Building components · Roof Carcass & Edges
Firring Piece
Explains the firring as the piece that separates the level the structure works at from the plane the water has to run down.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a firring is
A firring piece is a length cut or manufactured with a taper along it, fixed over the joists of a flat roof so that the deck bearing on it sits on a falling plane rather than a level one. Run along the joists it varies the height continuously; run across them it steps the fall from joist to joist.
Its value is that it separates two things that would otherwise have to be the same. The structure can be set level, which is how structure is built and how ceilings below are formed, while the surface above it is set to run water toward the outlets. Without something performing that job, a flat roof drains in whatever direction its structure happens to sit, and that direction is decided by other considerations entirely.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a joist. A joist spans between supports and carries the load of the roof; a firring is carried by the joist and supports only the deck, and its purpose is to change the geometry of a surface rather than to span anything.
- It is not a tilting fillet. The published tilting fillet is short and local — it lifts the tail of a first course or eases the angle into an upstand; a firring runs the length of a roof and sets the plane of the entire deck rather than correcting a single change of angle.
- It is not tapered insulation. Fall can also be created within the thermal layer above the deck, but the two arrangements leave the deck in completely different places: a firring puts the fall beneath the deck, so the deck itself slopes and the structure stays level, while tapering the insulation leaves the deck flat and forms the fall above it.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- firrings
- tapered batten
- cut-to-fall batten
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Which direction the outlets are in, since the firrings exist to deliver water to those points and nowhere else.
- Being run consistently, because an inconsistency between adjacent firrings appears as a depression in the finished deck plane.
- How the deck is fixed through them into the structure below, as the firring is an intermediate piece between the two.
- Whether their direction interrupts anything else the void depends on, since the direction they run and the direction air can move along the void are decided by the same choice.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The whole point of the element is the direction water takes, so the arrangement is decided by where the outlets are before anything is cut.
In a cold roof the void beneath the deck may depend on air moving through it, and the direction the firrings run either preserves that path or interrupts it.
Small inconsistencies accumulate into a visible depression in a surface that is meant to shed water, and depressions in a flat roof are self-reinforcing once water sits in them.
Cutting a long taper accurately and repeatedly is the practical difficulty; preformed firrings exist for that reason, and mixing methods across one roof is a recognised source of unevenness.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Clarifications
Commonly misunderstood
- That a flat roof is flat. It is a roof with a fall small enough that it reads as level, and the firrings or an equivalent arrangement are what create that fall; the description refers to appearance rather than geometry.
- That firrings can be run in whichever direction is convenient. Running along the joists and running across them produce different results for the void beneath, and the two arrangements are not interchangeable.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Where are the outlets, and how do the firrings deliver water to them from every part of the roof?
- Are the firrings run along or across the joists, and what does that mean for the void beneath the deck?
- How is the deck fixed through the firrings into the structure?
- Is the fall being formed below the deck, above it in the insulation, or both, and why was that chosen?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes an element and what it does. It states no fall, gradient, taper, size, spacing or fixing schedule, and none can be inferred from it.
- How a particular flat roof is drained, and what falls it is designed to, is a design determination made for that roof and verified against local requirements.
Roof Carcass & Edges
The battens, boards and edge elements that finish a roof structure and prepare it to receive a covering.
Browse all roof carcass & edges entries →