Roof lapping on a water reservoir

[ashtree]

I am hoping you can help me with an opinion the suitability of the roofing system installation.

I do process engineering and manage water quality for a water utility and recently we have had built a new glass fused steel treated water reservoir at one of the water plants i work at. Everything had gone well and i got onto the roof of the reservoir to collect a sample(the first time up there) and i noticed that every sheet of the aluminium roof was lapped wrong both length ways and cross ways.

When this was referred to the contractor/supplier they said they always did this and stated that because they put sealing between the sheets it was not an issue. They also stated that only half the roof is lapped incorrectly because the roof falls away both sides from the high point.The roof is screwed in the valleys not the peaks. The install seems to go against all the well known roofing practices, but they have trotted out statements from the tank supplier and from the building inspector saying its okay. The construction drawings also seem to at least partially confirm that screwing the sheets on the valleys was intended.

My argument is that in order to prevent run off from the roof going into the treated water , you become 100% reliant on the sealant around the screws and between sheets. This is a contamination risk. On some sheets the sealant is obvious but many are not so we have no certainty whether sealant was even installed. A photo of one sheet is attached.

The contractor and supplier have dug their heels in as expected and have given me all the reasons under the sun why this is okay, but i am not convinced personally. Just because this is the way they normally do it does not necessarily make it good practice. But i would be interested to hear your opinions on the matter.




Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[hokie66]

Those sheets don't even nest accurately. And I agree...the screws should be through the crowns, not the valleys.

By "lapped wrong", I take it that your photo is looking down the slope. If so, that is silly.

What profile aluminium roof was used? Manufacturer?

 

[ashtree]

Thanks for the reply.

You are correct in that this is looking down slope, although there is very little slope on the whole roof.
I don't know what the profile is and i suspect the manufacturer is from overseas, possibly UK.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Ron]

ash...you are correct. The sheets are reverse lapped. I'm not familiar with Australian Building Codes, but in the US this would be a code violation. I suspect it is similar in your area.

Sealants do not perform well on metal, particularly aluminum. Do not rely on them to make your roof work properly.

This is also against the recommended practices of roofing in every locale in which I have worked in North America.

Stick to your guns and have them replace the roof.

 

[ashtree]

Gents,
Thanks for your replies .
Just to follow up on this , the contractors have begrudgingly agreed to replace the roof , but have continued with the line that this is how they always do it.
Having withheld a couple of $100k from the final payments until the roof is fixed has managed to convince them of the error of their ways, but i would expect that there may be a lot of these type reservoirs out there that are "relying" on the sealant to prevent contamination.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"