Is it possible to expose ceiling beams while meeting fire regulations?

[MattD]

I'm designing a new build dwelling. I understand that Building Regs (part B) makes it difficult to have exposed beams in ceilings (i.e. exposing the joists of the floor above) due to the need to have 30 minutes' fire resistance. But is it absolutely impossible to meet the regs with exposed timber beams? I think this is sometimes also referred to as 'ceiling pockets'.

 

How do you estimate whether or not a particular design (material + thickness) has 30 mins fire resistance? I assume the final calculation would need to be done by a structural engineer, but is there a way to get a rough estimate of whether or not something will pass before we send it off to the SE?

 

Furthermore, we are in Wales and this requires us to have a sprinkler system, including a sprinkler in EVERY ceiling pocket, unless the "ceiling pockets are constructed of materials conforming to BS EN 13501-1:2018, Class A1, A2-s3, d2 and B-s3, d2." Does anyone know where to determine whether a particular type of timber falls into one of these classes? This website gives some clues, but doesn't actually provide guidance on what materials would meet the standards.

[saveasteading]

1 hour ago, MattD said:

there a way to get a rough estimate of whether or not something will pass before we send it off to the SE?

Yes, you ask on here.

 

If it is supporting the structure then it will need protection. The one that doesn't conceal the timber is intumescent varnish. Look it up on line. Envirograf tend to explain well.

[Mr Punter]

They can be exposed and left unprotected subject to the section size and loading.  It is possible to calculate the char rate on a timber beam.  You can also apply clear coatings to increase fire resistance if required.

[saveasteading]

1 hour ago, Mr Punter said:

left unprotected subject to the section size and loading.  

Yes but is that likely for a domestic sized,  fully exposed joist?

[ETC]

3 hours ago, MattD said:

I'm designing a new build dwelling. I understand that Building Regs (part B) makes it difficult to have exposed beams in ceilings (i.e. exposing the joists of the floor above) due to the need to have 30 minutes' fire resistance. But is it absolutely impossible to meet the regs with exposed timber beams? I think this is sometimes also referred to as 'ceiling pockets'.

 

How do you estimate whether or not a particular design (material + thickness) has 30 mins fire resistance? I assume the final calculation would need to be done by a structural engineer, but is there a way to get a rough estimate of whether or not something will pass before we send it off to the SE?

 

Furthermore, we are in Wales and this requires us to have a sprinkler system, including a sprinkler in EVERY ceiling pocket, unless the "ceiling pockets are constructed of materials conforming to BS EN 13501-1:2018, Class A1, A2-s3, d2 and B-s3, d2." Does anyone know where to determine whether a particular type of timber falls into one of these classes? This website gives some clues, but doesn't actually provide guidance on what materials would meet the standards.

The fire resistance is obtained by the construction of the complete floor in a typical domestic dwelling. If you want to expose the floor joists you will need to speak to British Gypsum to get a floor build up that will give you the required FR. I suspect you wont be able to.

Edited October 16, 2023 by ETC

[Mr Punter]

1 hour ago, saveasteading said:

Yes but is that likely for a domestic sized,  fully exposed joist?

 

I have left them exposed in a conversion, but they were large beams.  They need to be meaty for the char rates to count.  https://www.designingbuildings.co.uk/wiki/Sacrificial_timber

 

BC did not seem bothered but this was 8 years ago pre Grenfell. 

 

[MattD]

5 hours ago, Mr Punter said:

They can be exposed and left unprotected subject to the section size and loading.  It is possible to calculate the char rate on a timber beam.

 

Thanks all. We are of course free to spec the floor joists as we see fit. From my reading online, it looks as though we just need to know the charring rate for the material (softwood), which gives the amount of material that would be lost from each exposed face, per minute, for the required duration. I see varying figures online – can anyone point to an authoritative source that Building Control would refer to when making this calculation? For example, if the charring rate is 0.8mm/min then for 30 min resistance that's 0.8*30 = 24mm of sacrificial timber required for each exposed face. But if the figure is 0.65mm/min, it's only 19.5mm. I assume that you then just need to check against span tables that the reduced member size is sufficient for whatever it's supporting. Can anyone confirm if my understanding of the calculation method is correct?

 

Also, thanks for the tip on intumescent varnish. Are there 'accepted' figures for how much time these varnishes gain you on the above calculation?

Edited October 16, 2023 by MattD

[Temp]

You can't just leave regular joists exposed because it's unlikely that would meet Part E Sound transmission.

 

We opted for a beam and block first floor with oak beams below it. They look ok. Will try and post pic later.

[MattD]

6 hours ago, Temp said:

You can't just leave regular joists exposed because it's unlikely that would meet Part E Sound transmission.

 

We opted for a beam and block first floor with oak beams below it. They look ok. Will try and post pic later.

Thanks – hadn't thought to consider sound transmission yet; that might cinch it.

 

...So your oak beams are purely decorative?

[Temp]

On 17/10/2023 at 22:02, MattD said:

Thanks – hadn't thought to consider sound transmission yet; that might cinch it.

 

...So your oak beams are purely decorative?

 

Sorry for not posting sooner but this is our dining room ceiling. Hope the cobwebs don't show. The smallest beams are 75mm wide and 60mm deep.

 

Yes all decorative. The only exception being the big beam on the right which supports part of the stairs landing. The floor upstairs is beam and block.

 

 

[MattD]

Thanks all – that looks great @Temp. In our case I don't think I can justify purely decorative beams, and 'beefing up' the actual joists would mean doubling their width which isn't economical. The house we're building is a traditional timber frame anyway, so that will all be exposed (and is chunky enough to do so).

 

Given the requirement for noise reduction, fireproofing etc (not to mention that this complicates fitting of sprinklers as required in Wales), we will just box them in!

[Glenn]

I've been able to expose the beams on my small house no problem at all - size did go up a bit (per comments above) so that protection is about char rates, but not excessively - certainly not a doubling. I wanted exposed beams for a feeling of height and for character (even though they are painted) - both would have been lost if I boxed them in.

 

In terms of sound transmission it's a bit of a moot point - there's a dirty great double height void in the lunge/dining area with open mezzanine and a stairwell. It's been about reducing impact noise not airborne

 

Regards

Glenn