Will my steel gutters rust if I drill holes in them? - Gutter Pumper

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Will my steel gutters rust if I drill holes in them?
No - and here are the reasons why they won’t.

Many types of material are used in modern gutters but by far the most common are the zinc/aluminium coated steel type. Probably 90% of the gutters now supplied are of this material and they can be either coloured or the original Zinalume finish. Below is a small table of the types of metal, the thickness required and the minimum cross sectional areas used in the gutters supplied today.

Let’s make this simple and just explain some of the terms of metal used in gutters. It would only be in very exceptional circumstances that the last four would be installed as guttering on a normal house. So now we are down to two types: ZINC Coated and ZINC/ALUMINIUM Coated – fairly simple as this is the coating over bare steel (which will rust). The diagram below shows roughly the base steel (which is the Base Metal Thickness or BMT) and the coating which is sometimes recorded as AZ or Z which simply means coated by an Aluminium/Zinc alloy or just Zinc by itself.

Most gutters supplied today are of the type covered in a Zinc/Aluminium coating over the base steel which normally would be about 0.42mm thick as in the left hand sketch above. The two coatings above are normally Zinc or Zinc/Aluminium and can be applied in three ways from as a thin layer of electroplated Zinc, a heavy layer of Zinc applied by hot-dripping (galvanized) or the technologically advanced zinc/aluminium alloy. Don’t worry too much as your gutter has a 95% chance of being of the last type of a Zinc/Aluminium alloy coated metal gutter. To explain why this is the normal practice – Zinc/Aluminium Alloy coated steel lasts up to four times longer than Zinc coated steel of the same thickness.

This Zinc/Aluminium coating serves two purposes –
   1.  By providing a physical barrier between the atmosphere and the steel
   2.  By giving a galvanic action

Painted bare steel if scratched or drilled will rust as the bare metal is exposed to the atmosphere as paint alone will not instigate a galvanic reaction. The Zinc/Aluminium coated steel if cut, scratched or drilled does not allow the steel to rust by a simple fact that the coating sacrifices itself to coat the steel from exposure to the atmosphere. So when a drill cuts a hole in the gutter, bare steel is exposed but this is quickly coated by the Zinc/Aluminium on the top and bottom surfaces through a galvanic reaction with moisture which builds up a protective Zinc coating over the exposed metal.

Galvanic reaction also protects the original downpipe drops when they are drilled or cut into the gutter. It is also exactly the same with roofing sheets that have all four edges initially exposed,  moisture provides the electrolyte that completes the galvanic reaction that soon seals up the base metal surface.

The most important aspect of roofing and gutting when considering coating is the compatibility of metals. There are many tables available that show which ones should not be used together. For example – you cannot use a Zinc/Aluminium coated roof that drains into a Galvanised (Zinc only) gutter. The colour bonded gutters are exactly the same – they have the Zinc/Aluminium coating under the paint layer and will behave exactly as the Zinc/Aluminium coated material.


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