In detail
What belongs in a properly considered specification?
The visible surface is only one part of a flat roof. Comparing proposals is easier when you understand how the materials, installation method, thermal design and building use fit together. These are the practical details to discuss with Robbie before agreeing the scope.
The underlay, intermediate layer and cap sheet
The exact products and attachment methods should be chosen as a compatible roofing system. Depending on that specification, an initial layer provides the starting waterproofing layer over the prepared substrate, with a further underlay or base layer adding to the built-up construction. Laps need suitable positioning and bonding. Layer names can vary between specifications, so the useful question is what each proposed layer does and how the complete assembly is installed.
The cap sheet forms the exposed weathering surface. A mineral finish is commonly used to protect the bituminous surface from sunlight and weathering, while providing the familiar granular appearance. The cap must still be correctly detailed at corners, changes of direction and drainage points. It should not be treated as a decorative finish capable of compensating for poor preparation beneath it. Sound decking and appropriate detailing remain central to the finished roof.
Torch-on work and careful junction design
Torch-on felt involves heating a suitable bituminous membrane to achieve the required bond. Hot work needs particular care around combustible construction, cavities and concealed materials. Some locations or details may require an alternative application method within the specified system. A considered installation plan addresses those risks before application, rather than assuming that the same technique belongs at every point on a roof.
Upstands, wall abutments and perimeter edges require purpose-made detailing within the felt build-up. Layer transitions should maintain continuity without creating an unintended barrier to drainage. Outlets need integration with the surrounding membrane, and any rooflight kerbs need suitable height and weatherproof connections. These areas often demand more thought than the open field of the roof, especially on dormers or extensions with several changes in level.
Decking, insulation and condensation control
Deck boards provide the working base for the waterproofing. Roofing-grade OSB or plywood may be appropriate, but board choice, thickness, support and fixing depend on the particular construction and covering requirements. A deck that looks reasonable from above may still contain local deterioration around old leaks. Where opening up reveals damage, the affected area needs assessing before deciding what can remain. New waterproofing should not be used to hide a weak, wet or inadequately supported substrate.
A warm roof generally places insulation above the structural deck, while a cold roof places insulation below it and relies on suitable ventilation arrangements. Neither approach should be selected from a simple label alone. Vapour control, moisture movement, available height and junctions with the existing building all matter. If insulation is being changed, the roof build-up and any applicable building regulations requirements should be considered together, rather than assuming that extra insulation automatically prevents condensation.
Extensions, garages, dormers and commercial roofs
An extension roof over a heated room needs particular attention to thermal performance, internal moisture and rooflight junctions. An unheated garage presents different conditions, but still needs sound decking and reliable drainage. Dormers can have restricted working space and complicated connections to pitched roofing. Garden rooms may introduce several exposed edges and prominent trims. These differences affect the detailing, so a familiar material should not lead to an identical specification for every building.
Commercial units may have larger roof areas, more penetrations or access requirements that need discussing before work is planned. Keeping entrances, deliveries and occupied spaces in mind can help establish a workable sequence. A standard waterproof covering is not automatically a walking surface, terrace or base for plant. Where regular access, equipment or a change of use is proposed, the roof structure, protective arrangements and waterproofing details need separate consideration.