(Written by Jack Milarch – NMHBA EVP/CEO – Originally published in the May 2012 Housing Journal)
Roof Venting Challenges in Residential Building Code
There has been a requirement in New Mexico’s International Residential Code, Section R806.2, for several years that was re-worded in the 2009 version and has led to new interpretation that increases the requirement for roof venting significantly. Designers and builders (especially those who haven’t built a house in a while!) may be surprised to see the new interpretation of the requirement for venting that amounts to a ratio of 1:150 of roof area. There is an exception to allow half of that amount if optimal vent positioning can be accomplished, reducing the requirement to 1:300, but even that is still extreme venting. Climate zones don’t matter in this requirement because it is designed to address the humidity-laden indoor air concerns as houses are built tighter and tighter.
In case we haven’t gotten your attention yet, let’s run through an example from a recently permitted home. This home has eaves all around, a large garage and two nice attached covered patios so the total roof area is 3930 square feet, with a heated area of 2413 square feet; obviously a bit more “roof intensive” than many homes, but certainly not all that rare as custom homes go. From the venting calculations point of view the entire roof counts. The roof venting notes from the plans for this particular home design read:
Roof shall be ventilated in accordance with IRC Section R806.2; 1/300 exception follows:
Soffit (eave) vents located in the overhang shall be LP “Smartside” Vented Soffit and provide up to 50% of the required area; 10 sq. inches per lineal ft. = .069 sq. ft. per lineal ft.
Roof vents shall be “Lomanco” model 750 and provide at least 50%, and not more than 80% of that which is required. Vents shall be located near the ridge and in no case less than 3′-0″ above the soffit (eave) vents; 50 sq. inches per vent = .347 sq. ft. per vent.
• Total area to be ventilated, including porches, garage and overhang: 3,930.0 sq. ft.
• Total vented area required at 1/300: 13.1 sq. ft.
• 50 % soffit (eave) vent required: 6.55 sq. ft.
6.55/.069 = 94.93 feet required; provide by installing 6: 16″-0″ sheets, or 96 lineal ft. for a total of 6.624 sq. ft.
Place no vents on raked soffits or adjacent to gas meters. Uniformly distribute on both sides of the building.
• 50% upper roof vents required at 1/300: 55 sq. ft. 6.55 sq. ft.
6.55/.347 = 18.65 vents required; provide by installing 20 total “Lomanco” model 750 roof vents or 6.94 sq. ft. Place within 24′ of high ridge over main roof and garage roof, equally spaced as follows: FRONT, 10 vents (3 over garage, 7 over heated area) and REAR, 10 vents offset from those on the front of the house (3 over the garage, 7 over the heated area of the house)
- Total roof area venting provided: 13.564 sq. ft.
- 624 sq. ft. (48.8%) in eaves
- 94 sq. ft. (51.2%) in upper roof
As an alternative you may be able to get extreme venting by way of gable end vent applications.
“Flat” roofed design challenges
Many builders have quit installing vents on the backside of parapets because these are notorious “leakers”, but if you build this way you might get stuck using the rule without the exception. In buildings with very low slopes you have to comply with the 1:150 requirements. Study the rule, the exceptions, and your designs very carefully. Let’s assume you use the 1:150 rule with a flat roof and typical round vents.
A common 4” round “flat hat” type vent yields about 12 ½ square inches of vent area. Therefore it takes about 12 of these to get a square foot of vent area. A quick math exercise for that same house with a flat roof (3,643 sq. ft. without eaves), calculated under the 1:150 rule potentially needs 25 square feet of vent area. You would therefore theoretically need 300 of these things to get to the total vent area! Obviously that isn’t a good solution. You may need to investigate power vent options or other creative solutions to solve this problem.
This issue has come to the forefront because code officials are also suddenly choosing to re-interpret and enforce the setback requirements for fire-resistance rated projections when less than 5 feet from the property line (Section R302.1). This is a big deal since there is no such thing as a fire-rated soffit assembly, and the new interpretation flatly requires that when the eaves project into the 5-foot setback from the property line. This means soffit vents won’t be allowed in eave areas and compliance with the ventilation requirement gets even more difficult.
Builders should consult with your designer and/or roof contractor to make sure roofs are being ventilated properly. You may also want to consider other alternatives like semi-conditioned attics or mechanical means of ventilating to reduce the number of roof penetrations required.
Special thanks to Jim Beverly for noting the problem and providing the examples for this article.
The actual code quotation is as follows:
R806.2 Minimum area. The total net free ventilating area shall not be less than 1/150 of the area of the space ventilated except that reduction of the total area to 1/300 is permitted provided that at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above the eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1/300 when a Class I or II vapor barrier is installed on the warm-in-winter side of the ceiling.