SH v SM Risk Groups

Single storey building with a shop/dairy/takeaway to front and living space at the rear - can this be treated as risk group SH? Or is it SM?

Hi Tim,
Where part of a building is another risk group the building/firecell cannot be SH. 1.1.2 of C/AS1 refers.


Thanks Gillian. In the now superceded C/AS2 fro SM risk group clause 2.2.10 allowed SH to be used but I see that clause has been deleted in the new C/AS2. Interestingly so has clause 2.2.11.I will proceed on the basis of SM. Cheers.

Hi Tim
I’m not sure which 2.2.11 you are referring to but I would suggest that the driving clause will be (in new C/AS2) 1.2.1(b) then steps 2 and 3.

Now I am confused. Maybe i did not ask the question correctly. The old C/AS2 (10 April 2012) had clause 2.2.10 that allowed SM units (such as the one I have) to be designed as SH. That clause is not included in the new C/AS2 (27 June 2019). So I am guessing that a SM unit is always an SM unit.

I noted (and this is unrelated to the original question) that clause 2.2.11 in the old C/AS2 (upper floor with SM always has to have smoke detection in the floor below) has gone from the new C/AS2 which I thought was strange.

Tim, The old C/AS2 referred to household units one above another in 2.2.10, so that wouldn’t apply to your dairy example.
2.2.11 (lower floors / risk groups require type 4 or 5) is still covered (although not as clearly worded in -
> Same risk group on different floors
*> 2.2.7 Where firecells containing the same *
*> risk group occur at different levels in the *
*> same building, the fire safety systems *
*> for the firecell having the most onerous *
*> requirements shall be applied to all firecells *
> of that risk group.

This project has resurfaced. I know that the shop and the accommodation will be two separate firecells. Maybe I did not make that clear earlier. CA for the shop clearly. But the accommodation firecell could be SH? Reason I am asking is because the external fire spread issues are considerably less onerous if it is SH rather than SM.

No, accommodation has to be SM, as per the replies above. SH applies to houshold units only, that are free standing, side by side or one above another and can’t include another risk group in the building. The building in this case includes both firecells, shop and accommodation.
There is no science around the 1m from the boundary/ 2m between facing buildings for domestic houses and in theory, a house fire should raze an entire block. It is a societal acceptable risk, since the alternative is ~12m between facing houses.

Geoff I really do get that, but then why does C/AS2 say that SM only applies where there are more than two household units.

I know we are getting into 'words" and not “engineering” but bloody hell it confuses the hell out of me

the requirements of paragraph 2.2.11 were moved to table 1.1

Is it known to you that two risk groups are different firecells or is it the requirement of the code ??


Oh yes of course two fire cells. Question is what risk group is the accommodation fire cell. Common sense says SM but definitions suggest SH. MBIE unable to answer.

Tim Pike

Sigma Consulting Engineers Ltd

clearly SM Tim. the building has sleeping plus another use. Interestingly it drives the alarm system requirements up to a T4 and T5…

Vaguely related to the topic heading - I see in the MBIE consultation on C/As1 and C/SA2 changes that SH Houses - C/As1) has been restricted to essentially 2*1 story flats on top of another.

) Low-rise multi-unit dwellings with no more than one household unit
above another (see Figure 1.1) and where each household unit has an
escape route independent of all other household units, and including
associated garages or carports whether or not they are part of the same
building. Where there is one household unit above another, the escape height shall be less than 4 m,

I don’t have an issue with it as such, but one to watch as developers and arcitects love to put a 2 storey flat on a single storey one.
I wonder how it will affect the likes of granny flats or separate house hold units built into the basement / ground of a (possibly existing) multi story house?

Hi Tim,
Make sure the architect has considered G15/AS1 para 2.0.1. Normally applicable to three storey apartment buildings. The requirements for compliance can affect the fire design if the architect proposes a solid waste chute, or doesnt want a chute and cant achieve the travel distance and instead wants a solid waste firecell within the building.

Hi All,

Any suggestions whether two apartments on one storey (above officefloor and retail shop) requires to have fire rated inter-tenancy separation wall (Floor area of each apartment <100m2). Thanks

The tenor of your question suggests that you should engage a fire engineer or fire designer.

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