Galvanic corrosion is our subject today and I am still talking roofs. This type of corrosion is also called bimetallic corrosion or dissimilar metal corrosion. Galvanic corrosion occurs when different metals come into contact with each other, with an electrolyte present and the result being corrosion. We all know rust which is the result of oxygen and iron reacting. The result of the corrosion on metal roofs is very similar.
What is the most common electrolyte we find in our homes causing the galvanic corrosion? Yip our favourite ……..WATER.
 |  |
Most people are unaware of the damage water causes in the home. Water accumulates in the home from cooking, breathing and poor ventilation. A general rule about water and homes is the following: Water that is inside must get outside and water outside must stay outside . if this cannot happen, damage happens.
Achieving this balance is critical for healthy living. I shall discuss this in a later post.
Back to the roof.
Most hardware store stock a variety of roof screws, bolts and other attachment devices for roofs. Not all roof screws are created equal and some are designed specifically for a type of sheeting. There are hammer-driven roof screws which are not really screws, but they have a diagonal thread, hence the name screw. You have the bolt headed screw which can be driven in through a socket drill drive and has the advantage that it can be removed without damaging the roof sheeting.
The shafts of these screws are usually coated with zinc or other metal alloys and marketed as rust-resistant roof screws. Let’s look at a typical Hex head roof screw and discuss its parts.
Looking at the parts of the screw and what we discussed earlier about Galvanic corrosion we have three different metals present and now this is driven through a metal roof sheet made of steel that has been galvanised. Here is a typical roof screw: a galvanised head, a washer that is probably only metal plated , a rubber washer which if installed incorrectly serves no purpose and a zinc coated thread. This comes into contact with a galvanised roof sheet and if the contact is not insulated against water penetration, corrosion will occur.
Besides choosing the correct roofing screws, how they are installed is of cardinal importance. Take a look at the pictures below of how screws are not supposed to be installed and the correct way.
Screws can be tightened loosely, which prevents the rubber from creating a proper seal with the roof sheeting. If they are tightened to tight the rubber washer gets squashed out and exposes it to the elements and a good seal is not created.
Tightening the screws too tightly can also harm the roof sheeting by causing a kink or dent.
To install correctly the head of the screw must be tight against the metal washer and the metal washer must be firm against the rubber washer. The rubber in turn must form a tight connection with the sheeting. This will be an example of a correct installation.
Replacing rusted roof screws is a crucial step in ensuring the longevity and integrity of your roof. By considering factors like roof sheet profile, screw length, diameter, and corrosion protection, you’re taking proactive measures to address potential issues. A proper roof installation should have all roof screws attached correctly. Hope you all have a happy leak-free roof. Rust - in-roof-sheets
If in doubt, contact me on www.proinspect.biz or find me on Facebook https://www.facebook.com/ProInspectbazil/
コメント