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Which roof ventilation system is right for my attic?

My attic space is 965 square feet. There are four 1ft x 1ft pots at the top of each of the four dormer roofs. There is also a chimney that comes through the attic and blocks the air path from the gable vents (approx. 1ft x 1ft on each side of the house). I'd really like to install a roof power vent. Should I seal off the upper 1ft x 1ft pots and install four lower pots for this?

Pittsburgh, PA

Well, let's figure out how much attic ventilation you should have as a first step. You have an attic space that is 965 square feet of floor area. Dividing that by 150 gives you a Total Net Free Ventilating Area of 6.4 feet. That figure is used because the Federal Housing Administration recommends that there be proper attic ventilation for every 150 square feet of floor space in the attic. However, that may increase to as much as every 300 square feet in older homes that aren't quite as airtight as newer construction.

To convert the 6.4 feet over into square inches we multiply it by 144 giving you a ventilation recommendation of 922 square inches. While a 50/50 split between intake and exhaust ventilation is considered adequate, many professionals suggest that intake should be 60 percent of the ventilation leaving 40 percent for exhaust. Using that formula for your attic, 553 square inches would be needed for intake and 412 for exhaust.

At the moment, you really don't have any vents properly functioning to pull air into the space as all of the existing units are high on the roof surface. My recommendation would be to install a product such as GAF's Cobra venting which allows application even if the building has no soffits. This should let plenty of outside air to enter at a lower point in the space which should create the cross ventilation that your attic needs.

If the pots in the dormers can exhaust inside air from the entire room, then you may already have more than adequate ventilation to the outside between them and the gable vents. That would give you 864 square inches of exhaust when you only need 412.

I really don't see the need for a roof power vent unless you really want to have one. If you do decide to install a unit, ventilation throughout the space should be more thorough if you lower the pots on the roof or install fascia venting.

As far as the chimney that divides the space, I don't really see that as being an issue. As long as you have adequate intake and exhaust ventilation in the two separate sections it creates, everything should be fine.

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Roofing vs. Reroofing

Roofing vs. Reroofing

If you come to find out that your roof has been damaged and needs to be fixed then there are two main approaches you can take. You can either decide to make a roof repair or replace the entire roof. Roof repairs will generally save you time and money, but in most cases a new roof is the best option. As a general rule of thumb, if the existing roof on is only one layer and there are no problems with the roof deck, a new roof is not necessary.

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