Flat Roof Refurbishment with Existing Asphalt and Ponding
A 1970s flat roof over a warehouse is to be refurbished. The existing covering is a 20-year-old asphalt system with numerous blisters and open seams. The roof deck is a 150 mm concrete slab with a 1:80 fall to internal outlets. The client wants a new single-ply membrane (1.5 mm TPO) fully bonded. During the site survey, you notice that several areas near the outlets have standing water 48 hours after rain, and the existing asphalt has a spongy feel in those areas. The roof area is 800 m². The building will remain operational during the work, and the client has a limited budget. The specification calls for a 20-year warranty.
1. What are the key preparation and substrate assessment steps you must take before installing the new TPO membrane, and why are they critical?
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The critical steps are: (1) Moisture survey: Use a moisture meter or infrared thermography to map wet areas in the existing asphalt and concrete deck. Ponding and a spongy feel indicate trapped moisture, which can cause blistering and premature failure of the new membrane. (2) Core sampling: Take cores to determine the extent of wet insulation (if any) and the condition of the concrete deck. (3) Pull-off tests: On the existing asphalt, perform pull-off tests to assess adhesion for the new membrane; if the asphalt is friable or contaminated, it may need removal. (4) Surface preparation: Remove all blisters, loose material, and contaminants; repair cracks and spalls in the concrete; ensure the surface is clean, dry, and smooth. (5) Fall assessment: Verify the existing falls; ponding areas may require additional insulation or screed to achieve the 1:80 fall, otherwise water will continue to pond and degrade the new membrane. (6) Compatibility check: Ensure the TPO membrane is compatible with any residual asphalt or bitumen; a separation layer may be needed. These steps are critical because the new membrane's performance and warranty depend on a sound, dry substrate and correct drainage.
2. The client wants to save money by installing the TPO directly over the existing asphalt without removing it. What are the risks and what conditions would make this acceptable?
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Risks of overlaying: (1) Trapped moisture: Any moisture in the existing system will be sealed in, leading to blistering, delamination, and reduced insulation performance. (2) Uneven substrate: Existing blisters and ridges will telegraph through the new membrane, causing premature wear and potential punctures. (3) Adhesion issues: Asphalt may be incompatible with TPO adhesives; proper bonding may not be achieved. (4) Weight: Additional layers increase dead load, which may exceed the structure's capacity. (5) Warranty void: Most membrane manufacturers will not warrant an overlay over a wet or deteriorated substrate. Overlay may be acceptable only if: a thorough moisture survey confirms the existing system is dry; the existing asphalt is sound, well-adhered, and free of blisters; a suitable separation layer or primer is used to ensure compatibility; the falls are corrected to prevent ponding; and the membrane manufacturer approves the system in writing. Even then, a 20-year warranty is unlikely without a full replacement.