Corrosion of standing seam aluminium roof

[Muddy]

I have a standing seam aluminium roof on a single storey part of my barn conversion, I fitted it in 2011. Because it's easy to keep an eye on, I noticed the exposed plywood of the deck was delaminating, and a fungus appeared. At first I blamed a bad drip detail and tried to fold the drip edge down further, but on a subsequent investigation I found I could lift the bottom of the panels because the plywood had degraded to compost. It was not going to cure itself, so I carefully unclipped the aluminium panels to reveal and replace the rotten ply and make any other repairs.

What I noticed is that where the aluminium is in contact with the stainless sliding clips there is evidence of corrosion, to the extent that there is complete perforation of the aluminium. The holes are small, about 3-4mm, but almost every panel of 15 is affected. I fitted the roof to the instructions of the suppliers. The aluminium sits directly on the roofing membrane, which is standard breathable Permavent Eco. I can see from the staining that condensation on the underside of the aluminium gets trapped in the membrane and soaks the plywood, but also allows a galvanitic corrosion between the stainless fixings and the aluminium. 

I was going to reinstall the panels, lifting them off a replaced breathable membrane to allow the condensation to drain. But it doesn't seem as straightforward as that anymore.

It is outside of any guarantees. My thinking is that this type of roof is used widely by other self builders. 9 years, or less is not a very good lifespan.

[epsilonGreedy]

Owners of aluminium hull boats are paranoid about galvanic and stray current corrosion. It might we worth testing to see if there is an electrical potential difference driving this corrosion.

 

Is the roof earthed together with some other metalwork in the house? In yachting lore there are stories about a metal penny being dropped in the bilges and then burning its way through an aluminium hull.

 

http://www.pcmarinesurveys.com/AC DC electrolysis.htm

Edited October 19, 2020 by epsilonGreedy

[ProDave]

Stainless / aluminium is not a huge problem, witness how many stainless still things bolt to an aluminium boat mast.  But you would normally use something like Duralac when mating dissimilar metals.

[Muddy]

21 minutes ago, ProDave said:

Stainless / aluminium is not a huge problem, witness how many stainless still things bolt to an aluminium boat mast.  But you would normally use something like Duralac when mating dissimilar metals.

I would have agreed with you before I lifted the panels and discovered the 'evidence' . If it's not corrosion caused by a galvanitic reaction between 2 dissimilar metals, what has caused it? The corrosion aligns with the clips, and even a couple of screws in the deck.

[Gus Potter]

Following this. Just a thought.

 

Are you close to the sea?

 

What kind of barn is it, traditional ? If not is the structure moving about and overstressing the fixings of the cladding causing the ovalling , hole enlargement?

Edited October 19, 2020 by Gus Potter
just to add a bit

[Muddy]

We are in Shropshire. The roof is a single pitch of about 5 degrees, and on a single storey sunroom facing southwest, originally stables. The rest of the barn is brick 2 storey.

Aluminium expands/ contracts about 5mm per meter ( I was told) hence the special sliding clips, and the panels are 5 meters. It's not a huge area and accessible. I remember reading on GBF about someone's bad experience with a standing seam roof and condensation so was primed, and keeping a closer eye on it. But the corrosion was entirely unexpected.

The area of plywood deck to be replaced is mostly above the wall, which is timber frame. The roof is, from the inside, plasterboard, intello plus vapour barrier, 9x3 joist with warmcell between, 50mm foil faced pu insulation, 18mm exterior ply, Permavent Eco membrane, then the Vieo roofing.

[scottishjohn]

I hear what you are saying but  maybe its the very low angle which is allowing the water to just sit and not run off ,so making it a very wet area most of the time 

how much gap is between the the roof and the plywood -- when I looked at zinc roofing --

a real air gap and a minimum slope was very important  to stop corrosion

 from what you say there is no gap -so water could just sit there by capillary attraction + never dry out

I am guessing that this aluminium roof is a coated one --so maybe also movement has worn through the coatings where fixing touch  --and then you loose some of your corrosion protection 

 

[Mr Punter]

If it is a zipped seam the halters holding the roof panels should space them away from the substrate.

 

If it is flat to the substrate you could maybe use a membrane like they do on some zinc roofs, where the membrane is covered in coarse plastic extrusions or dimples that support the panels and leave a gap for air and drainage.

 

 

[Russell griffiths]

I used one of the fancy membranes with the polypropylene mesh on top

i wouldn’t use a standard membrane. 

[Kevin J]

https://bssa.org.uk/bssa_articles/bimetallic-galvanic-corrosion/

[scottishjohn]

just done some searching and everyone is saying 5degrees is way too flat  for a metal roof 

 

 https://www.asktoddmiller.com/minimum-roof-pitch-metal-roofing/

would do some hard thinking before going any further

[markc]

We started erecting commercial buildings in the early 90`s with standing seen roofs.

Several failed prematurely due to corrosion with the poor quality stainless posts. The very low angle will not shed condensate making matters even worse.

We ended up recommending posts had a pvc (insulating tape) isolator between them and the aluminium sheets.

[Muddy]

I agree that condensed water has drained and collected to the bottom of the roof, because the membrane was a standard breathable and in too close contact with the aluminium covering. That was very apparent when I exposed the rot. A 'hairy chest' type membrane as described above should allow that drainage. If, when, I reinstall I will lift the stainless brackets up on osb squares to provide a 9mm gap.  I was following the suppliers installation instructions, and they haven't changed.

However, it's the corrosion that really worries me. Everything I read sort of says that a small contact of stainless with a lot of aluminium is ok. Assumptions are so easy, I've been bitten before.

[scottishjohn]

your main problem is the low angle --most recommend 14degree as minimum --you have 5 degrees 

Me --i would dump it and convert to GRP

 reusing corroded sheets ,then new plywood +fancy expensive membrane -- -- does not sound a good idea -and you still have an inherent problem of low angle- which no marker will warranty their product for 

If its looks your after--then maybe convert to GRP and refit your alluminium on top 

[Patrick]

On 20/10/2020 at 20:36, Russell griffiths said:

I used one of the fancy membranes with the polypropylene mesh on top

i wouldn’t use a standard membrane. 

That's how it's done.

[Kevin J]

On 19/10/2020 at 21:05, Muddy said:

I have a standing seam aluminium roof on a single storey part of my barn conversion, I fitted it in 2011. Because it's easy to keep an eye on, I noticed the exposed plywood of the deck was delaminating, and a fungus appeared. At first I blamed a bad drip detail and tried to fold the drip edge down further, but on a subsequent investigation I found I could lift the bottom of the panels because the plywood had degraded to compost. It was not going to cure itself, so I carefully unclipped the aluminium panels to reveal and replace the rotten ply and make any other repairs.

What I noticed is that where the aluminium is in contact with the stainless sliding clips there is evidence of corrosion, to the extent that there is complete perforation of the aluminium. The holes are small, about 3-4mm, but almost every panel of 15 is affected. I fitted the roof to the instructions of the suppliers. The aluminium sits directly on the roofing membrane, which is standard breathable Permavent Eco. I can see from the staining that condensation on the underside of the aluminium gets trapped in the membrane and soaks the plywood, but also allows a galvanitic corrosion between the stainless fixings and the aluminium. 

I was going to reinstall the panels, lifting them off a replaced breathable membrane to allow the condensation to drain. But it doesn't seem as straightforward as that anymore.

It is outside of any guarantees. My thinking is that this type of roof is used widely by other self builders. 9 years, or less is not a very good lifespan.

You are correct 9 years and corrosion is totally unacceptable

there should be an eaves flashing to prevent water tracking back onto the substrate, often referred to as a T Plate. 

If there corrosion further up the sheet it's even more worrying 

would like to know more on the VCL, Insulation, fixing method and standing Seam system / manufacturer though

[SteamyTea]

Not read in detail all the above, tl;dr.

What state is the stainless in?

Some grades of stainless disintegrate in the absence of oxygen. So the problem may be the interface between the aluminium and the retaining clips. 

I am no chemist, so can't give a break down of the reactions involved, but strange things happen in chain reactions and they are not always obvious.