Building envelope

Roof Assembly Systems

No roof answers one of those questions without answering the other. Put insulation above the deck and the deck stays warm; put it below and the void above it has to be ventilated; put it above the waterproofing and the membrane is protected but the drainage layer changes character. Meanwhile the covering itself is either lapped and shed, as tile and slate are, or joined into something continuous, as a membrane or a seamed metal sheet is.

Both halves are described here, which is why coverings and build-ups share one family rather than sitting apart from each other. The roof carcass that spans belongs to the primary structure family, and eaves, abutments, parapets and rooflight kerbs belong to envelope junctions, because that is where roofs actually fail. Falls, drainage layout, fixing patterns and wind uplift are design matters, and no entry claims a covering is watertight on its own.

15 system entries · educational assemblies reference · how a system performs always depends on the whole building and qualified professional design

Scope

What this category covers

Where the boundary of this part of the encyclopedia sits, including what it deliberately leaves to qualified professionals.

Warm, cold and inverted flat roof build-ups, and trafficked or water-attenuating roof decks
Ventilated and rafter-line pitched roof assemblies and the voids they depend on
Lapped and continuous roof coverings, from tile and slate to seamed metal and membranes
Not covered: falls and drainage design, fixing patterns, wind uplift or waterproofing specification

All entries

Every roof assemblies entry

The complete set for this category. Frequently referenced entries come first — a starting point, not a ranking.

How the parts relate

Warm Flat Roof →A flat roof in which the thermal layer sits above the structural deck and beneath an exposed waterproofing, so the deck and the spaces below it stay on the warm side of the insulation.

Cold Pitched Roof →A pitched roof insulated at ceiling level, so the roof space above becomes an unheated ventilated void that lies outside the envelope along with anything stored in it.

Single Ply Roof →A flat or low-slope waterproofing arrangement using a single layer of flexible sheet membrane, joined by welded or bonded seams and held down by adhesion, mechanical fastening or ballast.

Cold Flat Roof →A flat roof with insulation held within or below the deck structure and a ventilated air space kept between that insulation and the cold underside of the deck and its waterproofing.

Inverted Roof →A flat roof with the waterproofing laid on the structural deck and the insulation placed above it beneath a ballast or surfacing, so the membrane is shielded and rainwater reaches it from beneath.

Warm Pitched Roof →A pitched roof insulated along the rafter line so the roof space falls inside the envelope, with the thermal layer sitting above, between or below the rafters in combination.

Trafficked Roof Deck →A flat roof that is also a usable surface, where a wear or paving layer is carried above the waterproofing on supports or bedding and the roof drains at the concealed level beneath it.

Roof Void Ventilation →The designed air path that lets outside air enter, cross and leave a roof void, so moisture arriving in the space is carried away instead of settling on its coldest surfaces.

Roof Attenuation →A flat roof arranged to hold rainwater temporarily within or on the assembly and release it slowly through a restricted outlet, spreading the discharge into the drainage network over time.

Tile and Slate Covering →A lapped-unit roof covering in which overlapping tiles or slates are fixed to battens over an underlay carried on the roof structure, shedding water down the slope rather than sealing it out.

Standing Seam Roof Assembly →A metal roof of long trays joined by upstanding folded seams and held by concealed clips over a continuous deck and separation layer, so the covering is not pierced across the field.

Twin-Skin Metal Roof →An industrial roof assembled on site over purlins from separate layers - a profiled liner sheet, a spacer system, insulation and a profiled outer sheet - each fixed in its own operation.

Composite Panel Roof →A roof of factory-made panels in which an outer weathering sheet, an insulating core and an inner liner are bonded into one component, fixed to purlins with engineered joints between panels.

Built-Up Bituminous Roof →Flat or low-slope waterproofing formed from successive reinforced bituminous plies bonded together with offset laps, so the finished waterproofing is a multi-ply layer rather than a lone sheet.

Liquid-Applied Roof Build-Up →Roof waterproofing formed in place from a liquid cured over a primed substrate, usually incorporating a reinforcement fabric, giving a continuous layer that follows complex shapes without joints.

Elsewhere on Build Design Hub

Planning guidance for this subject

This encyclopedia explains how these assemblies are organised. These sections help with choosing between them and preparing for the conversation.

Roof planning basics →The planning side of how a roof strategy is settled.Roofing materials →The choosing-and-comparing side of roof coverings.Roof covering decision preparation →Preparation before a roof covering conversation.

Materials Library

What these assemblies are built from

A systems entry explains how an assembly is organised. These materials sections describe the substances themselves — what they are, how they behave and what governs their use.

Insulation →Reference entries for thermal insulation materials — mineral and glass wool, rigid foam boards, natural-fibre insulations, blown and sprayed products and reflective systems.Roofing →Reference entries for roof coverings and roof waterproofing — tiles and slates, shingles, sheet and standing-seam metal, single-ply and built-up membranes, liquid systems and green roofs.

Building Systems Encyclopedia

Reference entries for building assemblies — how each is organised, what role every component plays, how the parts interact and which junctions matter.

Browse all building systems →

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