Replies

1. GBA Editor
   Martin Holladay | Oct 03, 2018 04:51am | #1

   User-7187399,
   First of all, can you tell us your name? (I'm Martin.)

   The formula for determining the net free area required for soffit vents is as follows: Assuming that a roof system has both soffit vents and a ridge vent, the standard code formula requires 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the lower half of the attic (generally at the soffit) and half near or at the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area.

   More information on this formula can be found here: "All About Attic Venting."

   Manufacturers of aluminum soffit vents should provide the net free vent area per linear foot of their soffit vents on the packaging.

2. Expert Member
   Peter Engle | Oct 03, 2018 07:57am | #2

   The wood soffit should be removed completely, or at least sliced open along its length. Your rectangular vents probably no longer provide enough vent area, as the screens become clogged with paint and dirt over time. Even if not, they are unlikely to line up well with the new ventilated soffits.

   There are different styles of aluminum vented soffit, so it is difficult to post specifics, but I have seen a number of roofs rotted out during your process, because the perforations in the new soffit are not compatible with the rectangular vents. The net result is that little or no venting takes place. If the new soffit vent provides 5 square inches of free vent area per foot and your rectangular vents provide 20 square inches but are located 4 feet apart, then it would seem that they provide the same free vent area to the roof. But if the new soffit is tight to the wood, you might only get 18" of the new soffit vent lining up with the 18" of original vent, meaning you only get 7.5 square inches of total venting in 4', for a 75% reduction in total venting.

   The most effective soffit venting is continuous venting that vents each rafter cavity. The best way to achieve this during a retrofit is to remove the wood soffits before installing the aluminum. Follow the links Martin provided to calculate the net free vent area needed for your roof and make sure the specs on the aluminium provide enough. Finally, make sure that the insulation in your attic is not blocking the free flow of air from the soffits into the attic.

3. 
   Michael__R | Oct 04, 2018 11:42am | #3

   Hello!! My name is Michael.

   Thanks very much for you feedback.

   I will discuss these comments with the contractors that will provide me with quotes.