Design Considerations
New Roof Requirement
Lease negotiations with a new tenant required installation of a new roof system. Ownership contemplated a holding period of ten years or less.
System Alternates
We presented several alternate choices, including very long term solutions with warranties transferable to subsequent owners. Cost control proved to be the deciding factor. We designed a system to minimize expense and maximize performance.
Roof Drainage and Slope to Drains
The existing built up “cap sheet” roof over a plywood deck was designed with very good slope to the drains. Minimum requirement is one quarter inch in twelve, yet this roof was closer to one and one half in twelve or more. It was designed without a “ridge” line and the entire volume of rain water was directed to the back wall and drain sump pairs located along the base of that wall.
Polyester Reinforced Asphalt Emulsion System Design
While unreinforced maintenance systems are sometimes installed on roofs in otherwise good condition, with the roof at approximately twenty two years in age, we specified polyester reinforced emulsion with acrylic surfacing. The same warranty is available with one or two reinforcement plies. To economize we put additional strength where it was most needed, by specifying one ply polyester in the upper half of the roof, with a second ply around air handling equipment, and two plies across the lower half of the roof.
Roof Removal at Drain Sumps
While we did notice some incipient seam separation in the field, most problem and repair areas were located at the waterways, crickets, and drain sumps along the rear wall. We required old roofing at the drain sumps be removed down to the deck, and replaced with polymer modified membranes heat welded in place. This turned out to be the right idea, as during this process concealed sections of water soaked materials were uncovered and removed.
Four Ply Polyester Roof Reinforcement Areas
Because the aged elasticity of the oxidized asphalt used in built up roofing is less than one per cent, these materials are subject to cracking and splitting at directional changes in the roofing. The cold process asphalt emulsion we specified is never heated or oxidized during manufacture, and remains more pliable and flexible. We added an additional two (2) plies of polyester to the drain sumps and waterways bringing the total in those areas to four (4) plies.
This extra reinforcement significantly strengthens the new roof in the most sensitive and critical areas that receive the bulk of rain water, while adding only marginally to the cost of the system.
Extra Work and Change Order
Because one of the new tenants was a food market, additional roof mounted equipment plus a building addition, were added after the Pre-Bid Contractor Job Walk was conducted, Certified Contractor Bids received, awarded, Contract Documents drawn, delivered and signed. Corrections for these alterations were included in a Work Change Order approved separately.
The Project was completed successfully, on time and on budget.
Roof reconstruction and replacement in areas where equipment was removed following the Contractor Bid Walk, were addressed in the Change Order.
The building addition roof was sealed with two layers of polymer modified asphalt membrane, with emulsion and acrylic surfacing.
APP modified membranes were used to seal equipment flashings and drain sumps.
We specified gypsum board beneath the modified membranes to level the roof areas removed, and increase fire resistance over the combustible roof deck.
Drains sump areas were removed down to the deck, new modified membranes installed, then sealed with asphalt emulsion, polyester reinforcement and acrylic surfacing.
An extra ply of polyester reinforcement was installed beneath and above the emulsion in the waterways resulting in four plies in those areas.
Any areas with defects or imperfectly sealed reinforcement seams were corrected with a “three course” reinforced acrylic flashing cement method.
Prior to correction, areas were marked for identification.
Asphalt emulsion and polyester fabric installation completed with correct application quantities.
Energy Star® reflective surfacing was applied in two separate cross direction passes to complete the process.