Metal Roof Leaks

Dirt Seal Effect and Leaks on Metal Roofing

Metal Roof Leaks

Identifying and isolating the source of metal roof leaks can be frustrating and time consuming. Leaks can originate from the opposite side of the building and make its way across the purlins. A thorough understanding of the metal roof construction will help you pinpoint the cause of the roof leak.

A majority of metal roof replacements are done because of leaks. These replacements are done prematurely and can be prevented.

History

In 1997 to 1999, I was a consulting with one of the largest metal roofing manufacturers in Asia. I was frequently called to troubleshoot recurring leaks on their newly installed projects. Most jobs were less than a year old and some were barely 3 months old when they started leaking. I spent a great amount of time under a roof during a thunderstorm with a flashlight in tracing a leak. During this time, I constructed a couple of roof sections over a driveway using skylights of the same profile as the ones I was frequently examining. I wanted to see the progression of roof leaks as they developed.

The Dirt Seal Effect (DSE)

The picture on the right shows you how dirt accumulation on the vertical laps creates a seal. The width of the seal varies but it is always thicker near the purlins. Water enters through breaks in the seal and travels the length of the vertical seams until it encounters resistance near the fasteners. The water backs up and goes over the corrugation and down the screw hole.

Caulking the fastener does not help in this situation. As you can see in the following pictures, the water is between the metal sheets (below) where caulking could not reach.

DSE occurs very slowly. Rainy season starts with a drizzle. Just enough to moisten the dirt but not enough to wash it down.

Replicating DSE with Paint or Tape

Ever wonder why a metal roof leaks shortly after it is painted? Paints are not elastic and crack when the roof flexes. A thin coat of paint can seal the seams similar to accumulated dirt.

Most leaks occur near the gutters. Most roofers will attempt a repair using roofing mastic or seam tapes and aggravate the leak. When seam tapes are installed halfway up the roof, it replicates the dirt seal effect. Because of this, always install seam tapes along the entire length of the vertical seam. Tar or asphalt based roof mastics dry out and crack over time, making thing worse.

Do not tape horizontal seams without taping the verticals. If you tape the horizontal seams exclusively, you are replicating the dirt seal effect along the intersection of the horizontal and vertical seams. The water dams up behind the horizontal seams, causing more leaks than before.

Conclusion

Compared to other roofing types, metal roof leaks are still the easiest to troubleshoot and fix. DSE is common on corrugated profiles similar to the picture above but also occurs on R-panels to varying degrees.

Roof pitch affects the severity of the DSE directly. We worked on several churches built in the 1950’s and they had very steep roofs. It has been painted several times and the DSE was caused by the build up of paint which was cracking. Rainwater was coming down this roof at a higher rate and the leaks were a steady stream of water. It did not matter if it were a light rain or a hurricane (typhoon).

A full seam seal and metal roof coating application can prevent DSE. Seaming can be done with a fabric backed seam tape or polyester roofing fabric and roofing adhesive. Unlike paints, roof coatings are designed move with the substrate.