BS 6229:2018 is the key British Standard governing the design, construction and ongoing maintenance of flat roofs.
Its aim is simple: to help create flat roofs that are durable, well-drained, energy efficient and long-lasting.

A major part of meeting this standard involves achieving the correct fall (slope) — and this is where timber firrings, especially made-to-order sizes, become essential.

Below is a clear breakdown of what the standard requires and how firrings help you meet it.

Key Principles of BS 6229:2018

1. Proper Roof Falls (Minimum 1:80)

One of the most important requirements in BS 6229:2018 is ensuring the roof doesn’t hold water.

• Finished falls must be at least 1:80

• Designers should aim for 1:40 to account for onsite tolerances

• Prevents ponding, leaks and unnecessary structural loading

Where firrings come in:

Timber firrings are the simplest and most cost-effective way to create the precise fall required for a compliant flat roof.

2. Construction Guidelines

The standard provides best-practice guidance for installing:

• Waterproofing membranes

• Insulation

• Vapour barriers

• Protection layers

The aim is to improve energy efficiency, reduce thermal bridging and prevent condensation.

Correct falls work together with correct detailing — poor slope = guaranteed long-term issues.

3. Maintenance & Longevity

BS 6229:2018 stresses the importance of:

• Routine inspections

• Checking for ponding

• Keeping outlets clear

• Identifying early signs of blistering or membrane failure

A properly constructed fall dramatically reduces maintenance issues over the roof’s lifespan.

4. Material Requirements

Materials used in flat roofs should be:

• Durable

• Weather-resistant

• Compatible with adjoining layers

• Suitable for long-term performance

Timber firrings play a role here too — straight, accurately machined firrings help keep insulation and membranes sitting flat and correctly aligned.

How Timber Firrings Help You Meet BS 6229:2018

Creating a correct fall isn’t optional under the standard — it’s fundamental. Timber firrings provide a simple solution for achieving the required slope on warm roofs, cold roofs and tapered systems.

Why firrings are essential:

✔ Create the required fall from 50mm–0mm, 100mm–0mm, 150mm–0mm, or any bespoke size
✔ Provide a predictable, consistent slope across the roof structure
✔ Allow correct drainage toward outlets or gutters
✔ Reduce the risk of ponding, water ingress and membrane failure
✔ Ensure insulation and waterproofing layers sit correctly
✔ Save time and labour on site compared with improvised solutions

Made-to-order firrings (the big advantage)

Every roof is different. By ordering firrings cut to your exact sizes, you:

• Avoid unnecessary site cutting

• Speed up installation

• Ensure a uniform fall from end to end

• Reduce waste and improve accuracy

• Meet BS 6229:2018 drainage requirements with confidence

For contractors and DIYers alike, ready-made firrings mean you get a perfectly tapered finish without the hassle of calculating or cutting on site.

Why Falls Matter Under BS 6229:2018

A compliant fall protects the roof from:

• Standing water

• Accelerated membrane wear

• Freeze–thaw damage

• Structural loading

• Long-term leaks

Even the best waterproofing system will struggle if the roof is completely flat. Timber firrings remain the simplest way to build a reliable slope without altering joist depths or structural elements.

Using BS 6229:2018 in Real Projects

Planning & Design

• Incorporate firrings to achieve the correct drainage gradient

• Ensure proper integration of insulation, vapour control layers and membranes

Construction

• Install firrings directly onto structural joists

• Ensure a smooth transition to full thickness at the high side

• Achieve consistency across the entire roof

• Lay insulation & membranes onto a well-prepared slope

Maintenance

• A correctly installed fall greatly reduces maintenance frequency

• Less ponding = fewer membrane problems over time

Conclusion

BS 6229:2018 provides the essential framework for designing and constructing flat roofs that are safe, resilient and long-lasting. At the centre of these requirements is proper drainage — and the most practical way to achieve this is through accurately machined timber firrings.

Whether you're a roofer, builder, architect or DIYer, using made-to-order firrings ensures:

• Compliance with the standard

• Faster installation

• Better long-term performance

• A cleaner, more professional finish

Need Timber Firrings Cut to Size?

At Nottage Timber Merchants, we machine made-to-order flat roof firrings, including:

• 50mm → 0mm taper

• 100mm → 0mm taper

• Custom tapers to suit your fall requirements

Order online or call 01656 745959 for advice on the sizes you need.