Mass timber structures and the planning fire narrative

A mass timber fire statement has to do something unusual: explain why a structure that burns is safe. That is a harder argument than the one a concrete scheme makes, and it is harder for a reason that has little to do with the physics. Timber frames are well understood, charring behaviour is codified, and tall timber buildings exist. The difficulty is that the planning submission has to satisfy a Regulator, an insurer and a warranty provider whose appetites differ from each other and from the Building Regulations.

Design teams who treat this as a Building Regulations problem to be solved later tend to lose the scheme, or lose the timber.

Why structural timber draws early scrutiny

For a relevant building under Article 9A of the Development Management Procedure Order 2015, a fire statement accompanies the planning application and the Health and Safety Executive, as the Building Safety Regulator, is consulted. On a cross-laminated timber scheme that consultation response will ask about the structure, because the structure is combustible and because the questions it raises affect matters that planning does control.

Two of those matters appear on the planning drawings. Floor zone depth, which affects building height and sometimes storey count. And the external wall build-up, which affects the elevations and the material palette the design officer sees.

So the fire narrative and the design narrative are the same conversation on these schemes, and holding them separately produces a scheme that satisfies one reader and not the other.

Where the external wall restriction bites

Regulation 7 of the Building Regulations 2010 restricts the use of combustible materials in the external walls of relevant buildings, and amendments made in 2022 extended the building types caught and dealt with balconies. Verify the current text of Approved Document B and regulation 7, because this is an area that has been amended repeatedly.

The practical consequence for mass timber is specific. Using timber for the internal structure of a tall residential building is a different question from using it within the external wall construction. Where the timber forms part of the external wall of a building caught by the restriction, the restriction applies to it in the same way as to any other combustible material.

That is why many mass timber residential schemes resolve as a timber structure with a non-combustible external wall system, and why the facade zone on those schemes is deeper than architects expect. The elevations have to be drawn with that depth from the start.

Encapsulation and the exposed timber decision

The single biggest fire engineering decision on a mass timber building is how much timber stays visible.

Full encapsulation treats the timber as a hidden structural material, protected by plasterboard linings designed so that the timber does not contribute to a fire for a defined period. It is the conservative route and it discards the main architectural reason for choosing timber.

Exposed timber keeps the soffits or walls visible and accepts a contribution to the fire load. That contribution has to be quantified, and the analysis rests on charring behaviour under Eurocode 5 part 1-2, together with the harder questions about adhesive line delamination and whether the fire self-extinguishes once the contents have burned out.

The honest position is that this is an area of active research and engineering judgement rather than settled codified practice, and a fire strategy that presents exposed mass timber as routine will be challenged. Where the architecture depends on visible timber, expect to pay for analysis and expect to compromise on extent.

Insurance and warranty appetite as the real limit

Here is the part that catches commercial teams. The binding constraint on mass timber residential is usually not the Building Regulations. It is who will insure it and who will warrant it.

Construction phase cover is the one most often overlooked. An exposed timber frame on site, before any encapsulation, is a different risk from a finished building, and the market prices it accordingly.

Test all four before the planning application is submitted. A permission for a scheme that cannot be insured is of no value, and changing the structural material after permission usually means revised drawings, a revised fire statement and an amendment application.

What belongs in the planning submission

Keep the planning submission to the matters planning decides, and be specific about them.

1. The structural system, named, including where timber is used and where it is not.
2. The encapsulation strategy at a principles level, with the extent of exposed timber stated.
3. The external wall approach, confirming how regulation 7 is addressed for the building type and height.
4. Fire appliance access, hardstanding and water supplies for firefighting, with dimensions.
5. The evacuation strategy principles, including stair numbers and positions as drawn.
6. Sprinkler or suppression provision, which on timber schemes is usually part of the case rather than an option.

A Fire Statement covers the Gateway One submission itself. A fuller Fire Safety Strategy to BS 9991, with a framework under BS 7974 where a fire engineered approach is used, sits behind it. Both are fire-engineering documents requiring a competent author, and PlanScope's draft is a structured starting point for that engineer rather than a substitute for them.

What does not belong in the planning submission is detailed compartmentation drawing or connection design. Those are Gateway Two matters.

Peer review of the fire engineering

On a mass timber residential scheme, commission an independent review of the fire strategy before submission rather than after the Regulator comments.

Three reasons. The Regulator's response arrives inside the determination period, so there is little time to react. A reviewed strategy is a stronger document in front of a committee. And the insurer's technical team will ask whether the strategy has been independently reviewed, so the review has commercial value beyond planning.

Choose a reviewer with mass timber experience specifically. General fire engineering competence does not cover delamination behaviour or the test evidence for particular encapsulation build-ups.

Being honest about residual risk

Mass timber is chosen for carbon reasons as much as architectural ones, and the carbon case is real: a Whole Life Carbon Assessment prepared to the RICS methodology usually shows a substantial embodied saving against a concrete frame. Councils normally expect a suitably qualified author for that work, and professional review is strongly advised.

That carbon case is not an answer to a fire question, and presenting it as one damages credibility with the Regulator. Keep the two arguments separate in the submission.

Three disclosures strengthen a timber application. State the extent of exposed timber and the analysis behind it. State the insurance and warranty position as it stands, including anything unresolved. And state which elements of the strategy depend on test evidence for a specific product, because that is the element most likely to change between planning and construction.

A design team that names its uncertainties is treated differently from one that appears not to have found them.

Enter your postcode on PlanScope to see which reports a tall residential scheme is likely to need, then brief the fire engineer and the insurer against the same list.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.