HI


PLEASE HELP.
HOW DO YOU VENT SOFFITS IF YOU DON'T BOX IN THE EAVES?
WE DON'T WANT TO BAX IN THE EAVES BUT DON'T KNOW HOW TO VENT THEM. WE WILL HAVE RIDGE VENT BUT WE HAVE ATTIC BOX TRUSSES SO WE NEED TO VENT THE EAVES TOO.


THANK YOU

Depending upon how the fasica is attached, either flush to the buildings sidding or protruding out past the buildings side hung on the end of the rafter tail, you have two options. If the fascia(the board behind the gutter) is flush with the sidding you need to remove the gutter and instal fascia vents. Small circular vents about one and a half inches round between the rafter bays. Before reinstalling the gutters pad out the fascia with 1/2 inch(non rotting material) and reattach the gutter. If your eve boards extend beyond the side of the house and there is a small soffit(four to eight inches) you can use the small round louvered vents also, between the bays. Or you can choose to remove the soffit board, apply a continous soffit vent(a long narrow piece of perferated aluminium) down the middle of the soffit, cut the existing soffit to accomidate width of the vent and install over the flat tabs of the vent.

Inch-and-a-half vents will not provide NEARLY enough free area for proper venting of a roof. For an average size roof. they need to be more like 6 inches in diameter, and even then two per bay would be required.

A continuous screened vent might be a better solution, but it is difficult to describe without a detail drawing. Basically you would install a rough fascia on the rafter tails, but leaving 2 inches free at the top. Then you would install intermittent 5/4 blocking, and to that you install the finished fascia. Somewhere in there you have to get a strip of stainless steel screening.

Venting, this is staright form the book: "the total net free ventilating area should be at least 1/300th of the area of the space being ventilated, with 50% of the required area being at or along the ridge."
Openings should be protected against the entrance of rain, snow and insects.
If we use the exanple of an 12 foot by 16 foot house that has a gable roof, any pitch. The roof has a continuous ridge vent of 11 feet by three inches. The eave boards have 18 one and a half inch round circle plug vents. One gable end has a gable end louvered vent at two feet wide at its base. I will save you the math. You have no problem using the small vent.

Twelve by 16 is a mighty small house. If the house is 24 feet deep and sixteen inches wide, for purposes of sizing vents for one rafter bay, the area is about 32 square feet. One one hundredth of that is 0.32 square feet. Half of it is 0.16 square feet, or 2.3 square inches. A one-inch diameter minivent has a free area of approximately 40: of total area. or about 0.3 square inches. Therefore you would need 8 one-inch mimivents per rafter bay.

The gable vent is useless for the purpose of keeping the roof cold in winter, and is not necessary.

There. The math is done. One-inch minivents won't do it. Use the continuous screened vent.

Mr. Hetzel, mabey our vigourus debate will help educate the others here about net free area. Meaning that the infow of air at the soffit/eave must be equal to the outflow of airat the ridge . Extreamly important for a healthy ventaled attic or roof. I believe our point is that, to have adaquate eave and soffit ventilation. Personally, I would never suggest the installation of vents behind the gutter, but someones mistake is somebodys soultion. A better soultion for flush eave boards is a continoust venting drip edge. But that may take a great deal of effort.
This time I will not save you the math. Using Mr. Hetzel's example of a 24 foot house by 16 inches wide:
Total area: 32 SF
Net free area cal. x/300 15.3 SI
50% area at soffit/eave x/2 7.6 SI
two side of a roof x/2 3.8 SI
area of circle = pie x radius squared
3.14(3/4 x 3/4) 1.8 SI
3.8 SI/1.8 SI 2.1 units

Mabey, a two inch louver might be better, thanks for making me do the numbers again.

You forgot that the free area of the louver is in the range of 35 to 40 percent of the gross area, so you have to more tham double the louver size.