EPISODE: 016 - APPROVED DOCUMENT B - FIRE SAFETY - PART 3 OF 4

BYTNAR - TALKS

EPISODE 016 - APPROVED DOCUMENT B - FIRE SAFETY - PART 3 OF 4

This episode is for people who want to know more about Approved Document Part B – Fire Safety.

You should like this episode if you ask yourself questions like:

• What are the requirements for maintaining a building's structural stability during a fire under Approved Document B?

• How does compartmentation work to prevent fire spread in dwelling houses and between separate structures like garages?

• What fire resistance standards must structural elements of new additions to existing buildings meet?

• Why are sprinkler systems required for certain flats, and at what height does this regulation apply?

• What measures are necessary to inhibit unseen fire spread within cavities and concealed spaces in flats?

• How should openings and penetrations in fire-resistant compartments be protected to prevent fire and smoke spread?

• What are the criteria for external walls and roofs to resist fire spread between buildings, and how does distance to the boundary affect this?

• How should fire spread over roof coverings be managed, and what are the European directives related to this?

• What guidelines does Approved Document B provide for determining unprotected wall areas based on proximity to property boundaries?

LET'S TALK ABOUT YOUR PROJECT

---

This is Bytnar Talks: The Engineer Takes on Construction, Episode 16.Hi, I'm Piotr Bytnar. Each day, I help my clients plan and design building projects through Bytnar Limited — a consulting Chartered Structural Engineers practice.My biggest passion, and the cornerstone on which I've built my business, is finding clever solutions for construction projects.

I am a Chartered Structural Engineer and a budding software developer, so you can rest assured that I will strive to talk about the best practices and the use of new technologies in the industry.And if you're embarking on a construction project, or are involved in planning, designing, and building the world around us, you'll find this podcast useful.

Approved Document B: Fire Safety — Part Three

Here we go again.Welcome back to Bytnar Talks, your favorite podcast on all matters of architecture, engineering, and construction.

It is Thursday, the 16th of May, 2024, and I'm here back with you for the 16th episode, with further information on fire safety.

Fire safety is a meal best served cold — and in a tasting menu fashion — so you can appreciate its flavors more. Therefore, I’ve split it into several episodes.

Last week, in Episode 15, I closed Sections 2, 3, and 4, talking about the escape routes of dwelling houses and flats, as well as the internal fire spread through linings.

Approved Document B provides detailed guidelines for means of escape in dwelling houses and flats:

• Section 2 emphasizes direct exits for ground floors and protected stairways for higher levels, with specific criteria for escape windows and doors.

• Section 3 focuses on containment within flats and controlled evacuation, highlighting the separation of living and sleeping areas, and regulated evacuation distances.

• Section 4, Requirement B2, addresses internal fire spread, mandating materials for linings to resist flame spread and heat release, particularly scrutinizing insulated panels and thermoplastic materials for fire resistance and their application suitability.

Now I’ll carry on to cover Requirements B3 and B4 — so:Internal fire spread: structure, and external fire spread — Sections 5 to 12.

So, without further ado, let’s dive into the fire safety requirements B3 and B4 as treated in the Approved Document.🎵 [Music] 🎵

Requirements B3 and B4 deal with fire spread, taking into consideration the structure and external fire spread, outlined in Sections 5 to 12.

In this episode, you’ll find out what to take into consideration and when — including compartmentalization, cavity barriers, fire stopping, and more.You’ll also learn how distances between buildings matter in light of fire spread between them.

So, again, without further ado, let’s get on with it.

Generally speaking, before we get into the interpretation and guidance of the Approved Document, let’s have a look at what the regulation actually requires of us.

🔥 Requirement B3 — Internal Fire Spread: Structure

There are four points to consider:

Point 1:The building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period.

Point 2:A wall common to two or more buildings shall be designed and constructed so that it adequately resists the spread of fire between those buildings.For the purposes of this subparagraph, a house in a terrace and a semi-detached house are each to be treated as a separate building.

Point 3:Where reasonably necessary to inhibit the spread of fire within the building, measures shall be taken — to an extent appropriate to the size and intended use of the building — comprising either or both of the following:

• Subdivision of the building with fire-resistant construction, or

• Installation of suitable automatic fire suppression systems.

Point 4:The building shall be designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.

So — simples:

• Separate buildings should be isolated from each other. That’s a no-brainer, right?

• Separation means fire spread is slowed down.

• In the event of fire, the structure is to survive reasonably long.

• Within the building, spaces should be either or both compartmented and protected by sprinklers.

• And concealed spaces should restrict the spread of fire within them.

Can we go home now, you ask?

You can — if you don’t want to know more.But if you want to speak the same language as your Building Control Officer, you’ll want to stick around.They will certainly refer to what’s coming next...

So, starting with Section 5 of Requirement B3 – Internal Fire Spread in Dwellings.

When we start thinking about the required protection, we ought to go to Appendix B of the Approved Document to assign the required protection time to a given situation.

Generally speaking, we’re talking about 30 to 120 minutes resistance for the maximum fire scenario, providing protection toward the regulatory cause elements of structure in the way.

The fire should be able to reach its full potential and decline without collapsing the building.

Naturally, such considerations may be reasonably relaxed for buildings of low consequence, where collapse would not endanger other buildings or people in close vicinity.

And that concerns all the structural elements of the building — frames, floors, walls, buttresses, beams, roofs, and so on.

So what is not considered an element of structure?

It’s all elements that do not form part of the load-bearing or stability system of the building, or escape route.

Also, the lowest floor of the building, when we add an additional storey to a two-storey house — that new storey needs to be REI 30, as do all floors forming part of the existing escape route.

The existing first floor structure should be of sufficient structural capacity, but some relaxation on integrity and insulation of fire is acceptable.

In the event of fire, the first floor should be no less than R30, if the extension is sufficiently small — only one more storey, up to two rooms of less than 50 m² in total.

Compartmentation is important.

It's used to avoid the spread of fire between separate houses, like in the case of terraced or semi-detached properties, but also between houses and garages.

The garage should be made in a way that, if there is a door accessing the house directly from it, there is no way for fuel to get into the house.

So think of:

• Sloping away from the doors, and

• Doors raised a minimum of 100 mm over the garage floor.

For compartments to be effective, they need to form a complete barrier, so they must be considered together with all the other details of elements attached to them — like the party wall under the roof, or with a fire barrier in the form of a parapet wall.

Any penetrations of the compartment should be:

• As small as possible,

• Fire-stopped, and

• Structurally isolated.

So for example, roofs adjacent to compartments should be of B ROOF(t4) classification, on an A2-s3, d2 substrate, for at least 1.5 m on each side of the boundary line.And that would exclude thermoplastic windows from the zone.

Some continuity of roofing patterns or substrate with limited contribution to fire, over the compartment wall, can be allowed in buildings up to 50 m in height.

But what do all of those abbreviations mean, really?

Let’s explain:

B ROOF(t4) is the highest performance classification for a roof.It means the roof has been tested to Type 4 fire testing, which measures:

• External fire exposure resistance,

• Flame penetration, and

• Spread of flames.

So, a B ROOF(t4):

• Will resist flame penetration for 60 minutes,

• Will self-extinguish within 5 minutes of removing the heat source, and

• Will not allow more than 0.38 m of flame spread — meaning approximately 19 cm either side of the ignition point.

As for A2-s3, d2 — let’s break that down:

• A2 = limited combustibility,

• s3 = high smoke production,

• d2 = may produce flaming droplets/particles.

I explained that in more detail in the previous episode, so if you’d like to listen to that, you’re more than welcome — more than invited.

The next ever-so-important aspect of fireproofing is the cavities in buildings.

Cavities can allow the fire to spread unnoticed, and — interestingly — due to the chimney effect, that spread can happen very fast.

To address the issue, we introduce cavity barriers in key points of our buildings, especially between the compartments, to ensure that the cavity is not a bridge between them.

If the construction is a typical cavity wall construction, it is sufficient to close the space around openings and the top of walls with any material — or not at all in case of full-fill insulation.

Otherwise, cavities need to be closed with materials rated E30 for integrity and I15 for insulation — meaning:

• 30 minutes resistance to fire and flame spread,

• 15 minutes to prevent excessive heat transfer.

Common cavity barrier materials include:

• 0.5 mm steel,

• 38 mm timber,

• mineral wool, or

• 12 mm cement board (CES board).

These are readily identified by the document. Any remaining gaps between the cavity barrier and the surface it abuts should be fire-stopped.

Openings in cavity barriers can be done, but are restricted, as addressed in

Section 6 — Loadbearing Elements of Structure – Flats.

Now we move to flats, where the approved guidance is similarly structured to that of dwellings.It follows the same philosophy of structural integrity and division, and highlights the possible further consideration of such bearing compartment walls, which are expanded on and addressed in several other sections.

When considering conversion of a dwelling house to flats, any shortfalls of the existing construction should be addressed, but may be acceptable within REI 30 fire resistance, especially for buildings of less than three storeys, if a protected stairway exists or is proposed.

Section 7 — Compartmentation & Sprinklers in Flats

Basically, unless it is a wall or floor dividing parts of the flat itself, it will need to have fire-rated construction — including between the flat and:

• the refuge chute, or

• places of special fire hazards.

Compartment walls of masonry or concrete construction can be bridged by structural elements, but only to a minimum extent, and must be fire-stopped.

When it comes to sprinklers:

• Blocks of flats with a top storey above 11 m, measured from ground level, should be fitted with them.

• The consideration starts with systems inside the flats and extends to the escape route, if it is not fire sterile.

Separation between buildings should be in a continuous vertical plane, in the form of a wall — no "flying freehold" or overhang-type scenarios.

Similarly, in division of separate parts of buildings in other cases, it is enough if the compartment walls are full storey height,but if they enclose the top storey, they should form a fire parapet overhead.

There are further provisions for all elements that are built into or supported by the compartment walls or floors:

• to either limit their impact on the compartment, or

• to restrain their failure to the connection with the compartment.

For example, a curtain wall-type scenario should not detach and fly away from the building, essentially creating a “fire kite”, spreading danger far beyond the immediate environment.

All perforations must be fire-stopped and built to certain specification.

Shafts — of any use — should be protected and compartmentalized,although there are rules for penetrations, access, and ventilation.Within certain limits, some of the enclosure can be glazed, and external walls often do not need to be fire-rated.

Section 8 — Cavities in Flats

Cavities in flats generally fulfil two functions:

1. Divide space, restraining the spread of fire within the cavity, and

2. Enclose the space to prevent ignition from outside sources.

Cavities mean all hidden spaces — most obviously:

• cavity walls,

• suspended ceilings,

• floors,

• and attics.

Cavities should respect the compartmentalization of the building,and when they are large, they should also be divided into smaller sections.

Section 9 — Protection of Openings & Fire-Stopping

In essence, all imperfections and perforations through compartment construction should be fire-stopped, to inhibit the spread of fire and smoke.

When it comes to pipe penetrations, these should have:

• Proprietary sleeves, or

• Be of small internal diameter, or

• Be sleeved in metal.

Ventilation systems should:

• not aid the spread of fire and smoke,

• be fitted with fire and smoke dampers where needed, and

• in certain circumstances, provide smoke extraction.

When ducts pass through fire compartment elements, they should ideally:

• be fire-resistant, or

• be in fire-resistant enclosures, and

• have automatically engaged fire and smoke dampers.

Look into the ASFP Blue Book — that’s the Association for Specialist Fire Protection Blue Book — for the best approach to these matters.

Interestingly, flue enclosures should have half the fire rating provided by the compartment wall — otherwise, in effect, you’re dealing with two walls, so the total still equals the required fire protection.

A great wealth of knowledge can be accessed through the Association for Specialist Fire Protection, so you are encouraged to visit their resources and devour all that good knowledge.

And this is it for Requirement B3 – Fire Spread: Structure.

Now we're moving on to Requirement B4 – External Fire Spread.

So what does the requirement actually say?

As written in the letter of law, Requirement B4 has two points:

Point One:The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height, use, and position of the building.

Point Two:The roof of the building shall adequately resist the spread of fire over the roof and from one building to another, again having regard to the use and position of the building.

In the gist:The wall should not be susceptible to ignition or becoming a torch, and fire should not spread between buildings — via walls and roofs, or through radiant heat from openings in walls and roofs.

All right, class dismissed!— No joking!There’s still more to go.

Let’s see what the guidance has to say in:

Section 10 – Resisting Fire Spread Over External Walls

Walls should meet the performance criteria given in BR Report 135, or the provisions in the Approved Document.

Materials should generally have limited combustibility or low to medium contribution to fire.The exact reaction to fire class of materials will depend on the type and height of the building.

Interestingly, cavity walls are excluded from this specific consideration — so they can contain combustible material within the cavity,— at least outside of Regulation 7(2) consideration, that is.

The guidance also directs us to other publications for more answers regarding green roofs and walls.

Cavities are obviously important to limit the spread of fire.

There are additional provisions for external balconies in buildings with a storey over 11 m in height.Notably:

• The balcony cannot be turned into a barbecue — I know!

• The surface should not be perforated, and

• The balcony should not contribute to fire.

There is also mention of the restriction of metal sandwich panels, which consist of materials with more than 35 MJ/kg of gross calorific value.That means: how much heat the material will release upon ignition.

When the building falls under the remit of Regulation 7, most of the materials used in the external envelope, or things attached to it (excluding items listed in Regulation 7, paragraph 3),should be of very limited combustibility.

Section 11 – Resisting Fire Spread from One Building to Another

The risk of fire spread depends on:

• the use of the building,

• its compartmentalization, and

• the suppression systems present.

The consideration begins with:

1. Establishing which boundary is relevant, and

2. Determining the impact a fire in the building may have on the neighboring building.

Depending on the distance of the wall to the boundary, different provisions apply regarding how much of the wall can be unprotected.This can result in most of the wall being:

• Fire-enclosed, or

• Completely unprotected, if far enough from the boundary.

When the wall is far away from the boundary,the unprotected area consideration starts at Class C materials and below,for materials 1 mm thick or more.

Some unprotected areas in the wall may be ignored.

The distance to the boundary is typically:

• within 1 m, or

• over 1 m

This determines the allowable areas and distances. The guidance includes several diagrams and rules, but also allows for the use of BR Report 187 as an alternative approach.

Generally speaking:

• For small buildings, the amount of unprotected wall area at or closer than 1 m from the boundary is 5.6 m² —

  
  

  That’s quite a bit!

• Smaller openings that can be ignored may be added to that, without decreasing the amount allowed for the larger ones.

The amount of unprotected wall area becomes unlimited for walls that are:

• 6 m or more from the relevant boundary,

  
  

  or in buildings of certain classifications.

In bigger or more complex buildings, we take into account the percentage of wall area,starting at 4% at 1 m or less from the relevant boundary.

In buildings with sprinkler systems, the boundary distance can be:

• halved to a minimum of 1 m,

  
  

  or

• the allowable area of unprotected wall can be doubled.

Section 12 – Resisting Fire Spread Over Roof Covering

It’s important to say here that roof covering does not include the roof structure as a whole.

Covering products accepted under European Directive 2001/95/EC (or formerly Directive 91/106/EEC) are fine for use —as are rooflights, with some limits, obviously.

In general, any material can be used on a building that is at least 20 m away from the relevant boundary.

However, the designation of the roof increases to the maximum of B ROOF(t4) for roofs that are 6 m or closer to the relevant boundary.

So — there you have it:

Approved Document B – Requirements B3 and B4. ✅Internal fire spread (structure), and external fire spread — covered! Under the Belt 🎵 [Music] 🎵

Let’s recap.

Requirement B3 focuses on maintaining a building’s stability during a fire event, emphasizing measures to inhibit fire spread both within the building and between separate structures.It mandates:

• Subdivision of buildings with fire-resistant construction, and

• Installation of automatic fire suppression systems, where necessary.

In addition, it requires the inhibition of unseen fire and smoke spread within concealed spaces.

Section 5 deals with internal fire spread in dwelling houses, stipulating protection times for structural elements to ensure stability during fire incidents.New additions to existing structures — such as additional storeys — must meet specific fire resistance requirements.Also, the compartmentation plan plays a crucial role in preventing fire spread between separate houses and between houses and garages.Garage design must prevent the transfer of fuel to the house — ensuring the door slopes away and has a raised threshold.

Section 6 addresses loadbearing elements of structure in flats, emphasizing the importance of structural integrity and division, particularly regarding compartment walls.

Section 7 underscores the necessity of fire-rated construction in flats, extending to refuse chutes and areas of special fire hazards.Sprinkler systems are mandated for flats with top storeys above a certain height.

Section 8 delves into cavity protection in flats, emphasizing the need to divide and close cavities to prevent fire spread.

Section 9 focuses on protecting openings and fire-stopping, to inhibit fire and smoke spread through imperfections in compartment construction and due to perforations.It highlights the need for proper pipe penetrations and ventilation systems.

Requirement B4 addresses external fire spread, emphasizing the need for external walls and roofs to resist fire spread between buildings.It outlines criteria for materials used in wall construction, including provisions for external balconies.

Section 10 discusses resisting fire spread over external walls, emphasizing material performance criteria and the exclusion of cavity walls from certain regulations.

Section 11 tackles fire spread from one building to another, considering:

• Building use,

• Compartmentalization, and

• Suppression systems.

It provides guidance for determining unprotected areas in walls, based on distance to the relevant boundary.

Section 12 focuses on resisting fire spread over roof coverings, emphasizing compliance with European directives and specific requirements based on distance to relevant boundaries.

In summary:These regulations aim to ensure:

• Structural integrity, and

• Inhibition of fire spread within and between buildings.

They provide detailed guidance on:

• Fire-resistant construction,

• Materials,

• Compartmentation, and

• Protection measures to mitigate the impact of fire incidents.

Compliance with these regulations is essential to enhance overall fire safety in buildings.

In the next episode, I will cover Requirement B5: Access and Facilities for the Fire Service,and see what’s hiding in the Appendices A to G. 🎵 [Music] 🎵

Now, a few words from me:

I hope you enjoyed this episode and that the considerations of fire safety Requirements B3 and B4 are making more sense to you now.

If you have any questions, reach out to me on LinkedIn or send me an email — I’m more than happy to help you out.

At Bytnar, we deal with planning, designing, and managing your projects.We are always glad to facilitate a free initial consultation to steer you in the right direction.

Visit www.bytnar.co.uk for a piece of non-obligatory advice.

At Bytnar, we help our clients design and execute their dream homes or investments.If your building is falling apart, we can also help investigate the reasons behind it, and provide you with an appropriate:

• Strategy,

• Design, and

• Specifications for the repair.

Thank you again for listening!Please voice your opinions — I’m waiting for you on LinkedIn, and I want to hear from you.

Remember:Bytnar designs the world around you.

Toodloo! 👋

---

Piotr Bytnar BEng (Hons) MSc CEng MIStructE

Chartered Structural Engineer who deals with the Architecture of buildings. His Master's Studies led him to an in-depth understanding of risk and contract arrangements in construction as well as specialist knowledge in soil mechanics.

He and his team help homeowners and property developers to design and deliver construction projects reducing waste in time and the cost. He believes that the construction project is an iterative process that can be well managed and it is best managed if all the aspects of the project definition and management are dealt with in-house or coordinated by one organisation. His team works to all stages of RIBA and ISTRUCTE stages of work and enables contractors to deliver projects on-site providing risk evaluations, methodologies for execution of works and temporary works designs.

---