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dpreznik 04-12-2007 11:35 AM

Ridge vent question
I invited a contractor to install a ridge vent. But I noticed theat the vent doesn't reach edges of the gable, about 5' on each side. The contractor said that was supposed to be that way. But later I have never seen anywhere a ridge vent with that much left. Could anybody please explain to me what is right way to do it, and whether I was wronged by the contractor.
Thank you.

cibula11 04-12-2007 12:08 PM

How much does your soffit overhang? Typically we install ridge vent about 2 ft from the end. But if you have a 24" soffit then you would start it probably 2ft from that so you are looking at 4ft from the end. I'm sure it has been installed properly. Did you ask him why it was 5ft?

cibula11 04-12-2007 12:09 PM

you also would want to start back from the edge so that a high wind gust doesn't lift the ridge vent out .

Ed the Roofer 04-12-2007 01:21 PM

The correct length of ridge vent is derived from calculating the NFVA per lineal foot of exhaust vent product installed. The ranges are from 12 square inches to 18 square inches per lineal foot dependant upon the specifications from the manufacturer.

You need to comply with the shingle manufacturers specifications and the local building code ordinances.

You need One square foot of total exhaust ventilation for every 300 square feet of the attic floor space, only if you also have an equal amount of soffit/eave intake ventilation or if you have a proper continuous vapor barrier under all of the attic insulation facing the warm side of the ceiling.

If you do not meet those two criteria, then the calculation for exhaust ventilation changes to One square foot for every 150 square feet of attic floor space.

Example: 40 foot long house, from gable edge to gable edge and 30 feet deep from front eave edge to rear eave edge.
This equals 40' X 30' = 1,200 square feet

Now, either divide 1,200 by 300 or 150 depending upon which scenario existed on your home.
That is the total amount of Square FEET of exhaust ventilation you need.

Wait a minute! Now, you have to convert the square feet into square inches.

Lets presume the minimum amount necessary. Divide 1,200 by 300 = 4 square feet required. one square foot is 12" X 12" = 144 square inches, so you need a total of 4 X 144 = 576 square inches.

Divide 576 square inches by the amount of NFVA from the ridge vent product, and that will tell you how many feet of ridge vent is necessary.

Using a ridge vent providing 18 square inces of NFVA per lineal foot would determine; 576 sq in / 18 sq in = 32 lineal feet required.

But, for aesthetic continuity, I would prefer to offer additional ridge venting running the entire length of the ridge line from gable edge to gable edge. This enhances the overall finished look of the roof view and also allows for additional exhaust ventilation to be provided for.

Your contractor may have installed the ridge ventilation correctly for functionality, but not for aesthetic appeal.

The previous poster is incorrect about the ridge vent being susceptible to wind blow off, if the proper sized nails and the correct nailing pattern are followed.

Some roofers use a roofing nail gun to install the ridge vents, which only allow a maximum of a 1 3/4" nail, which barely penetrates the deck sheathing. I use 2 1/2" hand nails to ensure a secure attachment, and go from end to end. It looks nicer and provides more ventilation.


dpreznik 04-12-2007 02:03 PM

Thank you everyone who answered. I didn't mentioned that my house has cathedral ceiling, it is 47' long and 26' wide, the roof has two layers of shingles, and the ridge vent for gable has 1" down each side. Maybe you can add something?

Thank you very much.

Ed the Roofer 04-12-2007 02:28 PM

Do you have a continuous air space above the insulation in the individual rafter bays?

Do you have 100 % continuous soffit ventilation toallow for the ridge ventilation to have air to be exchanged with?

If you did not have an air space in each of the rafter bays above the insulation by a minimum of 1" to 1 1/2", were insulation baffle vents installed on top of the insulation to provide for this clearance of an air pathway?

Their are conflicting studies regarding the potential benefit from providing a balanced ventilation system in houses which contain a cathedral or vaulted ceiling, but the building code and the manufacturers specifications still take precedant.


dpreznik 04-12-2007 02:37 PM

Yes, I have soffit vents and plastic vent channels between the roof and the insulation each of the rafter bays.
I had a big problem with condensation during winter time which I discussed in the "Water in the insulation" thread. I hoped the ridge vent would help, but still not sure.

RooferJim 04-13-2007 03:03 PM

This is a big pet peeve of mine. The ridge vent should ALWAYS ! run the full lenth of the ridge. you can always keep you cut back from the edge or a chimny but you should always run out the vent itself and the caps the full lenth . Otherwise it is not symetrical and throws off your ridge line and makes the whole job look like sh#t. Truth is some roofers are just too lazy and cheap to cut into another roll or lenth of vent and just split the difference. There are still one or two jack asses in my territory that will do this but not any real roofing company. Have him fix it


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