Recently our specification engineers have been taking a lot of calls from specifiers asking about the use of zinc metals on rolls for use as DPC or cavity tray.
Our stance is clear. Zinc although acceptable in roofing applications should never be used in brickwork cavities as a cavity tray.
PD 6697:2019 states that in buildings exceeding three stories, if a product is In contact with or embedded in an outer leaf of an external cavity wall, that its material specification should be limited to austenitic stainless steels, aluminium bronze, phosphor bronze, or copper. If the building is located in a coastal area this should be limited further to just austenitic stainless steels containing molybdenum. Other metals are not suitable for durability, low resistance to aggressive environments, and bimetallic corrosion problems.
There are a plethora of issues with the use of zinc in this way, but the most common question we are being asked is:
“Will these zinc roofing products react with Masonry Support, Wall Ties, Lintels, Channels etc?”
The answer is a resounding “Yes”.
Zinc and metals like stainless steel are not intended to be used in conjunction with each other. In the presence of any moisture, the two materials have a very large difference in potentials. The water acts as an electrolyte and creates an electrical current between the two materials. As stainless steel is the nobler of the two materials, the more active material, zinc, will corrode.
In brickwork cavities, DPCs and Cavity trays will almost always be in close proximity to stainless steel products such as Masonry Support, Wall Ties, Lintels etc. When zinc products interface with these products, failures can occur, which in this case could end in a non-sealed cavity, potentially causing damp problems on the internal of the building.
| Galvanic Cell | Environment | Area Ratio | Corrosion rate
(mm/a) |
|
| Stainless Steel | Carbon Steel
Zn 99.9 Al 99.9 Cu-DGP Ti |
Drinking-Water, aerated |
1:1 |
0.47 0.26 0.17 0.07 <0.01 |
| Stainless Steel | SF-Cu | Artificial Sea Water |
1:1 1:10 10:1 |
0.12 0.07 1.00 |
| Carbon Steel |
1:1 1:10 10:1 |
0.38 0.25 1.10 |
||
| Zn
Ti |
1:1 1:1 |
0.61 <0.01 |
||
| Galvanic Cell | Area Ratio | Corrosion rate
(mm/a) |
|
| Stainless Steel | ZnCuTi |
1:1 1:5 |
4.39 1.43 |
| Stainless Steel | ZnCuTi |
1:1 1:5 |
3.88 0.91 |
Speaking and working with Architects, Engineers, and Facade technicians about G-Tray, we frequently have to combine cavity barriers, wall ties, masonry support or lintels, and cavity trays in a very small area between windows. It is almost impossible to design a solution avoiding contact between the cavity tray and these stainless steel products.
ACS advises all clients to avoid the use of these roofing products sold as DPC or cavity tray alternatives.
For more information please contact our team of expert structural, mechanical or civil engineers.
Article originally published on 1st April 2020