The Case of the Rubber Boot Flashing and the Metal Roof

By Martin Aubé
Technical Trainer, SBI (Stove Builder International)

Note: Wood Energy Technology Transfer Inc. (WETT Inc.) is a non-profit training and education association. WETT does not endorse nor recommend any specific manufacturer, distributer, retailer and/or appliance type or component. This information is being provided for informational purposes. The provision of this information should not be seen as a recommendation or endorsement by WETT Inc.

Metal roofs are becoming more and more common. With that comes a different approach to the flashing. Everyone should be aware of the standard shingle roof and standard cone flashing, but things tend to change when the metal roof is added to the equation. Some manufacturers have their own specific metal roof flashing. Some of these consider a provision for the ribs of the steel roof. Some also add in their own wedge/snow splitter for a more durable flashing. These flashings are made available from chimney manufacturers and should always be the first choice at keeping all components with matching brands.

Now enter the rubber boot flashing, widely available from most steel roof companies through some hardware stores and other avenues. Rubber boot flashings are designed for low-temperature vent stacks, not chimneys.

We have discussed previously rubber boot flashings in general terms, mostly referring to the standard black rubber boots we see. They have introduced a higher temperature red silicone rubber flashing. Even with its higher heat temperature rating, although roofing companies may support and provide these flashings for their 50-year-no-leak warranty, these rubber boot flashings are "NOT" approved by the chimney manufacturers.*

*Note that there is one manufacturer that has made a black rubber boot version available for their chimney. This is different, as there are spacers to install to keep the rubber off the chimney.

What, again, is the real issue with rubber boot flashings?

Well, the rubber is the issue. Yes, the red silicone rubber flashing has a higher temperature to help resist breaking down from heat. However, the temperature ratings are still below the code requirements for certification with a manufactured chimney.

So, what does this mean? In terms of a WETT Inspection™, these rubber boot flashings are currently not approved for use with insulated chimneys.

Metal roofing companies are complicating the issue by stating that only their rubber flashings will provide their roof-no-leak warranty. All rubber boot flashing may provide great water-shed protection but they do not have any strength against impact or pressure. The rubber flashing cannot only slide down the chimney breaking the seal, it can also be reversed with enough weight and become a funnel, channelling water down the outside of the insulated chimney and possibly into the conditioned living space.

What do we do about chimney installation requirements? For instance, many installations require a ventilated flashing. Remember that manufacturer with the rubber boot and spacers? That one has ventilation. None of the other standard rubber boots do, regardless of the colour of rubber. If the installation requires ventilated flashing and any non-ventilated flashing is used, you will have an issue from a WETT Inspection™ standpoint.

In short, we need to contact chimney manufacturers and metal roofing companies and find a compromise. Chimney manufacturers have a superior flashing for metal roofs; these flashings should be used according to brand.

What this really means is that roofing companies do not want to learn about chimneys. They only install what is easy. This is not an easy industry.