Need input on a roofing quote

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sommot

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#324262 19-Mar-2026 18:38

So I'm getting a new roof put in and I've gone for a Colorsteel metal roof but I've asked them to add a layer of plywood sheathing as well as the normal underlayment. They quoted the install of 15mm untreated T&G plywood sarking at ~$8000. The entire roof is ~150m2 and will cost a bit over $30k (inc GST) including the plywood. Does this look about right? I really have nothing to else to go on so any input appreciated.

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#3471835 19-Mar-2026 19:23

We got a new roof a couple of years back. I did some research back then. For colorsteel metal roof the rate was 140/sqm (Auckland) . So for your roof 140*150 = 21'000 + plywood 8000 gives us 29000.

Did they quote 0.4mm or 0.55m colorsteel? and for safety roofers usually get scaffolding around the edges which may add easily 1000 if not more, that might be already included into your 30k

sommot

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#3471838 19-Mar-2026 19:49

They didn't specify thickness of the steel but I will double check with them. Thanks for the heads up. The quote does not include scaffolding as it is being provided by the reclad company.

eracode

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#3471840 19-Mar-2026 19:58

IANA roofing expert but an alarm bell sounded when I read ‘untreated plywood’.

Is that the norm when plywood goes on under steel? To me that means that if there’s any leakage, moisture or condensation gets on the ply, it will start to rot. So all the timber framing - trusses, purlins etc up in the roof need to be treated - but they’re suggesting untreated ply?

Sometimes I just sit and think. Other times I just sit.

sommot

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#3471841 19-Mar-2026 20:00

There will be an underlayment between the steel and plywood but I will definitely ask for treated plywood.

eracode

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#3471842 19-Mar-2026 20:03

sommot:

There will be an underlayment between the steel and plywood but I will definitely ask for treated plywood.

Please do that but it will be more expensive. They likely specified untreated to keep their quote as low as possible.

As mentioned I have no experience in this but I’d be surprised if there is no reg/spec covering the type of plywood if ply is being used.

Sometimes I just sit and think. Other times I just sit.

eracode

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#3471843 19-Mar-2026 20:14

Disclosure: I have now done an AI search on this and FWIW this is what I found.

TL;DR Treated ply is required except for in very specific situations.

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tweake

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#3471844 19-Mar-2026 20:18

sommot:

So I'm getting a new roof put in and I've gone for a Colorsteel metal roof but I've asked them to add a layer of plywood sheathing as well as the normal underlayment. They quoted the install of 15mm untreated T&G plywood sarking at ~$8000. The entire roof is ~150m2 and will cost a bit over $30k (inc GST) including the plywood. Does this look about right? I really have nothing to else to go on so any input appreciated.

installing roof sheathing with an iron roof is often not a straight forward conversion. it depends on what purlin setup they use, quite often it makes the roof height go up a bit and the roofing iron doesn't fit the guttering. you need to raise the gutters to suit. also i recommend a vapor port on the ridge line just to make sure there is sufficient outlet for moisture from the house.

if your sheathing the roof are you looking at doing a hot roof? ie lift the ceiling insulation up to the roof line. if so, need to air seal the sheathing and tie that into the wall, ie seal onto the top plate. also do the gable ends.

sommot

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#3471845 19-Mar-2026 20:25

They are installing ridge vents and vent and drainage roof battens if that's what you mean by vapour port. As for sealing the roof I don't think that's part of the quote and we never intended to do that.

eracode

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#3471846 19-Mar-2026 20:26

I would question the bona fides of the roofing contractor. Unless you meet the very restricted situations where untreated ply is permitted, why have they suggested untreated? It’s against the code/rules.

If me, I would make sure they are registered and in good standing with the industry body that covers roofers - whatever that is.

Sometimes I just sit and think. Other times I just sit.

sommot

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#3471848 19-Mar-2026 20:28

I just had a look back at some other quotes they gave us for asphalt shingle and they do specify "15mm H3.2 Treated CHH TG Plywood OR Viking 16.3mm H3.1
Treated Superstrand Sarking board" in that quote so maybe it's a mistake. I will definitely follow up on this. Thanks.

mattwnz

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#3471854 19-Mar-2026 20:45

If it does not need plywood, why are you installing it? Normally it is only needed where the manufacturers requires it, such as with a euro type tray profile. Or with butynol

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sommot

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#3471855 19-Mar-2026 20:47

Mostly for better acoustic insulation during rain/hail etc. We have a 25 year old pressed metal tile roof currently and it sounds like your inside an oil drum when it rains. We have a special situation which makes less noise important which I won't go into.

tweake

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#3471856 19-Mar-2026 20:51

sommot:

They are installing ridge vents and vent and drainage roof battens if that's what you mean by vapour port. As for sealing the roof I don't think that's part of the quote and we never intended to do that.

make sure they put the drainage part in, which is under the batten not on top.

if your not doing sheathing for insulation or air tightness reasons, then you can simply replicate a normal roof and leave a gap under the ridge for ceiling ventilation. a simple vapor port is to cover that gap with underlayment to air seal it.

tweake

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#3471857 19-Mar-2026 20:54

sommot:

Mostly for better acoustic insulation during rain/hail etc. We have a 25 year old pressed metal tile roof currently and it sounds like your inside an oil drum when it rains. We have a special situation which makes less noise important which I won't go into.

in that case, i would air seal, air seal and air seal. hard part is down onto the top plate.

sommot

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#3471860 19-Mar-2026 21:03

Thanks for the suggest, I will discuss with the roofer.

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