Long span garage roof deck, more thoughts

[Wildman]

Hi all.

I have been thinking about this garage roof deck some more and have come up with some other options that I would like to bounce off you all.

The span is actually 6.5 m x 7.5m so 6.5m short span.

I have discovered the properties of Posi-Strut beams. Has anyone used these before? They are truss beams made up of timber top and bottom flange with galvanised webbing between them. A 302mm total depth beam with 45x90mm F17 flanges will span 6500mm at 450mm centres to Australian flooring loading standards. Apparently 400mm centres give a more solid floor with less "bounce" though. A 412mm beam with 45x90 F5 flanges will also span 6500mm as will 40x70mm F8 flanges. These beams are lightweight (2 blokes could get them on to the roof and in place) and strong and will be cheaper than the steel and 180x45 KD joists I was planning. They are, however, not designed to be in the weather. The top flange can be nailed directly to, as can the bottom flange for flooring material or ceiling material.

To get around the weather problem, I did a bit of a Google search which led me to the website of Bob Vila in the US. The Yanks have (what I think is) a great solution to the roof decking system. Once the joists are in place (in my case, the Posi-Strut beams), they lay 19mm exterior ply down over the joists and nail it down properly. They then glue down huge sheets of rubber approx 4-5mm thick over the entire roof to the ply forming a waterproof membrane. This even runs up any walls to stop ponding finding its way in eventually. Onto this rubber, they glue hard plastic foam blocks down every few feet that they then can nail lightweight decking joists to that can then take decking. The foam blocks dont mind having their feet wet and allow the water to drain away easily from under the decking surface. I even saw a video of large square reinforced concrete tiles (30-40mm thick) glued to the foam blocks with a 12mm gap between the tiles to allow water to drain through in the same way.
I have been thinking that this would be fairly easy to do myself, far easier than the steel beam option and would solve the problem of the roof under the decking as well as allowing me to remove the existing roof so that the overall height of the garage did not rise at all which is what I want. The glued rubber and ply roof would be very low maintenance and should last at least as long as I want to be in the house for (10-12 years depending on kids).
Does anyone know of this rubber membrane technique being used in Australia?
On second thoughts, it might just be easier and probably cheaper to strengthen the walls and put a concrete slab roof in. I have access to any concreting gear short of a concrete pump so if I formed it and finished it all up myself, it would not be too expensive.
Cheers
Wildman

[duckman]

Hi all.

I have been thinking about this garage roof deck some more and have come up with some other options that I would like to bounce off you all.

The span is actually 6.5 m x 7.5m so 6.5m short span.

I have discovered the properties of Posi-Strut beams. Has anyone used these before? They are truss beams made up of timber top and bottom flange with galvanised webbing between them.

<snip>

Cheers
Wildman

G'day Wildman,

Posi-strut or Long Reach joists, the name depends on who fabricates them, are very popular here in Victoria for both floor joists and roofing purlins in commercial and semi-commercial buildings. They work extremely well and one man can lift them in the size and length you're talking about. The webs within the joists can be either galv. steel or timber.

They are very easy to install due to both their light-weight and the fact that with 90mm wide top and bottom chords they will stand upright, on their own to facillitate positioning and fixing, in all but the strongest winds.

As for your particular application, I suggest you ring a couple of fabricators and ask their advice. Chances are that someone, somewhere has done it before and the fabricator that you contact supplied the joists.

HTH,

Mark.

[seriph1]

bituthene sheet is the product - uncertain if it is available in oz ..... maybe contact the building industry cente in your state..... and google google google

[seriph1]

in fact ............. http://www.infolink.com.au/browse_su...p?catcode=0306

[Wildman]

I thought about Bitumen sheet, but these ones I saw in the US were full rubber sheets that used a rubber cement to glue it to the ply. I would think that rubber would be longer lasting and better sealing than bitumen as long as it was kept out of the sun. It would also be easier and nicer to work with I would think easier and cleaner to cut and shape.


Cheers
Wildman