membrane mechanically attached to deck studs through a layer of 1-in. thick Owens Corning Durapink extruded polystyrene insulation board. The roof would be finished with new curb flashing, metal coping and pipe boots. The new roof was expected to not only prevent leaks at least for the life of its 15-year warranty, but to improve the effectiveness of the building's air conditioning by as much as 20%. According to the manufacturer, the lightweight system adds less than 30 lbs. per sq. ft. the deck loading of existing BUR materials.
The existing roof was a gravel-surfaced BUR on top of perlite insulation on lightweight concrete over a metal deck. Baker began by removing the gravel. "Until you sweep the rock you can't see what shape the underlayment is in," says Andy Hill, project manager for Baker. After the gravel was removed, the crew patched blisters in the existing BUR with hot asphalt and several plies of roofing paper. The old wall and curb flashing were torn out and the overall surface was cleaned and smoothed out as necessary. One inch of insulation board was needed to accommodate irregularities in the existing roof.
Durapink extruded polystyrene insulation was selected because it is impervious to moisture, maintains a lifetime R Value of R-5 per inch of thickness and offers good compressive strength. The material is also lightweight and easy to handle, score and cut. With the roof surface prepared, the Durapink board was laid down in a staggered pattern which helps avoid wind shear. The rigid board was fastened directly into the deck studs with eight mechanical fasteners in each 32-sq.-ft board, one every 4 ft. Then the boards were taped.
"Once we have the board down, we roll out the TPO membrane," says Hill. "We get the match right, put in fasteners every 18 in. and seal the seams with a robot hot-air welding machine." Hot-air welding literally fuses the overlapping sheet edges, creating one homogeneous bond. According to Stevens, this seam has four times the strength of most taped seams. The membrane offers the performance characteristics of rubber, and is reinforced with an encapsulated, woven polyester scrim.
When the last membrane was sealed, the roofing crew constructed a walkway to the roof-mounted HVAC equipment. New flashing and pipe boots were installed, fastened and sealed. The entire project took a six-man crew 11 working days, which included two days for gravel removal, blister removal and patch-up, and clean-up.