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Metal Roof Overlapping: To Seal or Not to Seal

24 March 2021

There are three words that strike fear into the hearts of anyone involved in metal roofs… Cut Edge Corrosion, which, if left untreated, will lead to the exposed metal surface corroding and eventually perforating.

This is perhaps the most favoured opinion when you speak to the majority of roof sealant manufacturers out there, and one which we, at Alltimes Coatings, support.

In simple terms, adding sealant under the overlaps of the metal roofing will prevent water spreading up between the two sheets.

However, it is commonly argued that by using a sealant, you are preventing condensation from the cold metal roof being released, causing inevitable corrosion anyway. With this in mind, let’s take a look at an old roof – where two metal sheets have had the seal and coating.

lifted sheet with corrosion

As you can see, there is visible rust, but it stops where the seal was applied, demonstrating that condensation isn’t coming down the metal sheets, only going up.

What happens if you coat the metal roof but don’t seal it?

If you only coat the roof, you can actually make things worse. Any movement between roof sheets of a coated-only roof can crack the coating at a join. This capillary action will result in water ingress and, you guessed it, corrosion. It’s worth noting that rainwater contains pollutants which will accelerate this process even further.

Treating Cut-Edge Corrosion with the Right Sealant

Of course, sealing under the roof overlap will only be effective if the sealant is of high quality. There are many different ways to seal the lap from mastics to tapes to embedding a fleece or glass into a liquid, so it’s important that you choose one that is both effective and easy for your applicators to get right.

Two of the main reasons why lap sealing fails is because the strength and flexibility of weak silicone mastics or tapes that lift on the profile corners allowing water ingress and retention.

Sealant Application

If the product isn’t installed to specification, it could result in a smaller bead of sealant and/or insufficient coating, meaning the product would not have the strength needed to cope with the movement of the roof sheet overlap.

Similarly, if too much sealant is used, you’ll end up with significant costly implications, which was explored in one of our recent articles .

This really does drive home the saying, “a product is only as good as the installer.”

Conclusion

To seal or not to seal: that is the question.

Clearly, there are a number of considerations before making that decision, and as you’ll see, there isn’t a simple yes or no answer.

To leave you with a lasting thought…

If you choose to seal overlaps of sheets before you apply coating, you must choose the right product and ensure your applicators are sufficiently trained.