Under -deck "roofing"

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neb

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#1885891 18-Oct-2017 19:17

neb: At some point I'll post a photo of the single rusty bolt that was holding up one entire side of the deck.

The Jesus nut:

That's a single 10mm bolt. The nut had corroded into place, but the hole was big enough to just pull the entire bolt out through it.

You don't want to know what the "foundations" for that deck are like.

neb

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#1885915 18-Oct-2017 19:24

mattwnz:
I wonder if water coming off the treated timber has cuased that corrosion to be far worse than it should me. If you put a galvanised plate onto treated timber, it can corrode quite quickly.

Yeah, you can now get electrocoated and other exotically-treated screws for dealing with that. Needless to say my deck is nailed with hot-dip galvanised, I periodically go round and re-nail the ones that have corroded into nothingness.

Before anyone asks, it's not worth sinking a pile of money into patching things up piecemeal, long-term plan is to rip the whole deck out, replace it with one half the size, and do it properly from the ground up.

nickb800

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#1885916 18-Oct-2017 19:24

Steel sheet is completely unsuitable for this, as you will end up with unwashed areas that corrode quickly. The area around the joists is unwashable - both the ridges which will be hard up against the joists, and the troughs which will collect leaves. Even if you run water from the top periodically, decomposing leaves will build up under the joists, and you can't brush the surface. That's to say nothing of the runoff from the timber treatment in the joists, which will occur regardless of DPC being inbetween. And to maintain headroom underneath you probably can't achieve an appropriate fall for rain washing to be much help.

Even though bad workmanship was involved in neb's photo above, it gives a pretty good idea of what is to come with steel sheet.

Plastic is your best option here - corrugated polycarbonate, or maybe flat fluted polycarbonate like this http://pprpenrose.co.nz/roofing

Aluminium may be also be an option, as it forms an oxide layer that is more resistant to all the ills that kill steel - but you probably want to check it with someone used to installing aluminium, and don't expect a warranty

neb

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#1885918 18-Oct-2017 19:30

nickb800:
Even though bad workmanship was involved in neb's photo above,

Hey, I must thank the previous owners for one thing, I've got an endless supply of crappy workmanship stories to share with other house owners :-). And virtually any maintenance I do ends up as a net improvement.

MikeAqua

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#1886085 19-Oct-2017 09:38

nickb800:

Aluminium may be also be an option, as it forms an oxide layer that is more resistant to all the ills that kill steel - but you probably want to check it with someone used to installing aluminium, and don't expect a warranty

Aluminium will undergo poultice corrosion (piles of leaves etc) faster than steel. It's far more active than steel and in the presence of a build up of organic mater won't get the chance to protect itself with an oxide layer. Even marine grade aluminium alloy doesn't with stand up to poultice corrosion for long.

You can buy anodised aluminium cladding I'm not sure if that is any more resilient.

OP didn't want plastic due to it getting stained and ugly.

The fluted polycarb sheet tends to grow algae in the cavities - especially in shaded areas. If going down the fluted plastic route then there is a range range of coloured fluted plastics used for sign age. Light and cheap.

An issue with any kind of flat sheet fixed to the underside of purlins is ensuring and debris water can escape ... corrugate leaves gaps.

Mike

kryptonjohn

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#1886120 19-Oct-2017 10:20

Yeah at this rate I may end up with PolyC and paint it.

That said, the old under deck roof was heavy galv steel formwork and it was still looking very sound and there wasn't a whole lot of muck in it when it was demolished..

nickb800

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#1886230 19-Oct-2017 12:36

MikeAqua:

nickb800:

Aluminium may be also be an option, as it forms an oxide layer that is more resistant to all the ills that kill steel - but you probably want to check it with someone used to installing aluminium, and don't expect a warranty

Aluminium will undergo poultice corrosion (piles of leaves etc) faster than steel. It's far more active than steel and in the presence of a build up of organic mater won't get the chance to protect itself with an oxide layer. Even marine grade aluminium alloy doesn't with stand up to poultice corrosion for long.

You can buy anodised aluminium cladding I'm not sure if that is any more resilient.

OP didn't want plastic due to it getting stained and ugly.

The fluted polycarb sheet tends to grow algae in the cavities - especially in shaded areas. If going down the fluted plastic route then there is a range range of coloured fluted plastics used for sign age. Light and cheap.

An issue with any kind of flat sheet fixed to the underside of purlins is ensuring and debris water can escape ... corrugate leaves gaps.

Good to know about aluminium - I knew it was more corrosion resistant in general but didn't know how it would respond in this context

Sure the bottom of steel corrugations might fare better, but the leaves will catch behind the joists and create dams. I wonder if fitting purlins beneath the joists, running parallel to the ribs (rather than the usual perpendicular) would enable better clearance of leaves - both for corrugated steel to extend it's life and for polycarbonate to reduce unsightly buildup

For direct mounting to joists, opaque polycarbonate sounds like the better option, or just painting transparent polycarbonate as the OP says.

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kryptonjohn

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#1886234 19-Oct-2017 12:43

Remember that to get into the under deck catchment anything has to fit through the 4mm gap between the decking... so shouldn't get a lot of leaves but will get accumulated dust that could eventually start growing grass seeds etc and the ensuing mat of roots would dam it up pretty quick and be hard to dislodge.

kryptonjohn

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#1886281 19-Oct-2017 14:41

Found a "proper" solution. Like it.

http://www.underdeck.com.au/

Kickinbac

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#1886434 19-Oct-2017 20:34

kryptonjohn:
Found a "proper" solution. Like it.

http://www.underdeck.com.au/

Interesting that its colorbond, which is basically colorsteel. The fall is the only approx 10mm per metre.
And I quote from their website...
'All components are maintenance free - will not Rust, Rot or Corrode.'
'Blue Scope Steel Colorbond Warranty'

The colorbond warranty may be voided depending on their terms and conditions but I cant be bothered finding and reading them!

The system does not easy to easy to clean out.

kryptonjohn

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#1886440 19-Oct-2017 20:41

Don't see any problem with the fall. The water can't get in fast through the deck gaps and the wide channels have more volume than corrugated profiles. Remember your spouting carries a signficant amount of water yet has a smaller recommended fall due to its volume and high sides.

The system uses galv top-hat purlins so the roofing material itself is not in contact with the timber.

No sign of a NZ distributor though.

blakamin

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#1886481 19-Oct-2017 22:00

Kickinbac:
kryptonjohn:

Found a "proper" solution. Like it.

http://www.underdeck.com.au/

Interesting that its colorbond, which is basically colorsteel. The fall is the only approx 10mm per metre.
And I quote from their website...
'All components are maintenance free - will not Rust, Rot or Corrode.'
'Blue Scope Steel Colorbond Warranty'

The colorbond warranty may be voided depending on their terms and conditions but I cant be bothered finding and reading them!

Also available in aluminium, stainless cor-ten, zinc, copper...

cadman

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#1886842 20-Oct-2017 11:41

mattwnz:

I wonder if water coming off the treated timber has cuased that corrosion to be far worse than it should me. If you put a galvanised plate onto treated timber, it can corrode quite quickly.

As per this, it depends on the type of treatment. See here.

wally22

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#1888008 23-Oct-2017 10:32

kryptonjohn:

Don't see any problem with the fall. The water can't get in fast through the deck gaps and the wide channels have more volume than corrugated profiles. Remember your spouting carries a signficant amount of water yet has a smaller recommended fall due to its volume and high sides.

The system uses galv top-hat purlins so the roofing material itself is not in contact with the timber.

No sign of a NZ distributor though.

Weak point I can see is where mounting brackets (top hat purlins) attach to deck joists. Could mitigate that by using DPC between top hat and joists. Still will have dirt etc buildup on the colorsteel too.

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