Flat Roofing Timber Firrings

🪵 Creating the Correct Fall for Flat Roofs

Timber firrings are used to introduce a controlled slope (fall) into flat roof constructions. Despite the name, flat roofs are not completely flat — a slight fall is required to help direct water away and reduce the risk of ponding or standing water.

Sawn timber firrings are typically fixed to roof joists to create this fall before the roof deck and waterproofing system are installed.

What Are Flat Roof Firrings?

Flat roof firrings are tapered timber sections, cut so that one end is higher than the other.

They are commonly:

  • Fixed on top of existing roof joists

  • Used to form a consistent slope across the roof area

  • Installed before decking and waterproofing

Firrings allow roof structures to remain level while still achieving the required drainage fall.

Cut to Specification

Our flat roof firrings are cut to order based on your requirements.

Typical standard heights include:

  • 50mm

  • 100mm

  • 150mm

However, we can also cut firrings outside these sizes, both larger and smaller, depending on the needs of your project.

Lengths and tapers are produced to match the specification you provide.

“To 0mm” Firrings Explained

For most DIY projects and general building work, firrings are commonly specified as “to 0mm”.

A to 0mm firring tapers from a set height at one end down to zero at the other.

For example:

  • 50mm to 0mm

  • 100mm to 0mm

This is the most common specification for straightforward flat roof installations.

Alternative: Firred Joists

An alternative approach is to cut the fall directly into the roof joist, creating a single structural component with a built-in slope.

⚠ This method is only recommended for professional use, where:

  • Supporting structures are known to be perfectly level

  • The joist size and design have been specified or approved by a qualified engineer

This approach is not suitable for general DIY use.

Important Guidance on Building Regulations

While we’re happy to explain how firrings are commonly used, we cannot advise on building regulations.

Please note:

  • Any information provided online or in person is general guidance only

  • We do not verify compliance with current building regulations

  • References to building regulations or standards (including BS6229) are based on publicly available information, which may not reflect the latest revisions

Because of the number of variables involved in roof design, our staff cannot confirm whether a particular firring specification will meet current building regulations.

If your project is subject to inspection or approval, we strongly recommend consulting:

  • A qualified builder or roofer

  • An architect or structural engineer

  • Your local building control department

Ordering Flat Roof Firrings

You can order your flat roof timber firrings online here:

👉 View Flat Roof Timber Firrings

⏱ Lead Times

Please note:

  • All timber firrings are made to order

  • Minimum lead time is 24 hours from ordering

  • During busy periods, lead times may be longer

If your project is time-critical, please contact us before ordering.

Need Help Choosing Firrings?

If you’re unsure what size or taper to order, we recommend speaking with a trade professional who can confirm the correct specification for your roof design.

We’re always happy to cut firrings to your supplied dimensions — but design responsibility remains with the customer or their appointed professional.