Metal Roof – Peel and Stick Underlayment for a vented roof deck?
I am in the process of contracting for a Metal Roof on my current house and have a question of using what type of Peel and Stick (SA) underlayment to use. The house was built in 1986 and is in Zone 6A (MSP). It has a vented roof deck over a conditioned attic containing A/C ductwork. The underside of the roof assembly is plywood attached to the roof rafters and covered with 2″ of ccSPF. The roof vent has continuous ventilation on eaves and a vented ridge. The topside (roof deck) is one-half inch OSB (circa 1986) is currently with shingles. Pitch is 6/12. The AccuRite sensors in the attic show a range of 50 – 80% RH during the winter. For the replacement roof, one of my options is a 138T panel with ASV clips – which hold the metal panels 3/4″ off the roof deck (think rainscreen for a roof). The plan is to remove the shingles and place a peel and stick on the topside of the roof deck. Lots of choices but two example products are: (1) Platinum FT SA (0.035 perm); (2) VaproShield Slope Shield Plus SA (30 perms). The underside of the roof deck OSB is exposed to the vent – so that does provide a path for water vapor to escape if the OSB is exposed to water. Currently, I am re-reading through the BSI material (BSI 038 and CP 1101). Any thoughts/suggestions/recommendation from the GBA experts?
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Replies
BirchwoodBill,
If you have a ventilation channel under the roof sheathing, I think any benefits of drying to the top side by using a high perm underlayment are outweighed by the increased risk of water intrusion from above. So I would choose an impermeable underlayment rated for use under metal roofing. This underlayment doesn't have to be peel & stick.
I would also not include the small vent channel above for a few reasons:
- The vent channel encourages moist air to condense under the roof panels, which you then have to remove by ventilation, or a drainage path. No air gap means no moist air to condense.
- If there is enough water to drain, what does that drainage path look like at the eaves? How would it integrate with your drip edge?
- Suspending the panels makes them structural. They can no longer rely on the sheathing below for support. This makes them weaker and more susceptible to damage from debris or workers moving on them.
Birchwood,
If you have a ventilation channel below the sheathing, it is moot which vapor class underlayment you are considering. Any moisture that makes its way to the vented channel will be whisked away by the air convection. Any moisture that gets to the bottom of the sheathing in this channel can dry to the interior of the vented gap. A vapor closed membrane would be necessary by code for low slope roofs, but you don't have this issue. I would choose whichever membrane is more cost effective or available in your market.
I would avoid the small vent below the metal roofing. See this response I penned to a Q&A Spotlight about rigid insulation on roofs. It touches on this issue in more depth:
https://www.greenbuildingadvisor.com/article/off-gassing-worries-upends-a-renovation
The underlayment doesn't have to be peel and stick, but I would recommend it. Having something fully adhered to the sheathing will be more resilient to bulk water intrusion.
Josh