New C/AS2 vs C/AS1 SH scope definition

We are currently drafting a fire report for a 3 level building, 2 level major dwelling above a single level minor dwelling (both with independent escape routes) and noted a change to the SH scope wording in the new C/AS2 which seems to clash with the still current C/AS1. The new C/AS2 has added wording “…no more than two levels …” in table 1.1, whereas C/AS1 just limits you to no more than one unit above another. Fig 1.1. in C/AS1 IMPLIES that you can’t have more than two levels?

Perhaps something else that MBIE needs to clarify in the next amendments? In the meantime we’re left wondering whether this house is risk group SH or SM (with the added requirement of a type 5 since the travel distances off the intermediate floor are greater than 20m when applying the 1.5 multiplier!).

If we elect to show compliance with C/AS1 someone could point out that MBIE guidance states it is clearly risk group SM so I guess we’re stuck with SM for now.

I would have thought when using C/AS1, you should use the scope definition within that document (C/AS1) as it should still stand until amended or withdrawn.

For more clarification on floors vs units, the old commentary to C/AS1 - C/AS7 states in part:

"Risk group SH Risk group SH applies to detached houses and to buildings containing
a number of separate residential units, provided there is no more than one unit above another.
Therefore, the Acceptable Solution covers the fire safety requirements for a row of townhouses
and maisonettes as well as two-storey apartment blocks.
While each household unit may have more than one floor, it must still have its own independent
escape route. If the building provides a shared escape route, then C/AS2 will apply."

As such, I would suggest this falls squarely within SH, but perhaps a discussion with Council to get their buy in to the logic.

Hi Rob
C/AS2/4.13.8 removes all the ‘intermediate floor’ requirements therefore although Table 1.1 refers to two units above each other 4.13.8 permits intermediate floors within each unit. You could likely have two 3-level units above each other provided the escape length works out.
In fact Table 1.1 even defines what it means by virtue of the bracketed clause meaning that a level is actually a household unit within which there may also be 1 or 2 intermediate floors.

Thanks Luke, I agree, definitely worth a chat with Council. The excerpt from the old commentary is very helpful, confirms my understanding of how many levels you were permitted under C/AS1.

Hi Rob,

Please find attached Auckland Council Advice Memo - it makes for some interesting reading however I do not agree.

Paragraph 1.1.1 (b) of C/AS1 states that multi-unit dwellings are permitted with no more than one unit above another and where each unit has an escape route independent of all other units .

In household units it is permissible to have an upper level (bedrooms etc…)

Therefore in my opinion, a single multi-unit dwelling can have two levels and another multi-unit dwelling above can also have two levels as long as each multi-unit has independent means of escape.

Your thoughts?

Only C/AS2 has specific text in para 4.6.11 allowing an individual household unit to have upper floors. There is no corresponding paragraph in C/AS1 allowing household units to have upper floors, but is commonly thought to. But if C/AS1 was only for household units with a single floor level then all multi-storey single household unit detached dwellings in NZ would be Risk Group SM and not Risk Group SH.

What could be a catch is the independence of escape routes. An escape route terminates at a Final Exit. A Final Exit is the point that gives direct access to a Safe Place. A Safe Place is an open space where people may safely disperse after escape the affects of fire. Therefore; if external escape routes from separate household units need to converge before reaching a Safe Place (like down a narrow walkway or driveway), then the escape routes wont meet the definition of Risk Group SH, even though the building will in all other respects.

Also note that there is a limit of application on fire fighter access to multi-unit dwellings. NZBC C5.3 to C5.8 is limited from applying to detached dwellings and WITHIN household units of multi-unit dwellings. So, although C/AS1 and C/AS2 require a 75m fire hose run from the hardstanding location, the Building Code doesnt have any such performance criteria for fire hose coverage for Dwellings and within household units of Multi-Unit Dwellings.

Even better, since a Final Exit is not located immediately outside a building, the external escape route between the building and the Safe Place would meet the definition of an Exitway as it is protected by distance or fire rated construction. NZBC F6 and F6/AS1 require Exitways to be provided with emergency lighting. There is a limit of application in NZBC F6.3.1 that exempts F6 from applying WITHIN household units of Mult-Unit Dwellings, but there is no such exemption outside of household units of Multi-Unit Dwellings. Therefore; this would imply all external escape routes from Multi-Unit dwellings require emergency lighting from unit doors, over footpaths, carparks and driveways until they reach a Safe Place.

The best snag is the measurement of escape route lengths on upper floors in Risk Group SM compared to Risk Group SH. There is no exemption from the upper floors in household units in Risk Group SM from being treated as intermediate floors, therefore the escape route length on upper floors within the same firecell in Risk Group SM is factored by 1.5. This makes achieving a complying escape route length in Risk Group SM very difficult without a NZS4512 or NZS4514 smoke detecting fire alarm or NZS4515 sprinkler system. Risk Group SH is of course silent on how to measure escape route lengths on upper floors, as Risk Group SH is silent about even having upper floors.

one unit above the other, with or without an intermediate floor in the fire cell (household unit) and compliant egress etc is SH. The wording of the AS is clear. It simply says Units NOT floors.

Thanks for posting Paramjit. My take on this stance by Auckland Council is that they acknowledge a 3 level townhouse has a much different risk profile to a 2 storey dwelling above a single storey minor dwelling. If you have no control over what the occupants of the minor dwelling are doing, and maybe even don’t know them all that well (thinking of a rental property), when they start a fire and you’re on the 2nd floor of the main dwelling above with only type 1 domestic smokes to tell you of the fire in the minor dwelling, it may be too late for you (especially considering there is only a 30 minute fire rating between you). ALTHOUGH I agree that the scope definition in C/AS1 theoretically allows for a 3 storey dwelling above another 3 storey dwelling… see post by Luke de Schot above referencing the old commentary.

From the AC memo, MBIE are fairly clear and explicit on their interpretation so unfortunately I believe you’re stuck with C/AS2 for this building.

I don’t see your logic Rob. The wording is one unit above the other and each can have 2 floors…

“small multi unit dwellings that have no more than 2 floors (one household unit above another)”

this reads as the unit having no more than 2 floors and two units, one unit above the other…

I agree with Alan.
C/AS1 states in para 1.1.1b, ‘Multi-unit dwellings with no more than one unit above another…’
The definition of a Household Unit is not restricted by the number of floor levels of that residence.
There is nothing in C/AS1 restricting a household unit to no more than 1 level high when stacked one unit above another.

What is stated in C/AS2 Table 1.1 about Risk Group SH adds to the C/AS1 definition but is outside the scope of C/AS2, therefore only C/AS1 can be used to define what a “Low Rise Multi-Unit Dwelling” is. But the term “Low Rise Multi-Unit Dwelling” doesnt exist in C/AS1.

Good practice… Risk SM would require a greater fire rating between firecells, more protection of other property, and structural stability during a fire… values which should be adopted if the building is say, 4 levels high comprising of 2 units stacked vertically and only a few metres from relevant boundaries.

What would be nice is if the definitions of Risk Group SH were consistence across the Acceptable Solutions, but they are not. Hopefully the rewrite fixes this.

We may have to agree to disagree Alan, my reading of that sentence in table 1.1 of C/AS2 is the building overall, or “multi-unit dwelling”, is two floors high maximum, whereas the “household unit” refers to the single level unit on each level… which aligns with the new Auckland Council guidance. My original post referred to the contradictory definition in C/AS1 where you COULD have a two or even three level household unit above another in a 4, 5 or even 6 (?!) floor building/multi-unit dwelling (C/AS1 1.1.1 b) “no more than one unit above another”). The whole point is, as I believe Grant is pointing out below, if you have two, two level household units in a four level building, the safer approach would be to adopt C/AS2 (if you’re outside Auckland Council’s jurisdiction and have the option of making that choice).

Hi Rob,
Since there are no specific height limitations in C/AS1, and since Safepaths don’t exist in C/AS1, the maximum height of an apartment in C/AS1 is limited by the length of the open path escape route from the top floor of the apartment to the exit door leading to the External Escape Route.

Using C/AS1, a two or three level apartment above another apartment will likely either require a NZS4515 Type 6 and NZS4512 Type 4 system, otherwise C/AS2 would be required so that Safepaths could be included in the design, or the design be based on something other than an Acceptable Solution.

totally agree Grant, would definitely struggle to keep within 25m for a three level unit, even 35m is a struggle for most three level places I’ve done fire reports for.

you are assuming that the guidance by AC is correct in your reply… :slight_smile:

In accordance with “Commentary for Acceptable Solutions C/AS1-C/AS7” under Multi-Unit Dwellings, it states that you are permitted to have no more than one unit above another (regardless of how many floors are within each unit) as long as each unit has independent means of escape and can be designed to C/AS1.

that is encouraging. I was convinced I was correct, but then started to doubt myself. Good to see the MBIE Commentary which clarifies the Acceptable Solution allows the multi levels in a single unit. That agrees with all of the advice I have ever given out…Phew…

As I put together table 1.1 C/AS2 I may be able to provide some background. The scope in C/AS2 relative to C/AS1 limits multi unit dwellings to 2 levels. The thinking at the time was that when escape heights reach 4m C/AS2 imposes requirements for external spread of fire where C/AS1 doesn’t. Back then C/AS1 was going to get a facelift and whilst the scope in table 1.1 C/AS1 was known to be different from that in C/AS2 I It was to supposed to be aligned relatively soon. Then we all left MBIE, someone else did something different and so these tables now are different. This said C/AS1 is a stand alone document and it implies that you can have multi level household units only limited by travel distances. Over to the fire engineers to use their engineering judgement to design buildings that are safe enough.

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cheers Mike, great to have some input ‘from the horse’s mouth’.

Regarding your final sentence, this has been my point for the whole discussion. Personally, I agree with Council’s guidance/requirement and that you should probably design a 2 + 1 level (definitely a 2 + 2 level and above) to C/AS2.

After I have “judged” I also step back and check against the Building Code to see if it can still comply, bearing in mind that minimum code is not necessarily good design.