Detailing a Metal Roof
mikeysp
| Posted in General Questions on
Hi. I live in Zone 4a – Nashville. I am helping my son build a small office building (storage shed). We have the roof sheathing and underlayment down. Next is 1×4 purlins and metal.
I have two questions:
1. How do you deal with that large 3/4″ gap between the metal and the roof (Bug paradise)? Not to mention the three ridges on every 3ft section of metal roofing.
2. Doesn’t the underlayment need a way to drain AND have air flow? How does that work with 1x4s running every two feet?
I am not worried about bugs getting into the vented attic as I have screen to seal the soffits and ridge from critters.
Thank you.
-Mike
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Replies
Most people consider the air flow at the raised ribs to be adequate although I've heard of schemes to leave gaps in purlins, stagger purlins, etc. Same for a path for drainage. In the summer your roof is going to get plenty hot to evaporate what little moisture manages to get by your roof. I've owned several buildings with metal roofs in cold and hot climates. I haven't seen insects other than paper and mud wasps hanging on the eaves. I don't think they have an incentive to crawl up between the roof and underlayment.
My roofing manufacturer recommended solid decking covered by high temperature ice and water shield. molded foam closures fit between the deck and the roofing at the eave and between the roofing and the ridge cap.
Mike,
What advantage is there in using purlins over your sheathing?
Thank for all the replies.
I assumed just like the rain screen verticals I am using under lap siding on walls, it provides a way for air flow to dry and drainage so any infiltration of moisture is dried out quickly.
I am pretty new to building science kung-fu...call me grasshopper; and, please correct me is I am off base.
-Mike
Mike,
The answer isn't entirely straight forward. Most roofs are designed with an air-space (an attic or ventilated channels) so that the sheathing can dry to the underside. There is some debate as to whether providing a small gap (as you propose), and using a permeable underlayment will allow some drying to the top side. My own feeling is that without a dedicated path between the eaves and ridge, and a gap of at least a certain depth - say at least 1" - you don't get any appreciable benefit.
For metal roofing Matt Risenger recommends a solid roof deck covered with impermeable peel and stick membrane. On top of this he suggests two options. One is a layer of plastic fiber mesh about 1/4" thick. This provides a path to exit for any leaks thru the metal roof, but provides little ventilation beneath the metal roofing. The better option is to install 1x4 purlins, creating a 3/4" thick air space. Purlins parallel to the eave seem called for, as the metal roof panels run perpendicular to the eave. But purlins perpendicular to the eave seem called for, to provide a path for ventilation by the chimney effect; with air entering at the eave, heating up, and exiting at the ridge or gable. The answer is to run the purlins at a 45 degree angle to the eaves, interrupted by gaps between the ends of purlins in the same run. Any leaks drain downward along the top edge of the angled purlins until they reach a gap and drop down to the next purlin, and so on until exit. Ventilation air follows exactly the opposite path: rising along the underside of the angled purlin until it reaches a gap, then rising up to the next purlin, and so on to exit at the ridge or gable. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels.
Both mesh and purlins reduce summertime heat penetration to the interior somewhat, but I believe the significant advantage of the more effective ventilation gap provided by purlins is the reduction of the summertime temperature of the metal roofing itself. Dr. Joseph Lstibrick says reducing the average annual temperature of a building material by 20 degrees effectively doubles the lifespan of the material.
I remain to be convinced. A well installed metal roof should never leak. if it did, an impermeable membrane provides a secondary protection. To then add an actual drainage path for leaks seems to show a lack of faith in either the roofing or installation.
Purlins at whatever angle won't provide appreciable ventilation unless there is some mechanism to move the moist air. It may rise, it may not. At a conceptual level, it seems to me that that you are doing is trying to vent a gap you didn't need in the first place. If the roofing is tight to the sheathing , there is no air to vent, no condensation to dissipate.
Once you have provided a drainage path, you now have to reinvent how the eaves are detailed, as your path runs below the roofing. What a detail that will channel that water to the gutters looks like eludes me.
This really seems like something that has to be tested in various climate conditions. And over either a vented attic (where I agree roof venting is likely a waste), and over a cathedral ceiling (where I'm not so sure).
Any studies anyone?
What did you use for an underlayment? How are you flashing the eave, ridge and sides?
One thing I've always wondered is when you install a metal roof (either right on the sheathing or on furring strips) most people are told to stuff a molded foam closure into the edge gap to prevent insects, etc from getting in under the metal.
How does that affect the air ventilation? How does any water drain out if the edge is full of foam?
Scott,
How does water get out from under asphalt shingles? The answer is the same: What water? Why would there be water under either shingles or metal roofing if that roofing is directly attached to the sheathing below? Where has this water come from?
Well, as the great building science God Joe Lstiburek says, water will ALWAYS get in. Into walls, into foundations, into roofs, everywhere. Perhaps it gets blown in past the flashing during a storm (flashing standing seam metal roofs at the ridge is especially tough). Or perhaps through poorly sealed and flashed chimney or plumbing vents.
Shingles are typically nailed right to a roof deck so there shouldn't be much of an air gap beneath them but with a metal roof there will be voids (such as under the standing seam). Since a lot of metal roof manufacturers suggest that foam enclosures be installed at the eave and ridge of the roof to prevent insects from getting under the panels doesn't the foam impede the ventilation under the panels (and also allow water to drain)?
I mean, why bother installing furring strips at all if you're going to block the eave openings under the metal panels with foam enclosures?
Scott,
I've used foam closure strips at the ridge, but never even heard of using them at the eaves. I agree it sounds like a dumb idea.
In a wildland fire area, foam seems like a really bad idea, compared to stainless steel spark arresting mesh & intumescent closures.
Three additional points:
1) I should have mentioned that any ventilation gap openings should be enclosed with mesh or perforated angles of stainless steel or copper. Of course, foam closures would block ventilation.
2) The primary reason for the purlin ventilating space is extending the lifespan of the metal roofing by reducing it's average temperature; creating a path for leaks is way secondary, but it does no harm.
3) I agree with Scott that no roof is perfect, water will get in eventually. How much money and work one is willing to expend at installation in order to delay that eventual leakage and failure is a judgement call based on budget. Fiberglass shingles directly on tar paper is a quite reasonable option, and will last about 25 years. A metal roof directly on an impermeable membrane will last about 40 to 50 years. That same roof on purlins will last an additional 10 to 20 years.
"A metal roof directly on an impermeable membrane will last about 40 to 50 years. That same roof on purlins will last an additional 10 to 20 years."
I've looked for some research that supports the increased longevity by installing on purlins, but can't find any. Can you point me to where you saw the evidence of prolonged roof life?