Building in fire category areas

[Loki429]

Hi Everyone

I'm new to this forum and have been enjoying reading and looking at pictures of everyones decks over the last few days.
I'm in the process of designing a deck and pergola to go on the back of our house in Frenchs Forest, Sydney, NSW. We are close to the bush and the site is classed as a Category 2 for bush fires. I have a couple of questions I'd like to ask please...

The house is brick veneer, deck will be approx 11m x 4m, ranging from 300mm to 1100mm of the ground (slightly sloping block.) I'm planning on a covered pergola over the full length.

1. It appears the council or RFS have rules about attaching a deck to the house - stipulating that the deck must not be attached to the house. I was hoping to attach a ledger to the house and use this to support one end of the joists. Does anyone know why you cannot build the deck this way? I don't believe it is related to the house being brick veneer - it is to do with the cat 2 fire zone.

2. If I'm not allowed to attach the deck to the house, how do I build the pergola? Am I allowed to attach a beam to the house and attach the rafters to that? (The house is two storey so the pergola won't be anywhere near the fascia.)
If I do attach a beam to the house then what is the best method for attaching to brick veneer? Is it safe to dynabolt the beam to the bricks or should I be aiming to attach through the brick and to the timber frame (perhaps with long coach screws?)

Looking forward to your comments.

[ausdesign]

The Aus. standard that needs to be complied with is AS3959.
Threat levels are catagorised as medium, high & extreme with a corresponding construction method required - 'level 1, 2 or 3 construction'
You should read through the standard but in summary you'll find that a ledger can be bolted to the brick wall. [You cannot have tiombers penetrating the wall]
With level 2 -
The stumps have to be non combustable i.e. redgum or concrete. [not treated pine or cypress]
The area under has to be open - no plinths.
The decking needs to be spaced 5mm or more. & be fire retardant [merbau etc]

I'm going by memory so check it out - most councils will have a fact sheet that will give exact details.

[silentC]

In addition to the stumps - bearers and joists have to be non combustible. You can get flame-treated timber for this, or you can use one of the hardwoods from the list (get it from the RFS) or use steel, such as Duragal (that's what I did).

Info pertinent to NSW can be found here: http://www.bushfire.nsw.gov.au/dsp_c...cfm?CAT_ID=518

[silentC]

Actually, just looking at http://www.bushfire.nsw.gov.au/file_...2_C2DE0383.pdf in the Construction Standards table, it does say "no timbers shall be allowed to directly connect with the remainder of the dwelling". I think that's a bit open to interpretation, but it does seem to be saying that if your deck or pergola has timber components, they're not allowed to be connected to the house at all.

We used Duragal for posts, bearers, joists and verandah beams on our place. The rafters are Hyspan, so technically we may be in breach of that rule, yet the council happily passed it. However, the rafters are enclosed with eave sheeting, so it might be OK. All exposed components are steel or Merbau.

If you read it literally, any timber structure needs to be free-standing. Might be best to talk to the council fire officer (they all have one) about it.

[Loki429]

Thanks ausdesign and silentc

I spoke to the council but didn't get much help from there. I tried calling the RFS but the person I need to speek to is not available.

However further reading of AS3959:
Verandahs & Decks - "No timbers shall be allowed to directly connect with the remainder of the dwelling."
I will be seeking clarification of this from RFS. Sounds to me like its not allowed to be attached to the house at all, but I don't understand how that could help in a fire situation. If the deck is on fire and it is separated from the house by a 10mm gap it won't make much difference than if it was bolted to the brickwork!

Am I correcting in reading the following:
For the posts it does appear that timber mounted on galv shoes with 75mm clearance above the ground is suitable. The specifications give an "OR" situation with non-combustible or fire retardant treated timber being alternatives. However where I might have problems is with the 400mm above ground they specify. One corner of the deck will be approx 300mm from the ground, meaning the bearer and joists will be just about touching the ground. I was thinking of using unseasoned hardwood for the bearers and joists (better span capabilities and works out cheaper than treated pine). Hopefully they will be sufficiently naturally fire resistant? I haven't been able to find 90 x 90 hardwood posts yet.
Note that there is no specific mention of bearers/joists in the Verandas and Decks section.

FYI We are not directly connected to the bush. There is one house beside us then there is bush. At the rear of the property is some low scrub and a duck pond. The low part of the deck is against the house.

Best Regards

[silentC]

From my reading of it, you only need flame treatment up to 400mm above ground level for Level 2. If you use Merbau or Blackbutt or similar for your bearers and posts I think you are OK but best to check.

Regarding the 300mm, my understanding is that if it is at the top of a slope - ie. it's 300mm at one corner but slopes away, then you are OK regarding clearance. In general it's meant to be minimum of 400mm from ground to bearer.

You definitely don't want to use TP for bearers and joists. You can get flame treated pine I think but not sure I'd want to use it.

Duragal has some advantages if you aren't on the coast, including the fact that termites don't like it. But you need to paint it (unless you like the zinc look) and decking has to be nailed, not screwed. It's pricey but when you start looking at hardwood or flame treated timber, it begins to look more attractive.

You can tekscrew it together (although I welded ours) and it satisfies all requirements regarding bushfire construction standards.

[MikeT]

If the timbers don't penetrate into the dwelling then surely it should be safe or is there a concern that the heat or the burning joists will crack the bricks and allow passage that way - if that's the problem then you'd need a gap of a meter or so and cracking glass would be more of a concern anyway. I guess ours is not to reason why...

I assume this doesn't effect existing decks being renovated - I'm going to be redoing mine sometime soon - we back right into the bush and the decks only five meters away

[silentC]

I guess ours is not to reason why...

Sometimes it is because there's a tendency for these things to be put into print without really thinking it through and if no-one challenges it, you end up with people doing things and not really knowing why!

I reckon it could be read as "no timbers shall connect directly with the internal structure of the dwelling". Logically, if a ledger is bolted to a brick wall, you should be safe. The point of the legislation is to stop people building in such a way that a deck that catches fire can allow the fire to travel inside the house. I can't see how it could do that through a brick wall.

Plenty of older houses have the verandah joists as continuations of the floor joists, or sharing a bearer at least, with the verandah joist side by side with the floor joist. Plenty of balconies are set up on floor joists that extend outside the house. I can understand them wanting to stop that practice.

I assume this doesn't effect existing decks being renovated

I don't believe it does but it's a good idea to keep these things in mind all the same.

[ausdesign]

I stand to be corrected [it happens a lot] but my interpretation is -
timbers can butt against but not penetrate the 'required' part of the external skin - i.e. with a level 1 catag. it can be the sarking behind weatherboards.
In level 1 any materials can be used except stumps/poles must be fire retardent. Decking spacing 5mm+ and area underneath open.
Level 2 the same but decking must be fire retardent - merbau etc
Level 3 all timbers must be fire retardant.

My understanding is that with level 1 & 2, bearers & joists are not required to be fire retardant with spaced decking. When the deck is fully covered with say tiles or t&g flooring, then yes.

[silentC]

The relevant part of the NSW RFS guidelines simply says "no timbers shall be allowed to directly connect with the remainder of the dwelling". That's all they give you to go on. Not sure what the Vic guidelines say, they might be more expansive.

For level 2, you only need fire retardant material for members that are within 400mm of the ground. This contradicts the requirements for flooring systems in general, where it states that having bearers within 600mm of the ground requires the entire floor frame to be fire treated or protected by non-combustible sheets - but that relates to the main part of the dwelling, not external verandahs or decks.

We originally planned to build to level 3 but had it downgraded to level 2. That's why I thought the lot had to be fire retardant or non-combustible. Still, I reckon it was worth using steel because of the other benefits.

Have a look at page 30: http://www.bushfire.nsw.gov.au/file_...2_C2DE0383.pdf

There might be differences between the VIC and NSW regs.

[ausdesign]

Greetings Silent.

My take on the reg's is from the Australian standard - AS3959 but as we know States can include their own requirements. I'm not sure what we as designers are going to do with our extra time when standards are uniform across all of Aus.!
Your take on the wording is possibly correct. The wording in AS3959 says - decking timbers shall not be allowed to connect with the remainder of the building unless measures are used to prevent the spread of fire into the building.
I'm still convinced that 'deck' requirements & subfloor reg's are 2 separate items.
The standard refers to 3 types of construction
Slab - suspended slab floor supported by posts, columns etc.
Sheeted or t&g solid flooring - requirements such as the 400mm clearance etc.
Spaced decking. - with the measures I outlined previously.

The protection of bearers & joists etc is referred to for 'suspended timber flooring' which to my mind refers only to the house subfloor area.

I thought I knew all the ins & outs of this topic. You've got me thinking now so i'll check it out thoroughly & get some concrete answers & clarification. - an important & timely topic currently esp. in Vic at the moment.

[silentC]

My take on the reg's is from the Australian standard - AS3959

I'd say that would be a more reliable source than the RFS publication I'm referring to.

"decking timbers shall not be allowed to connect with the remainder of the building unless measures are used to prevent the spread of fire into the building" - I think that is the important part missing from the RFS publication.

That's why I said "there's a tendency for these things to be put into print without really thinking it through and if no-one challenges it, you end up with people doing things and not really knowing why".

Someone has been given the job of condensing the standard into a table for the back of the RFS brochure and so they have probably skipped a few important points, which is why it's a good idea to refer back to the standard and why you and your ilk will be kept busy for a good time to come

[Loki429]

I've just spoken to a very helpful man at the RFS and he has confirmed that attaching a ledger to the brickwork will be acceptable. He referred to building cases where the beam or joist of the house is extended out to include the deck and said that is not acceptable (as described by silentc.)

I guess the brickwork is a "suitable measure to prevent the spread of fire into the building." Sounds like the full details from AS3959 is more clear than the RFS publication.

As for the bearers and joists - they don't appear to be mentioned for deck construction although there are clear requirements for the house flooring system. I mentioned that one corner of the deck will be close to the ground and was told that as long as the deck spacing of 5mm minimum is adhered to I would be ok. I will still use hardwood for the bearers and joists for their added fire resistance capabilities. Note that the 400mm fire retardant treatment mentioned only refers to the posts, and even that is not required if the posts are mounted 75mm above ground on galv metal shoes!

I was also told to check the distance of the deck from the bush as even though the house may be in a level 2 area the deck may be closer and could put it into a level 3 or flame zone (ie no timber.)

He clarified the reasoning for the 5mm deck spacing - it is to allow any embers from a fire to fall through the deck and harmlessly onto the ground. It also allows smoke from a fire under the deck to be seen more easily, and for firefighting water to penetrate the deck to a fire below.

In summary it sounds like the regulations for deck construction in a bush fire area are a bit grey. I think common sense, careful thought and adherence to the standards specified should result in construction approval and minimised bush fire risk.

Many thanks for your help ausdesign and silentc!

[silentC]

The other thing they mention is that you must not close off access to the area under the deck. They don't say why but I believe one of the reasons is so you can get under there and clear out any leaf matter or other combustible stuff that falls through. I can get to all of mine with a leaf rake so I can rake out anything that collects.

You're right, most of it is common sense, especially when you understand the reasoning behind the regs. The problem is that they're not always well understood and you come up against planners and building inspectors who have different interpretations.

We were told initially by the inspector that our house would be level 3. Our designer disagreed and specified level 1. They compromised on level 2!

Now what about the pergola?

[Loki429]

Pergola - Using the same reasoning for the deck then attaching a beam to the bricks shouldn't be a problem (with regards to bush fire.)

There is however the structural question regarding attaching to the brickwork of a brick veneer house.

Is it stong enough to support a pergola?

The pergola is probably not that much of a static load. Only a rafter every 600mm to 1000mm (any suggestions on a good rafter spacing?). Covering material will be laserlite (or equivalent) so very very little load from that.
However what needs to be considered is the wind load on the pergola. Is that going to be an issue?

Comments?