Can someone tell me what the min. and max. width is for a ridge vent opening cut in roof sheathing? I can't seem to find it.

The width of the opening in the sheathing on the house I did today was
1" - 2" wide.

I think it should be about 4" wide but was wondering what the spec's are.

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The amount of open vent area required will depend on several factors, such as:
is this a gable roof with gable vents, or hip roof, any other roof vents (power vents, or turbine), and of coarse sq ft of roof area.
But ridge vent openings are normally 2- 3"

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Rick Cantrell
Columbus GA

Wes,

http://www.airvent.com/pdf/installat...II-install.pdf

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i just inspected a large house with 1/4" to 1 " ridge slots on either side of the ridge board, it just doesn't look correct ! Is there a spec for this - it was a large attic
Thanks

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Peter Buss

Peter,

Rick posted a link to a site that should help.

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Absolute Home Inspections, LLC

Obviously it depends on the manf. of the vent. Years ago I wrote up oone that I thought was too small. The builder pull out the manf specs and there it was....3/4" TOTAL width. Looks like Ricks is 1 1/2" total.

JF

Wes,

I have been through this several times and as the link Rick gave you shows, 3/4 inch on each side is enough. It does'nt look right but it is.

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Let's do some math.

house is 40 feet long by 30 feet deep, or 1,200 sq ft. This is the 'attic area' which includes a 2 foot overhang, not the 'house area'.

Required ventilation is 1:300 if 50% or more is near the ridge, 1:150 if soffit vents only.

1,200 sq ft / 300 = 4 sq ft / 2 = 2 sq ft. If *at least* 2 sq ft of net free vent area is at the ridge, then we can use the 1:300, otherwise we need to use the 1:150 rule.

Okie dokie, the ridge is 40 feet long, or 480 inches long less 21 x 1.5 = 31.5 inches for a total open area of 448.5 inches.

The ridge vent slot in the sheathing is 3/4 inch to each side of the ridge board, or 1-1/2 inches wide total. 1.5 X 448.5 = 672.75 sq inches.

672.75 sq inches / 144 = 4.67 sq ft That makes it doable, now, though, what is the net free vent area of the ridge vent itself?

This will be given in sq inches per linear foot.

The net free vent area of the ridge openings are 672.75 sq inches / 40 = 16.8 sq inches per linear foot. Thus, as long as the given sq inches of ridge vent are less than 16.8 inches per linear foot, that 3/4 inch cut out to each side of the ridge is going to be adequate.

Anyone want to check my math and make I did it right? Thanks.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
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The minimum and maximum width of the slot are not usually specified by code but by the manufacturer's installation instructions. The code will let you figure out the minimum square inches of net free vent area NFVA) that you need.

The manufacturer makes a vent that has a certain amount (in square inches) of net free vent area per lineal foot of ridge. This sets a limit on how narrow the slot can be, so that the vent and not the slot is the limiting factor on air flow through the vent. The vent itself will be made to a certain width and only extends to cover the roof a few inches on either side of the ridge. The manufacturer will specify a maximum slot width which, if not exceeded, should ensure a watertight joint between the vent and the roof covering.

Let's go back to that minimum slot width again and see how narrow it could be. I've seen typical ridge vents with NFVA as low as 9 sq in per ft of ridge and as high as about 18-20. Take that low flow vent at 9 sq in/ft. If you use that one, you need at least 9/12 or 3/4" total width of a slot (clear of the ridge board) or 3/8" on either side of the ridge. This is to ensure that the vent, and not the slot, is limiting air flow. Take a higher flow vent at 18 sq ft/in. On that one you need 18/12 or 1.5 inches minimum total width (clear of the ridge board) or 3/4" on each side if the ridge. Cutting slots wider than the minimum based on the NFVA of the vent you are using does nothing to increase ventilation; it increases the chances that the vent will leak.

Check on Jerry's math:

2006 IRC 806.2 has the requirements.

Let's assume a gable roof and our ventilated space (inside the attic) is 40 feet long (at the ridge) by 30 feet wide (eave to eave) or 1200 sq ft.

Using 1/150, the required total NFVA is at least 8 sq ft. Using 1/300, it is at least 4 sq ft.

Let's assume worst case of NO soffit vents and either no vapor barrier or one with an effective perm greater than 1. If we cut a slot along the entire length of the ridge inside the gable end walls (assumed to be 40 feet), we would need a minimum clear slot width (8 sq ft / 40 ft) x (12 in/ft) = 2.4 inches (total on both sides of the ridge board). For this to work, we'd need to find a vent that provided at least 29 sq inch NFVA per foot of ridge. (Note - some manuf say you couldn't do this configuration with their vents, because their install instructions specify use of their vent in conjunction with soffit vents.)

Let's assume a more typical case where we have soffit vents and they provide at least 50 percent of the required NFVA. Now we can use 1/300 which requires 4 sq ft total. We are going to meet that by providing 1 sq ft at each soffit and 2 sq ft at the ridge. Our slot running the full length of the ridge (inside the gable end walls) can be reduced to (2 sq ft / 40 ft) x (12 in/ft) = 0.6 inches (total on both sides of the ridge board). For this to work, we'd need to find a vent that provided at least 7.2 sq inch NFVA per foot of ridge and run it the full 40 feet.

Let's say we want to use one of those low profile ridge vents with the filter inside, that provides NFVA of 9 sq in/foot. For this vent we would need a slot 3/4 inches wide and (2 sq ft x 144 sq in/sq ft) divided by (9 sq in/foot) = 32 feet long. Use one of those higher flow ridge vents (NFVA of 18 in/foot) and you need a slot 1.5 inches wide (3/4" on each side of ridge board) and only (2 sq ft x 144 sq in/sq ft) divided by (18 sq in/ft) = 16 feet long. In theory, you could use that higher flow vent on a 3/4 inch slot if you ran it for 32 feet. BUT code says you must follow the manufacturers instructions and they will tell you how wide of a slot to cut for their vent.

Most manufacturers recommend a minimum of 50% venting at the soffit. The system performance is significantly comprimised if there is no soffit.
Also the width of the slot vaies by manufacturer. Proper width is important to eliminate the potential for weather infiltration. Make sure you install a vent with "Dade County" approval.
Benjamin Obdyke inventors of the original ridge vent on a roll has a tech group that will run the numbers for you.

Residential Roof Materials and Ventilation, Wall System Products

Using the standard ridge vent and ridge cap the maximum size on either side of a 2x ridge is 1.5".Installation instructions will say less.

Michael,

Where does your 1-1/2" come from?

Then that would be the maximum - whatever is stated in the manufacturer's installation instructions.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
www.AskCodeMan.com

The slot width should reflect the NFA (Net free area) of the vent. If a vent is rated at 18 sq. in per lin. ft (most common) - ie 9" per side, a 3/4" slot assuming no restriction would be adequate. Typically 1" is specified to offset any restriction that maybe designed into the vent itself.
Put another way....The slot on each side does not need be any greater than the height of the vent off the deck as this is the "limiting factor". If the vent is 1" off the deck a 1" slot is adequate (each side).

Jerry,from my own installation experience.[1.5"] That is not the manufacturers number.The 1.5'" allows just enough room to nail the vent and cap.If in doubt follow the manufactures instructions.

As a somewhat related situation.... I can't stand it when a bath fan duct termiates at a ridge vent. It just doesn't work. The AHJs in my area allow it (no surprise - it's still the wild wild west here in a lot of ways) but it just doesn't work. There weren't too many ridge vents 20 years ago but as a % I sure see a lot of ventilation problems with them.

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GAF says to cut 3/4 inch from both sides of the ridge, measure each side and if the total opening is less that 1", cut additional amount from each side until the opening equals 1 inch. Stop the cut a minimum of 12 inches from hip and ridge intersection, the outside gable or other intersection.
Do not remove more than 2 1/4 inch from either side of ridge.

http://www.gaf.com/Content/Documents/RID0001.pdf

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Jim Luttrall
www.MrInspector.net
Allen, Texas 75002