I don't know if it's just me not looking in the right places - but whenever I get into specifying timber frame buildups for fire resistance (which generally means including plasterboard or calcium silicate board layers), it all gets a little sketchy.

For example, if you look at the British Gypsum 'white book' it has many variants of buildups for steel frame and masonry, and does give some systems for timber framed partitions but if you are looking at a load bearing, timber framed external wall, there is nothing at all. If you speak to their technical advice, they point you towards TRADA.

The TRADA timber frame book talks about 30 mins or 60 mins resistance being "normally obtained" by 1 or 2 layers of plasterboard but is not more specific than that.

You can find various info sheets that say something similar, but there's generally always a caveat that the actual fire resistance depends on the whole system - what it's fixed to and how. And they'll say that the performance of boards can vary by product - therefore, check with the manufacturer.

The problem is though, checking with the manufacturer reveals that they never actually give a resistance for the board itself - it is determined by the "system", ie. specific buildup. And there may be no "system" that matches what you want.

In practice, what generally seems to happen is that building inspectors simply accept that principle of 1 layer = 30 mins, 2 layers = 60 mins. But it feels unsatisfactory to me.

Maybe this is the sort of thing that most people just don't worry about?

Posted By: snyggapain our timber frame, regular 2 storey build the only fire rated plasterboard the BC wanted was to box around the two timber posts that supported a lintel that spanned the middle of the kitchen / diner downstairs (small room, approx 3.5 x 7 m)

The rest of it was timber studwork, ply sheathing outside, insulation in the void and over, and regular plasterboard on the inside. no calcs needed or eyebrows raised. your mileage may vary

The plasterboard will give you the necessary 30 mins resistance from inside. As I said, in practice generally simply accepted by BCOs without question. But if you actually had to prove that the specific plasterboard used, in that context, does really provide 30 mins resistance, could you do that? Is there a product datasheet or a test certificate or anything that actually says this?

Posted By: djhThere's a document athttp://www.structuraltimber.co.uk/assets/InformationCentre/eb7.pdf" rel="nofollow" >http://www.structuraltimber.co.uk/assets/InformationCentre/eb7.pdfcalled "Fire safety in timber buildings" by the Structural Timber Association that explains the situation and refers to all the relevant standards. In particular BS EN 520 seems to be relevant, and whatever plasterboard manufacturers literature say in relation to it.

Note that as it also says, fire resistance ratings can be achieved for timber components even wehn exposed simply by adding extra thickness. Timber subject to fire chars and the char provides fire protection.

Yes, that's one of the documents I've looked at.

But as I said, it says "in general" but then with caveats that "plasterboard manufacturer's literature should be consulted". And the plasterboard manufacturers will not commit to anything as general as "1 layer of this provides X mins resistance".

Unfortunately I don't have easy access to BS EN 520. Maybe in there it gives some general recommendations that would satisfy scenarios where a build-up doesn't quite match anything certified or tested by a manufacturer.

Euroclass A1 and A2 are classifications relating to reaction rather than resistance (ie, combustibility rather than how long it will resist the fire compromising whatever is being protected).

So the Euroclass doesn't tell you anything about how long a particular board will protect a structure it's fixed to.

Generally it's seen more as the architect's reponsibility to oversee aspects relating to fire safety, via consultation with whoever is undertaking building control inspection.

Posted By: djh
I didn't claim it did. But it does say plasterboard is classified without further test and elsewhere tells what the various classifications mean in terms of resistance, I think.

I'd be happy to be proven wrong - but I'm fairly sure it doesn't. Wherever they claim X minutes fire resistance, it's for a "system". That system might be an internal partition with metal studs, it might be an internal partition with timber studs, or it might be encasement for a structural steel beam. But it'll always be for a "system", not for the boards themselves.

Posted By: djhHmm, I'm pretty sure it was the engineer who sized our structural timbers. But even if it is the architect who's responsible then it's his (or her) to assess the fire resistance.

Well the structural engineer would size structural timbers yes. If they are timbers which are to be exposed, and which require fire protection, and the strategy for that fire protection involves char rates and so on, then yes I think the engineer would be involved.

But where the structure is behind the fire resistance layer - whether that's roof joists above a plasterboard ceiling, or a loadbearing timber stud wall with plasterboard on it, then I don't think char rates really come into the question - you are relying on the plasterboard stopping the timber from even starting down the path of charring, within the specified time of resistance. In those circumstances, it would normally be the architect specifying wall and roof buildups, which are required to provide the necessary fire resistance.

Posted By: lineweightBut where the structure is behind the fire resistance layer - whether that's roof joists above a plasterboard ceiling, or a loadbearing timber stud wall with plasterboard on it, then I don't think char rates really come into the question - you are relying on the plasterboard stopping the timber from even starting down the path of charring, within the specified time of resistance.

Sounds reasonable to me. Basically it assumes that the structural components provide no additional protection over that provided by the fire resistance layer.

Posted By: djhThere's a document athttp://www.structuraltimber.co.uk/assets/InformationCentre/eb7.pdf" rel="nofollow" >http://www.structuraltimber.co.uk/assets/InformationCentre/eb7.pdfcalled "Fire safety in timber buildings" by the Structural Timber Association that explains the situation and refers to all the relevant standards.

I think the following paragraphs from that document sum up the difficulty of trying to look at it in more detail or from the point of view of the whole structure contributing..

Fire resistance of assemblies

Furnace tests are used to determine the duration of time that a structural assembly retains its stability, insulation and integrity (Figure 3). The test standards for fire resistance are the BS EN 1365 series. It should be noted that the furnace tests are comparative and do not relate to survival time in a structure or represent the real behaviour of a fire. The loaded fire tests take the design load or percentage of the design load for the structure and this is declared in the fire report.

Calculations can be used as presented in BS EN 1995-1-2:2004 Section 5 Design procedures for wall and fl oor assemblies, to predict fire resistance, but the methods are under review and limited to a maximum of 60 minutes. The calculations are dependent on material information and position of materials in the assembly. The calculation approach can be complex and requires both knowledge of the behaviour of the structural system at elevated temperatures and the importance of the structure - taking account of the consequences of failure. However, it is a common tool for product assessments and for determining the fire resistance of assemblies that have undergone similar (but not identical) tests.

Posted By: djhI'm very surprised if the regs aren't clear as to whose responsibility it is. And worried! Perhaps that's part of the problems exposed by Grenfell et al,if the lines of responsibility aren't clear?

Very broadly, as far as I understand the legal aspect of things (which is not fully) it's ultimately the building owner's responsibility to make sure any work they carry out complies with building regs.

Of course in reality they will often employ someone to design and someone to build that work. In that case it must be implicit that those people excercise a reasonable amount of care (whatever exactly that is) in what they do, that they do not design or build something that contravenes the building regs.

If there is a 'lead designer' then it's obviously part of their job to make sure that building regs aren't contravened. The 'lead designer' may or may not be an 'architect'.

The lead designer will then rely to some extent on input either from their LA building control or an Approved Inspector, in deciding what complies.

I think that where there may be some lack of clarity in where the line of responsibility lies, might be around that - for example, when there is something on a drawing that doesn't comply, but the BC inspector doesn't spot it or raise it. Or if the BC inspector approves a design that doesn't comply, but it's arguable that it's something the designer knew didn't properly comply, or should have had doubts about. And to what extent does the BC have discretion and what authority does it have?

Coming back to what this thread is about - a situation where technical literature seems unable to give a definitive answer, is the BC inspector saying "yes that is acceptable" enough - or does this leave a grey area where, if it's subsequently deemed not to be acceptable, is the responsibility shared with the designer?

I think that some of these questions are indeed amongst problems highlighted by Grenfell and I'm certainly interested to see what the report eventually says.

Posted By: lineweightI think that where there may be some lack of clarity in where the line of responsibility lies, might be around that - for example, when there is something on a drawing that doesn't comply, but the BC inspector doesn't spot it or raise it. Or if the BC inspector approves a design that doesn't comply, but it's arguable that it's something the designer knew didn't properly comply, or should have had doubts about. And to what extent does the BC have discretion and what authority does it have?

As I remember, my contract with the building inspector contained something about them not being liable for errors (or somesuch). IMHO all such clauses should be ineffective. If somebody works on something then they should take responsibility for it.

If neither the technical literature, or the technical enquiries department of the country's biggest manufacturer of plasterboard is able to give me an answer I'm not sure whose CPD course is going to.

RIBA? LABC? Or even the plasterboard firm. If the trainers don't know the answer then they should be able to get it! See my first point.

Posted By: WillInAberdeenAIUI, this unclarity over responsibilities was the reason why the CDM Regulations were introduced. They place the overall legal responsibility firmly with the Client (including for domestic buildings).

The Client would normally transfer the responsibility to the Principal Designer, if they have appointed one, who must agree in writing if they're going to take it on. If you haven't appointed a Principal Designer with this written agreement, then the bottom line responsibility for the safety of the design, stays with the Client.

During the on-site phase, there's a legal presumption that, if the Client has appointed a Principal Contractor, the responsibility for on-site safety transfers to them, written or not.

http://www.hse.gov.uk/pubns/indg411.htm" rel="nofollow" >http://www.hse.gov.uk/pubns/indg411.htm

I think that's right as far as health & safety stuff is concerned - building regulations are a separate issue though.

Posted By: cjardGet a proper detection system sorted, get escape route sorted and get out of a burning building;

The key bit though is that a mister/sprinkler system should mean it never gets to the burning building stage.

If the kit is available and reasonably priced, it will certainly be part of the spec for the next house.

BTW anyone else picked up on these smoke alarms that use voice rather than just a high pitched noise to alert you? Kids especially have been found not to be woken by the noise of smoke alarms.

Posted By: cjardMeh, it doesn't really matter whether the fire takes 30 minutes or 60 minutes to burn through the plasterboard and consume theframe of the house; you aren't going to be sitting in it at T+25 minutes going "I hope the fire service get here in the next 5 minutes before that plasterboard burns through or I'm going be wearing this house" - you'll have died approximately 20 minutes earlier from smoke inhalation

Get a proper detection system sorted, get escape route sorted and get out of a burning building; worrying about how to make it last until the fire can be quenched only does the insurance company a favour. Build it to regs and they'll rebuild it regardless

There are situations where the 30 or 60 mins resistance could be highly important, for example if people are trapped in a part of a building waiting for rescue. Whether the resistance is protecting structure, preserving compartmentation, or both.

Of course there should always be a safe escape route, but things go wrong, and it seems wise to have measures in place that can increase chances of survival if for some reason an escape route becomes unusable.

Posted By: lineweightI don't think I'd feel safe in a building where protection is only provided by an active rather than passive system.

I'm not suggesting building out of tissue paper! I'm just pointing out that with a mister or sprinkler, I suggest you will never get to the point of a burning building that you will be trapped in. As ever, security/safety is provided by layers of protection - you start with something that does not easily burn, you apply alarms to alert you and then add to that a suppression system.

Posted By: RexWarmcel gradually charred but never caught alight; likewise Warmcell, gradually charred but never even started to smoulder.

Is there a typo there?