Edge Detail on a Parapet with Inadequate Upstand Height
You are installing a reinforced bitumen membrane (RBM) system on a flat roof with a parapet wall. The parapet upstand is only 100 mm high above the finished roof surface, and the existing render is sound but painted with masonry paint. The roof area is 200 m². The specification requires a minimum upstand height of 150 mm above the finished roof surface. The client is reluctant to increase the parapet height due to cost. The building is a school, and work must be completed during the summer holidays. The membrane is to be torched-applied. The weather forecast predicts temperatures up to 30°C during the work.
1. What are the risks of installing the membrane to the existing 100 mm upstand, and what options do you have to comply with the specification?
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Risks: (1) Water ingress: With only 100 mm upstand, wind-driven rain can easily overtop the upstand and enter behind the membrane, especially if the outlet or drainage is blocked. (2) Insufficient protection: The membrane termination is too close to the roof surface, increasing the risk of damage from foot traffic and UV exposure. (3) Non-compliance: The specification and manufacturer's instructions require a minimum 150 mm upstand; non-compliance may void the warranty and building regulations. Options: (a) Increase the upstand height: The preferred solution is to build up the parapet using compatible materials (e.g., concrete or render) to achieve at least 150 mm. (b) Use a proprietary edge trim: If raising the parapet is not feasible, a proprietary edge trim or drip edge can be installed, but it must still provide the required upstand and be approved by the membrane manufacturer. (c) Re-negotiate the specification: If the client insists, obtain written approval from the membrane manufacturer and building control for a deviation, but this is risky and may not be accepted. The best course is to explain the long-term risks and insist on raising the upstand.
2. Describe the correct sequence for installing the membrane at the upstand, including surface preparation and termination, considering the hot weather.
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Sequence: (1) Surface preparation: Remove all loose paint from the render (as it may not be compatible with the membrane adhesive). Clean the surface thoroughly. Prime the render with a bitumen primer appropriate for torching. (2) Detail reinforcement: Install a reinforcing layer (e.g., a bitumen-impregnated glass fibre or polyester mat) in the angle between the roof and upstand, embedded in a layer of bitumen. (3) Membrane application: Apply the base layer starting from the upstand, ensuring a minimum 150 mm upstand height. Torch-apply carefully to avoid overheating the render; in hot weather, the membrane may become too soft, so work in smaller sections and allow cooling if necessary. (4) Termination: Terminate the membrane at least 150 mm above the roof surface using a suitable edge trim (e.g., a metal drip edge) fixed mechanically. Seal the top edge with a bitumen sealant. (5) Weather considerations: In 30°C heat, the membrane may become tacky; avoid walking on it and use knee boards. Ensure torching is done safely to prevent fire risk. (6) Inspection: Check for voids and ensure the upstand is fully bonded.