Thread: top hat battens and galwanised sheeting

I would ask for technical advice to confirm but I am pretty sure you cant.Zinc is a definite no no but wether the blue coating changes that I don’t know
Is there a particular reason you want to use a metal battern? I have gal sheets on my house and opted for timber batterns as they are much quieter
Gal sheets are expensive now at about 10% above colourbond options

No!!!!

Originally Posted by Beardy

Is there a particular reason you want to use a metal battern? I have gal sheets on my house and opted for timber batterns as they are much quieter
Gal sheets are expensive now at about 10% above colourbond options

Using gal for heritage reasons, the metal battens are freebies, hoped they might offset the cost.

Originally Posted by rwbuild

No!!!!

Why not? The only references I've seen about compatibility say to avoid materials like bare metal, copper, lead, stainless steel and treated timbers. No warnings about galvanised steel.

Edit. Found this:

"There is a misunderstanding that Zincalume is incompatible with galvanised surfaces. Zincalume is an alloy of aluminium and zinc, which are adjacent to each other on the galvanic table. They are, as can be expected, compatible in contact."

Compatibility of metals | NZ Metal Roofing Manufacturers

I would strongly urge against using zincalume battens under gal roof.
In 1973 when zincalume was introduced we were told it was not compatible and over the years I have seen gal gutters rust out very quickly with run off from zincalume/colorbond roofs
My own home here has most of the old gal gutter brackets ( holding zincalume gutter ) very rusty because the colorbond roof water runs over them.
I would also take expert advice from Bluescope as to what screws you use for the gal roof because you may find the zinc coated screws rust very quickly.

The expansion and contraction of the sheets will rub through the coating which is only applied as an anti glare and identification for safety during construction and also rob through the zincalum to the bare steel. If you want to use it you will need to affix a suitable membrane between them. I assume that the roof will be insulated so an insulation material of your choice over the battens under the gal sheets should be ok

Hi all. In my experience, there is no problem with gal flashings running onto zinc roofing but you can't have things the other way round. The water carries the "ions" (or whatever they're called) from one material to the other, and this does the damage. However, in the OP's scenario, there won't be the same issue. I don't believe it is physical proximity that causes an issue but I may be wrong. As said before, a layer of insulation will make all concerns moot.

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The compatability issues are addressed on page 8 - lysaght-roofing-walling-installation-manual (bluescope.com.au)

The real issue is about the top hat battens being suitable for the specific installation to meet the existing span and roof wind loadings etc. The specification of battens, connections to supporting structures (trusses / rafters etc), roof sheeting to batten etc all depend upon wind region classification. Data is available at TOPSPAN® 40 | Lysaght Professionals

Consult with a building designer as DIY design decisions can become expensive to rectify if a certifying authority orders remediation works or may result in an insurer refusing to payout a claim for building / contents damage if the "roof design" does not comply with building standards & regulations.

This table would seem to indicate that there is no incompatability between the gal sheets and zincalume battens..

TABLE 2.9.1:
Acceptability of drainage from upper surface onto a lower metal surface and direct contact.
COMPATIBILITY OF DIRECT CONTACT BETWEEN METALS OR ALLOYS
ROOF DRAINAGE SYSTEM
COMPONENTS & ANY
CLADDING MATERIAL
ACCESSORIES OR FASTENER OR (UPPER SURFACE)



That didn't copy as a whole table, have a look at the link in the previous post for details.

When they started to mix Gal with Zinc way back in the seventies I've seen spouting stuffed in 6 months ,so I'm not fully aware f what changes have been made but the mix contrary to what you read is a no no.

Originally Posted by mature one

…contrary to what you read...

I always go with written advice over anecdotes if that advice comes from a manufacturer or industry body.

As it's structured the PDF clearly states there is a difference between 'direct contact' and 'drainage' situations.

The OP is planning on direct contact use, not a drainage situation

Here in the West that's all we use, Zincalume roof onto blue steel battens. Granted all new builds here must use Colourbond to meet local council aesthetic standards. Getting (stupid) quotes to reroof my place in zinc all installers were going to use 40mm steel. It's a bit cheaper BUT they are all straight, which is the major benefit.

I think much of the confusion around compatibility of galvanised iron and zincalume is to do with galvanic action and noble metals. In particular this issue is more acute when water is involved (guttering is the obvious sticking point). However, condensation forms beneath a steel roof so it can't be ignored. The foil backed building paper is probably sufficient insulation to separate the metals, but others are better qualified to advise on this.

Some links:

BlueScope Steel Australia: Avoid incompatible metals

corrosion-technical-bulletin-ctb-12-dissimilar-metals (bluescope.com.au)

BlueScope Steel Australia: Guidelines for using ZINCALUME® steel

Regards
Paul