Unprotected areas and cladding materials


(Geoff Merryweather) #1

Following on from a discussion today with another engineer regarding “unprotected areas” and what is common practice. What is the opinion of SFPE members?

Unprotected area In relation to an external wall of a building, means:

a) Any part of the external wall which is not fire rated or has less than the required FRR, and

b) Any part of the external wall which has combustible material more than 1.0 mm thick attached or applied to its external face, whether for cladding or any other purpose.
Unprotected areas include non-fire rated windows, doors, or other openings, and non-fire rated external wall construction.

The “non-combustible” test is pretty onerous, being put in a red hot oven for 20(?) minutes and can’t burn. Note that fibre cement boards don’t meet this requirement due to the wood fibre filler gives a small puff of heat as it burns. We never had this problem in the good old days of asbestos…

Consider a typical school or maybe a small apartment block <10m, more than 1m from the boundary with no other reasons to limit the cladding. Timber weather boards in schools are common and as above, even fibre cement doesn’t meet “unprotected area” if you want to be pedantic.

There are a couple of situations of interest – horizontal fire spread to a boundary, and vertical fire spread protected with spandrels

The requirements for horizontal fire spread

5.2.2 Protection shall be achieved by using one or more of the following approaches: …

b) Distance separation (refer to Paragraph 5.5), or
c) Limiting unprotected areas in external walls (refer to Paragraph 5.5…

From this, does this mean that you can’t have your wall fire lined inside (1 way fire rating) with weatherboard or even fibre cement board outside? This is more onerous than the cladding requirements as “A” and B” type or NFPA 285 tested cladding doesn’t comply with non-combustible limits. You basically couldn’t use anything other than metal or concrete for a wall that is in any way fire rated.

I can see an issue of the wall is on fire, that the radiation across the boundary would be exceeded. My unscientific opinion is that it is a technical non compliance that is not an issue in practice for e.g. a typical school and maybe needs some clarification or limits of application but I would be interested in other opinions.

Spandrels have a different objective, to stop fire from the unit below entering the unit above.

5.7.11 If the conditions described in Paragraph 5.7.10 occur, unprotected areas (see Figure 5.7) in the external walls of the Firecells shall be separated by no less than:
a) 1500 mm where any parts of the unprotected areas are vertically aligned above one another, or b) 900 mm where the unprotected areas on one level are horizontally offset from those on the other level.

A spandrel that catches fire probably isn’t going to be effective at stopping flames breaking through the window above. A “low combustibility” cladding such as fibre cement would surely be ok however. Perhaps this need clarification as well.


(Frank Micallef) #2

Hi Geoff,
Fibrous cement sheets have historically been accepted based on the ‘comments’ in the acceptable solutions,
The comments were present in C/AS1:7.11.3 (pre 2013); and in the still current (for a month or two) acceptable solutions C/AS2 - 6: section 5.8.4. All comments have been removed from the new C/AS2: 2019.

Even if bare boards are non combustible the paints will not be.
Further, I’m not sure if anyone ever checks or rechecks the 1mm limitation on paint finish thickness, or what happens when buildings are repainted?
Prohibiting repainting (maintenance) does not seem a good option?
Similar restrictions apply to to paint finish thickness for internal finishes and ‘group number’ compliance.

As you mentioned its being pedantic and i’d suggest not practical to monitor or enforce. The thickness of paints (internally and externally) will increase as buildings are repainted.
Hopefully some common sense will prevail.