I am in a townhouse and my body corporate bylaws allow me to lay down flooring in my roof space and use it for utility storage. An engineer has confirmed that the framing can support a floor, people walking on it, and the usual range of stored domestic goods. The property was built before current building codes and the joist span in the roof is wider than usual in some places, up to 900mm. We are allowed to modify joists and battens, as long as a licensed builder does it.
In addition to providing storage, I see an opportunity to beef up our individual townhouse's fire resistance with choice of flooring. There are no fire walls in the roof (again, the property pre-dates that requirement) and it is not feasible to retrofit them. The bylaw deals with securing the area without triggering a requirement to build firewalls by requiring that the storage area be sealed off with non-wall barriers such as cages, fencing, or removable partitions, and it's also not permitted to seal off the entire footprint, you have to leave a corridor for trades because there are common utilities in the roof. I'm not particularly seeking advice on that aspect, it's just to give context that the gold standard fire strengthening choice, a firewall, isn't available in this case, so I'm after the next-best-feasible option, which seems to me is choosing a roof space flooring with fire resistant properties. The flooring will be over the top of Earthwool insulation.
There is also a practical issue regarding getting flooring into the space. I have an opening hatch and ladder approx 1000mm x 900mm. I have solar panels on the roof, so lifting the roof sheets to get materials in would be non-trivial. This limits the size of material that can go into the space.
I am not building this myself, I have a friend who is a licensed builder who will do the work (this is a bylaw requirement). The last time we discussed this, he was leaning towards 18mm plywood or possibly formply, but at that time fire resistance was not part of the discussion. This isn't his usual sort of work (he subs on big-budget large commercial builds), so although he is qualified to do a job that will be structurally sound, he doesn't necessarily have the wide product knowledge of options for a budget/utility use case. So I thought I would throw it out here for thoughts before our next discussion. Appreciate any thoughts.
I think this page may help -
I think ply in the thickness range you are looking at has a recomended span of 450mm.
One method to add fire resistance to your proposed floor is to line your flooring area with fire-resistant materials such as Gyprock CSR 3600 x 1200 x 13mm Fyrchek Plasterboard RE 4.32sqm. Typical applications include Fire separation and smoke walls, inter tenancy walls and party walls, multi-residential unit ceilings, lift wells, plant rooms, and any non-wet area wall or ceiling where a fire or acoustic rated system is specified. It will however add to the thickness and weight of your floor. Please make sure to take that into consideration.
The other fire-resistant material is James Hardie 2400 x 1200 x 6mm 2.88m² Villaboard Lining. Villaboard is deemed non-combustible and can be used in fire and acoustic wall systems. It is more impact resistant than plasterboard making it suitable for high traffic commercial applications. Both sheets will need to be trimmed down so that they will fit through your opening hatch in the ceiling.
I suggest speaking to your builder about my recommendation about putting these materials on top of the plywood. In order to lower the weight on your ceiling frame, you could use a plywood material that's less than 18mm. But your objective is to have it strong enough that you can still stand on it without the floor bending too much.
If you need further assistance, please let us know.
Hi @deslea , I find this to be an interesting discussion/challenge. I am not sure how much fire resistance you are hoping to, or likely to achieve, so I guess you will need to keep an eye on possible benefit Vs costs. If the joists run over adjoining units, then a roof fire could burn under the flooring you add, and any timber attached to it could burn too, but at least you may slow the spread into your unit. You could go further and coat the joists and the added flooring too, in fire retardant paint first (https://www.coating.com.au/fire-resistant-coating-australia/) if you want full protection. Perhaps using a thicker flooring material such as https://www.bunnings.com.au/james-hardie-2700-x-600-x-19mm-secura-exterior-flooring_p0710430 would save having to use a flammable material under a fire retardant one. The joists spacing will need to 450mm though to support the loading. 600mm will fit through the access hole (up to 1200 should go through by feeding it on the diagonal) but the roof head space will limit how long a sheet will go through. But your builder will be able to cut lengths to suit the situation. It will be interesting to know what you decide on, after you and your builder weigh up all the considerations.