UK Building Regulations - Intermediate Floors / Loft Conversion Floors

Seeking the experience of any UK professionals who have recently submitted applications to Local Authority Building Control that involve either new intermediate floors or loft conversion floors.

My historic experience with my local Building Control for loft conversions was that they would pass the following:

A new structural loft floor that would typically be 125/150mm joists plus a ply floor deck screwed and glued to the new joists. The existing first floor ceiling would either be underdrawn with battens and fire resistant plasterboard or from above, chicken wire draped over the new structural joists into the space between and mineral wool laid over the chicken wire.

This would provide the required 30 minutes floor separation to the new loft level.

Never had any issues until early 2020 when the response from Building Control to a loft conversion application was that because the new loft floor joists were less that the 195mm as stated by British Gypsum, I would need to justify the proposed specification.

I queried this a number of times but didn’t get a reply.

By this time the build was underway and as far as I know got a completion certificate.

I was never party to what might have been decided on site between the inspector and the contractor.

Fast forward to early 2021 and on another loft project where the existing first floor ceiling was to be removed and effectively a new intermediate floor put in.

Because of the short span the engineer was able to specify 100mm joists and so as a precaution I entered into a dialog with Building Control and asked if they would accept 100mm joists with 15mm fire resistant plasterboard fixed to the underside with 100mm mineral wool between.

The general published guidance for intermediate floors appears to suggest any structurally sound joists plus plasterboard, mineral wool and floor deck would provide the required 30 minutes fire resistance.

Prior to an application my local Building Control confirmed this.

This particular project has just obtained conditional approval and one of the conditions is that the 100mm joists are less than the 195mm as stated by British Gypsum and therefore not compliant and I need to provide test data to show that 100mm joists, etc. will provide 30 minutes fire resistance.

So far the comment from the inspector is to confirm that British Gypsum have no test data for my proposed floor construction and that he won’t comment on what I was told earlier this year by his colleague.

I have spoken with an independent building inspector and from what I told him over the phone, he was puzzled by my local Building Control’s stance.

Could you perhaps get round the joist depth issue by specifying a layer of Fireline board between the joists and the plasterboard?

I think the despairing thing about this Dennis, is that prior to 2020 a general intermediate floor / loft floor specification that differed somewhat from the examples shown in the British Gypsum White Book, that was widely accepted as a compliant solution was accepted by my local Building Control.

And now it seems that they are deferring entirely to British Gypsum.

I haven’t put any other suggestions to them yet as it seems the problem they have is that any joist depth less than the magic British Gypsum 195mm will not be compliant regardless of the number of fire resistant linings - at least unless I can show them test data!

The actual specification I’m proposing is 15mm Fireline fixed to the u/s of 100mm joists with 18mm floor ply deck screwed and glued to the joists and 100mm mineral wool inbetween the joists.

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Paul, I don’t think I have a solution but I do have some observations.

I have always assumed that the fireproofing of something like a ceiling is derived entirely from the surface finish. I guess a fire would eventually break through that by intensity of heat alone and promote combustion in the timbers above. Whether that would happen inside half an hour, I am not sure. Maybe it depends on how hot the fire gets.

My guess is that the word has gone around local authority BC departments about this, maybe as a follow on from Grenfell. BCOs must all be very nervous about fire after that.

I never use local authority BC myself, mainly because I have built up relationships with private firms. I find them much more proactive. In a situation like this, they might actually come up with a solution.

That’s interesting about Grenfell Simon.

I never considered that it’s aftermath could possibly find it’s way into residential loft conversions.

Afterall there were some very specific things that were very wrong that led to that tragedy.

Ah, but isn’t it all down to what I call LUC (the Law of Unintended Consequences)? Or like the old chestnut of a butterfly wing flapping deep in the Amazonian jungle. You never know what the ramifications may be.

Seems a bit of a hammer to crack a nut but would coating the joists in intumescent paint before cladding solve it?

Building Control are insisting on test data and aren’t accepting any variation on a joist less than 195mm.

My understanding was also the length of fixing and what it was fixed into is critical…the screw gets very hot very quickly , chars the wood and drops out …letting the fireproof finish fall… I’ve had to use 70mm screws before now…! A real pain !

That’s interesting Dan - that is a pain!

When I did loft conversions the length of the screw was never mentioned by Building Control.

Does your code have any sort of interpretation manual? Do they have any provisions to allow component assembly justification? Here in the US we used to be able to justify up to 1 hour assemblies based upon the fire resistance of each component, but that was with the UBC.

Codes are definitely NOT getting easier everywhere.

Well, that’s pretty much what I feel the general guidance for loft conversion floors was Nick - fix 12.5mm plasterboard to the underside of the ceiling and skim and that’s your 30 minute protection - I done many loft conversions prior to 2020 like that.

As Simon says, it would seem Grenfell has made local authority Building Control very nervous when it comes to fire.

(I live in the Netherlands)
What are the exact dimensions? A195x38 beam would basically have the same area as a 100x70. One just has to know how much time it takes before it reaches the dimensions where it would collaps.

The gypsum should take care of the first 30 minutes, in our region, fire-compartments below 15m height must be able to resist 30 min , above that height or when it separates another compartment (neighbors) 60 min.

For sure, it’s easier to respond with the standard solution instead of interpreting the actual spirit of these regulations ( Does someone has a chance of getting out of there within a reasonable time, including waking up by the alarm, getting dressed, easy route, etc etc)
It’s all a matter of taking responsibility, I guess.

The form factor is important, as the fire progresses in timber at about 0.8 mm per minute, so a 38 mm piece of wood is completely burnt in less than 24 minutes while 70 mm takes almost 44 minutes.

Wood also starts to lose its strength when heated to over 100°C. The interior of a burning beam heats to that temperature in parts that are nearer than 15 mm of the burning surface.

In our parts, whatever the timber size, it would suffice to put two layers of gypboard under it.

Hi PM,
I’ve done many loft conversions in the past (UK) & the joists/upgraded joists have typically been required to be a min of 38mm th’k (more in practice) but I’ve not come across a timber joist minimum height requirement before. I’ll have to look in to this myself.

The link below from BRITISH GYPSUM provides various way’s of acheiving this for 30min FR, however, I can’t see where the joist height is specific to a noted fire resistance.

Hey Clayton,

Yes, seen that extract from the Whitebook but it’s only available at the Irish domain BG site even though it refers to British Standards.

UK BG have effectively told my local authority Building Control that they only have test data for 195mm joists.

The UK BG site is %@$&*!#.

My local authority Building Control is not budging at the moment, however, a private company I approached has confirmed that they will accept any structurally justified joist with the fire resistant lining installed below.

Hi PM,
I did note after I sent the pdf link it wasn’t strictly for UK (Ireland) so had a look on the BG website, which has been updated since my last visit, now I can’t find anything !

British Gypsum White Book C06 LOAD BEARING TIMBER FLOORS info must be burried somewhere ?

Anyway, I looked at my old 2019 records for the same & they do state 195x38mm s.w. joist at 600mm ctr’s min for most floor variations. I didn’t think much of it in terms of joist heights, as most literature, Building Reg guides only state joist width minimums.

I mostly use private Building Control myself, STROMA BUILDING CONTROL are very helpful & seem to adopt the traditional common sense width minimum for fire, as joist heights can vary subject to the span. All as part of the TRADA joist span tables, so very odd indeed your LABC officer doesn’t recognise this common sense direction which has been around for years !