Roof Ventilation
DC Metro Area
insights on residential architec-
ture have been published in
House Beautiful, the New York Times, Southern Living, the Washingtonian, Washington
Post, Colonial Homes and Other periodicals. Ask the Architect appears frequently in the Times Mirror news group, and has been featured in titles published by Media General, Network Communications and others.
“I am considering building a home with a shed roof. My concern is roof ventilation. How do you insulate a shed roof and make sure the roof is vented? What is a minimum pitch, using metal roofing?”
Answer:
First, you do not have to create a vented shed roof. Most building codes allow for unvented roof construction. However, creating a vented roof gives a thermal separation between the roof and the conditioned (heated and cooled) spaces of the house, so the sun baking on your metal roof won’t over-stress your air conditioning in the summer, and snow will melt off your roof slower in the winter, causing fewer ice dams and other water worries. The vented roof also gives a path for moisture in the air, whether from outside or inside the home, to escape the roof structure.
Vented Roofing
The typical vented roof has a vent at the lower eave and a vent at the higher. Air can move through the attic space freely, and the floor of the attic is insulated to thermally separate the unconditioned attic from the living spaces below. If you want to vent your shed roof and don’t want an attic space, you’ll need to allow air to travel from a vent at the lower eave to a vent at the higher eave between the roof sheathing and the metal roofing. There doesn’t need to be much space, but there needs to be a continuous path for the air to travel freely, and you’ll need some sort of air barrier on the outside of your roof sheathing to minimize any moisture getting into the structure. The continuous path for air travel can be achieved by furring strips, or a roof underlayment layer that air can move through. Most roofing manufacturers will happily share construction details with you, including recommendations for how to separate the roof from the structure.
Unvented Roofing
If you go with an unvented roof, you’ll need to make sure you control moisture traveling through your roof structure. With a decent roofing material, you shouldn’t worry too much about water coming in from outside, but you do need to consider how moisture already in your home will travel through into your roof structure. You don’t want warm, moist air to travel halfway through your roof structure and hit colder air, causing the moisture to condense and leaving water inside your roof structure. There is a lot of building science that goes into the successful design of an unvented roof, and you’ll want a local qualified consultant to help you with the design and construction.
There are many types of metal roofs and many manufacturers. Most typical is a standing seam roof, and they usually have a minimum roof pitch of 3 inches of vertical for 12 inches of horizontal (3:12). Flat seam roofs, where the seams are bent over and soldered together can have a lower roof pitch, typically 1:12. Metal shingle roofs will likely need a steeper pitch. These are just guidelines, and every roofing manufacturer will tell you the minimum roof pitch that their products can be installed at (and still warranty).
Have another question? Want to know more specifics about roof ventilation? Feel free to contact us!
About Bruce
Bruce Wentworth, AIA, is a practicing architect whose insights on residential architecture have been published in House Beautiful, Southern Living, Washingtonian, Colonial Homes, and other periodicals.
