Q&A – Adding asphalt primer

Recently, a homeowner reached out to ask for some help understanding the different quotes they got from roofers.

Deb, a homeowner from California, reached out to our experts with this question: 

Hi, I am having my roof replaced and have dutch gutters. The roofer is recommending to apply a coat of asphalt primer to the galvanized sheet metal then add a GTA torch roll over the inner ply as a final covering. Is that advisable? I have had several quotes but only one mentioned this.

One of our experts, John Kenney from Cotney Consulting, responded to Deb's question, saying: 

Deb, the roofer’s proposed approach is not unusual. A properly designed modified-bitumen system over a built-in or Dutch gutter can be a durable waterproofing method, and asphalt primer over properly prepared galvanized metal is commonly used where the roofing membrane will bond to the metal.

What matters is the complete assembly. The galvanized metal should be sound, clean, dry and properly secured and the modified-bitumen membrane needs to be integrated with the roof above, including the corners, outlets, seams and transitions. Sheet metal moves with temperature changes, so the membrane detail also needs to account for that movement rather than simply covering the metal with another layer.

The GTA product you mentioned is typically a granulated, torch-applied APP modified-bitumen cap sheet. Torch-applied modified bitumen has a long history in the roofing industry, but I would pay particular attention to fire safety in your case because the photo shows significant wood construction around the gutter. NRCA recommends proper fire-resistant backer details and trained torch applicators when torch-applied membranes are used around combustible construction.

There are also other proven modified-bitumen options that may be appropriate depending on the manufacturer and detail, including cold-applied SBS and self-adhered systems. Those can sometimes reduce the need for open-flame work in areas where wood construction makes torch application more difficult.

My recommendation would be to ask the roofer for the exact primer, base or interply and GTA products they intend to use, along with the manufacturer’s approved detail for lining this type of gutter. I would also ask whether they are CERTA-trained or otherwise specifically trained in torch-applied modified-bitumen work and what fire-protection measures they will use around the wood construction.

I would not reject the proposal simply because the other contractors did not recommend the same approach. It may actually be a more complete waterproofing detail. I would just want confirmation that it is being installed as an approved system and that the torch work is being handled safely.

Continue the conversation in our Forum.