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Topic # 223785 17-Oct-2017 11:26
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Hi - we've demolished and rebuilt a large deck (About 8m across and 5m out although the roof eave covers about 1.3m at the house end) which extends from 1st floor living room out over the driveway. There's room to park two cars under it making it an effective carport. The original 60yo deck had galvanised metal suspended underneath it to run the water off to spouting. I think the metal was actually re-purposed from concrete flooring formwork. 

 

The new deck has nothing underneath it, so currently the rain (and kwila bleed) is just pouring through and I need to construct something to catch it and feed it into the stormwater drain.

 

My initial idea is to screw polycarbonate roofing sheets to the underside joists. The joists run out from the house, and I plan to fix battens across them to create a fall, and fix spouting to catch it bear the house end.

 

If I can find it I'd probably use opaque sheets as it will probably get dirty and not look great underneath if it's clear?

 

Will probably wait until after the whole renovation has received it's CCC before I do this to avoid complications with the council - although I will run the water into the stormwater drain the old deck used to flow into.

 

Has anyone done this or have any advice? How much fall should I allow? Are there any risks with capturing dampness under the deck?

 

Thanks in advance,

 

JohnO

 

 

 

 

 

 


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  Reply # 1885020 17-Oct-2017 12:10
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It will get dirty. There's no doubt. Translucent roofing will end up dark/striped along the low spots so might look odd, but ymmv. I think gutter falls are set to about 1:500. It might pay to stick to that as a minimum pitch.




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  Reply # 1885023 17-Oct-2017 12:23
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Yeah. So far have not found opaque though. 

 

The specified fall for roofing use is is high at 5 degrees or 90mm/m - presumably the same when underneath? That's a lot more fall than spouting (which is deeper and wider channel).

 

The other question I can't answer is the spacing for screws. The specs are for holding it down not for supporting it's weight upwards.

 

 


 
 
 
 


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  Reply # 1885080 17-Oct-2017 13:15
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Do you want flat or corrugated surface?

 

My first thought was to use corrugated roofing steel.  Just make sure you screw through the concave surfaces when you fix it from underneath.

 

If you want a flat surface I would thinks about painting some exterior plywood and using plastic joiners to join it.  Paint the sheets both sides with exterior paint before you before you fix it up there.

 

I would go for for a 4 degree pitch.  Multiply degrees by -1.746 to get fall in cm/m. 

 

If you know the fall/m and your batten spacing you can calculate your batten heights - do the calcs for the 'uphill' edge of each batten.

 

I assuming that you batten are perpendicular to your joists.  If the are parallel to the joists then I would just mark you fall along the batten rip along the line.  If you choose your timber carefully you should be able to rip each length into two battens.

 

 

 

 





Mike



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  Reply # 1885085 17-Oct-2017 13:31
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MikeAqua:

 

Do you want flat or corrugated surface?

 

My first thought was to use corrugated roofing steel.  Just make sure you screw through the concave surfaces when you fix it from underneath.

 

If you want a flat surface I would thinks about painting some exterior plywood and using plastic joiners to join it.  Paint the sheets both sides with exterior paint before you before you fix it up there.

 

I would go for for a 4 degree pitch.  Multiply degrees by -1.746 to get fall in cm/m. 

 

If you know the fall/m and your batten spacing you can calculate your batten heights - do the calcs for the 'uphill' edge of each batten.

 

I assuming that you batten are perpendicular to your joists.  If the are parallel to the joists then I would just mark you fall along the batten rip along the line.  If you choose your timber carefully you should be able to rip each length into two battens.

 

 

Hi @MikeAqua was thinking corrugated so there would be channels for the water to run down and keep it away from the upside down "purlins". Steel would be great as it's stronger than poly and opaque but downside is it's presumably a lot more expensive... but will look that up.

 

Your assumption above is correct - the joists run perpendicular to the house facing. So idea is to put purlins perpendicular underneath the joists with packers to create the fall. The fall to be away from the house so it only has to drop across 5m rather than 8m.

 

 

 

 




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  Reply # 1885086 17-Oct-2017 13:37
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Looks like corrugated iron about the same as poly carbonate - so that's much better.

 

 


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  Reply # 1885090 17-Oct-2017 13:48
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At 4 degrees I get about 20cm fall over 5m.  Might be a bit much?  You could probably get away with less.  It's not roof in the open air.  It's sheltered to some extent and will catch less water.

 

With colour steel the pale underside will minimise light absorption in the car port.  Much more durable than plastic.





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  Reply # 1885092 17-Oct-2017 13:52
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Falls for roofing are based on a few different things. One being the ability to shed water quickly enough that the overlap of sheets doesn't allow standing water through and cause structural/cosmetic water damage. This isn't an issue for a carport.

 

The other is the tray profile is only so strong for vertical loading before it will crumple. But this is based on standard purlin fixing centers of a minimum of 450mm crs. and again not what i'd consider a real issue if it's under a deck where you can fix it as much as you want with battens.

 

Unless you've got masses of headroom I really wouldn't stress about meeting the roof products minimum pitch specifications.

 

 




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  Reply # 1885093 17-Oct-2017 13:53
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It does seem like a lot. If it's not enough it will not carry the water away at the rate it's falling in... the consequence being it will fill up between the joists and overflow on the sides. Not sure that matters really as it will eventally stop and there's no path for the water to get to the house itself. However the rain can only get in there by passing through the 4mm gaps in the decking and it's hard to imagine it can get in there so fast that it won't fall away. I'm going to take some measurements and see how the sheets and guttering will look hanging that low.

 

 




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  Reply # 1885095 17-Oct-2017 13:55
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Disrespective:

 

Falls for roofing are based on a few different things. One being the ability to shed water quickly enough that the overlap of sheets doesn't allow standing water through and cause structural/cosmetic water damage. This isn't an issue for a carport.

 

The other is the tray profile is only so strong for vertical loading before it will crumple. But this is based on standard purlin fixing centers of a minimum of 450mm crs. and again not what i'd consider a real issue if it's under a deck where you can fix it as much as you want with battens.

 

Unless you've got masses of headroom I really wouldn't stress about meeting the roof products minimum pitch specifications.

 

 

Thanks @Disrespective - that's what I thought too so good to get your opinion.

 

 




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  Reply # 1885098 17-Oct-2017 13:58
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Actually that's a good point about the weight if it starts filling like a swimming pool. If the downpour was so extreme that it turned into a swimming pool the steel/fixings could fail which would be bad! Remember the fixings are holding it up to the purlins - it's not sitting on them so the screw heads have to hold the weight.

 

 

 

 


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  Reply # 1885111 17-Oct-2017 14:12
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kryptonjohn:

 

Actually that's a good point about the weight if it starts filling like a swimming pool. If the downpour was so extreme that it turned into a swimming pool the steel/fixings could fail which would be bad! Remember the fixings are holding it up to the purlins - it's not sitting on them so the screw heads have to hold the weight.

 

If you are really worried about excess weight possibly stressing the screw heads you could always fix the heads through a minimal strip of something on the underside of the sheets.

 

you could use something like brace coil and fix it to the ends of the purlins to give it more support...

 

https://www.bunnings.co.nz/lumberlok-30m-galvanised-zinc-export-grade-coil-strip-brace_p08911790

 

 

 

 


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  Reply # 1885115 17-Oct-2017 14:15
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I was envisaging battens fixed to the underside of the joists, and corrugate fixed to the underside of the battens.

 

If you do it that way, in between the battens there will be gaps between the joists and corrugate.

 

The battens will contact the corrugate at the tops of the ridges.  That isn't ideal as the battens may hold moisture and corrode the steel.  Waterproof strips along the under side of the battens would help.





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  Reply # 1885119 17-Oct-2017 14:22
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MikeAqua:

 

I was envisaging battens fixed to the underside of the joists, and corrugate fixed to the underside of the battens.

 

 

You mean long battens inline with the joists or "purlins" perpendicular to the joists?

 

 

If you do it that way, in between the battens there will be gaps between the joists and corrugate.

 

The battens will contact the corrugate at the tops of the ridges.  That isn't ideal as the battens may hold moisture and corrode the steel.  Waterproof strips along the under side of the battens would help.

 

 

I can't see any way around this - isn't the steel galvanised underneath the paint?


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  Reply # 1885127 17-Oct-2017 14:43
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kryptonjohn:

 

You mean long battens inline with the joists or "purlins" perpendicular to the joists?

 

 

Sorry purlins.  I should have been saying purlins all along

 

kryptonjohn:

 

 

 

...isn't the steel galvanised underneath the paint?

 

 

It is but still vulnerable to 'poultice' corrosion.  I think a roll of the rubber strip that goes between fairing and concrete would do the job.

 

 

 

 

 

I don't think you will have an issue with rainwater over loading the 'roof'.  The tek screws used to fix roofing have enormous holding power.  Rain can only fall through the gaps between your deck boards at a certain rate.

 

 





Mike



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  Reply # 1885130 17-Oct-2017 14:45
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Alright thanks guys I am sufficiently emboldened. "Lets do this" !

 

 


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