Advanced Timber Knowledge

How can timber elements be included on roof terraces?

Introduction

The timber on roof spaces is sometimes a subject of concern due to the perceived fire risks and complicated legislation surrounding it. Timber is used in a variety of ways at the roof level including as decking cladding and external furniture. This article will focus on external timber at roof level legislation and advice.

Building Regulations – 2018 Amendment

There is a perception that timber is not suitable for roof gardens and is borderline illegal. This is not true. The confusion stems from amendments made to the Building Regulations in 2018 brought into force following the Grenfell tragedy. The changes made involved documents B (fire safety) and 7 (materials and workmanship). These changes specify that for residential buildings of 18m+, any material used as external cladding must have a European Classification of A2 (more on this below).

For buildings below 18m, the former regulations where cladding without a fire rating can be used. However, it is an industry standard to specify any material with a Euroclass rating of B or above even on buildings below 18m.

The key point here, which causes much confusion, is that external furniture such as benches is exempt from these regulations. These regulations apply to balconies, cladding, decking – essentially any area creating an external wall. The 2018 amendment does not apply to furniture placed at roof level.

European fire classifications for timber

Throughout this blog, we have referred to the European Classification system or Euroclass fire ratings. If this blog was regarding external cladding, this section would be at the very top as it is crucial to understanding the topic. For external furniture, it is not essential, although beneficial, to have some understanding of the fire ratings.

With fire protection ratings, many people still use the Class 0, Class 1 and Class 2 systems. This system is known as the British Standard system and has been phased out, and replaced by the Euroclass system. Some specifications still refer to it which is confusing, although it is becoming increasingly rare.

The Euroclass testing system is very different from the British Standard system, as it includes testing for smoke propagation and the release of flaming particles. The British Standard system was focused on how long the material could withstand fire before losing its structural integrity. This was often given in minutes, such as 30-minute resistance. Please note that the Euroclass system is not time-rated and therefore does not offer 30, 60 or 90-minute fire resistance.

The reason for introducing the Euroclass system was because internationally fire safety standards were becoming too complex for practical use so the Euroclass system was brought in to harmonise these systems.

Table showing the classification below. No timber can meet Euroclass A1 + A2 requirements, only steel or aluminium.

Euroclass	Euroclass Definition	British Standard (where applicable)
A1	Non-Combustible Materials
A2
B	Combustible Materials: Very limited contribution to fire	Class 0
C	Combustible Materials: Limited contribution to fire	Class 1
		Class 2
		Class 3
D	Combustible Materials: Medium contribution to fire
E	Combustible Materials: High contribution to fire
F	Easily Flammable

Advice regarding timber on roof terraces

As above, it is legal to place wood-related products on roof terraces. Without timber elements, a roof space can appear cold with no natural materials besides small plants. However, timber is a combustible material, and it is irresponsible to disregard the risk it presents. So is there a way to specific timber furniture and mitigate the fire risk?

If care is taken at the design stage, it is possible to mitigate the fire risk of timber furniture. Measures you could take include:

2. Fire retardant timber treatments

Fire retardant timber treatments are the obvious solution to minimise fire risk. There are a vast variety of formulas available on the market. Methods of application range from simply painting the formula on the timber surface to impregnating the timber with the formula in a specially designed vacuum tank.

Fire retardant treatments is a term used interchangeably with fire resistant (intumescent) treatments. Although they both serve the same purpose, the methods of fire protection differ. Fire retardant slows flame reaching the surface of the wood by releasing gas when it gets hot, whereas fire-resistant treatments cause the timber to swell when they get hot creating an insulating coating protecting the timber underneath.

These treatments do not stop timber burning. The purpose of these treatments is to slow the fire growth period to 30 or 60 minutes to allow people time to evacuate the building in the event of a fire.

It is impossible to say for definite what rating a fire retardant can offer due to the huge number of variables at play such as quality of the treatment, method of application and timber type. However, as a general guide untreated timber has a Euroclass D rating. Treated timber would usually have a class C or B rating. If the specification specifies minutes of integrity, it is useful to be aware that it is impossible to treat timber to a 90-minute integrity standard.

1. Design techniques

To understand the design techniques that reduce the risk of fire, it is helpful to examine the De Pass Gardens fire in 2019. This fire engulfed a 6-storey residential block in London. Although there was no external furniture involved, the conclusions drawn from the event provide a good example of how not to use external wood-related products. The reasons the fire spread so quickly was because the timber cladding covering the balconies had a small cross-sectional area (very thin), had large gaps for air and oxygen to circulate between the timbers and the timber was not treated with any form of fire retardant. This advice was given by the Ministry of Housing, Communities and Local Government, the Timber Trade Federation (TTF) and the Wood Protection Association (WPA).

Learning from this event, it is therefore advisable to use timbers with a large cross-sectional area (such as the Pueblo collection of seating) and for the timbers to be closely spaced together.

Also, another technique you can use is to enclose the timber in a non-combustible material such as powder-coated steel. View our Plano integrated seating to see how this can be achieved. This will provide a physical barrier for the fire to overcome by restricting the area of flame spread.

Conclusion

In conclusion, the use of timber in external furniture at roof level is legal and given its aesthetic appeal, can add another dimension to a project. Ultimate responsibility for how and if it is used lies with the health & safety officer of the project.

There are numerous ways to reduce the fire risk timber presents. Primarily using fire-retardant treatments, but also through careful and considerate design.

Timber elements can be included on roof terraces or podium designs, however, it is crucial to get the design right. You can contact the technical team to ensure compliance with your design.