Roof Vent Locations

[Aaron Hayward]

First time post!

1000 sq ft home, hip roof 5/12, truss span approx 30'. Perforated soffit isntalled along the entire eave of the home. The problem potentially is that the roof vents (two) are installed about 1/3 down the top chord of the truss and i worry that they may not be high enough, to accomodate the 25% venting at the top rule. What i am unsure of exactly is what categorizes the top portion of the roof.

My gut tells me that the hip helps, wind from all direction, and that the volume of attic space is minimal and that what is installed may work.

Any ideas?

Aaron

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[Wayne Carlisle]

Welcome to the forum Aaron.

Where are you getting your information that 25% needs to come from the top?

R806.1 Ventilation required.

Enclosed attics and enclosed
rafter spaces formed where ceilings are applied directly to the
underside of roof rafters shall have cross ventilation for each
separate space by ventilating openings protected against the
entrance of rain or snow. Ventilating openings shall be provided
with corrosion-resistant wire mesh, with 1/8 inch (3.2
mm) minimum to 1/4 inch (6 mm) maximum openings.

R806.2 Minimum area.

The total net free ventilating area
shall not be less than 1/150 of the area of the space ventilated
except that reduction of the total area to 1/300 is permitted, provided
that at least 50 percent and not more than 80 percent of
the required ventilating area is provided by ventilators located
in the upper portion of the space to be ventilated at least 3 feet
(914 mm) above the eave or cornice vents with the balance of
the required ventilation provided by eave or cornice vents. As
an alternative, the net free cross-ventilation area may be
reduced to 1/300 when a vapor barrier having a transmission rate
not exceeding 1 perm (5.7 × 10-11 kg/s ⋅ m2 ⋅ Pa) is installed on
the warm-in-winter side of the ceiling.

[Aaron Hayward]

Thanks for the reply Wayne,
the 25% comes from the National Building Code of Canada, i see that you are in the states so i guess you are using the international code with which i am not familiar. One would think that the requirements are similar.

NBC makes no reference to whether or not vapor barrier is installed. Interesting concept though, certainly can see why it would be more restrictive if it were not installed.

As my earlier post has shown the requirement says 25% and the case i have does not meet that requirement,it is within the top 33%. However if the space was larger, then the actual distance of the vent from the peak would meet the regulation. For instance 5.5' from peak on this house does not meet, 5.5' from the peak on a house who's top chord is longer than 22' would meet the requirement.

Before i require the builder to cut a hole in the roof and disturb shingles etc. i want to make sure its actually necessary.

The regulation in the code you have referenced says at least 3' above the eave. If i have interpreted properly than this house in your locality would meet the regulation.

[Jerry Peck]

the 25% comes from the National Building Code of Canada,

Aaron,

Do you have the exact wording of the National Building Code of Canada?

That would let us read and understand what it is requiring.

[Wayne Carlisle]

Sorry about that. Not knowing the Canada Building Code I'm not much help however let me throw this out at you.


You are saying the vents are 1/3 the way down. Now 1/3 the way down is different fro the upper 25%.

Based on your post the ridge is about 75" tall. The rafters are approximately 16'3" run. If the vents are 1/3 the way down then that would put them approximately 25" perpendicullar from the ridge. So that would make them real close to the upper 25% of the attic space.

[Aaron Hayward]

Wayne, nice i hadn't considered it that way, my train of thought was stuck on the distance along the rafter(chord as it is a trussed roof) this might work I'll have to confirm when i go back for the reinspection as there were other deficiencies. thanks Wayne

Guys,

The requirement is as follows;

Section 9.19. Roof Spaces

9.19.1.1. Required Venting
(1) Except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the movement of air from the space to the exterior.

9.19.1.2. Vent Requirements
(1) Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area.
(2) Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area.
(3) Required vents are permitted to be roof type, eave type, gable-end type or any combination of them, and shall be distributed,
(a) uniformly on opposite sides of the building ,
(b) with not less than 25% of the required openings located at the top of the space, and
(c) with not less than 25% of the required openings located at the bottom of the space.
(4) Except where each roof joist space referred to in Sentence (2) is separately vented, roof joist spaces shall be interconnected by installing purlins not less than 38 mm by 38 mm on the top of the roof joists.
(5) Vents shall comply with CAN3-A93-M, "Natural Airflow Ventilators for Buildings"

[Aaron Hayward]

I was misinterpreting the code, it only says that 25% of the vents need be in the upper portion. The question still remains what is the upper portion exactly?

Sometimes you really need to discuss things

Aaron

You'll have to forgive me, i have only been a building inspector for about two years now. Still a rookie!

[Wayne Carlisle]

Well it looks like you can throw all of my figures out the window...unless my formula is how you determine upper portion.

Does your NBC have a definition for "upper portion"? How about commentary?

Aaron trust me...we've all been there...in some way or another we are all still rookies! The codes are a continuing learning experience!

Good luck!

[Jerry Peck]

(3) Required vents are permitted to be roof type, eave type, gable-end type or any combination of them, and shall be distributed,
(a) uniformly on opposite sides of the building ,
(b) with not less than 25% of the required openings located at the top of the space, and
(c) with not less than 25% of the required openings located at the bottom of the space.

Aaron,

First, note that the code is stating "uniformly distributed on opposite sides of the building, then that 25% or more shall be located "at the top" and 25% shall be located "at the bottom" of the space.

I see your code is as precise as our codes are at times ... "at the top" and "at the bottom", yeah, right, okay.

One could interpret that to mean precisely and exactly "at the top" and "at the bottom", or, to mean "somewhere near the top and somewhere near the bottom" - in which case the upper 1/3 or upper 1/4 could be considered "the top" and the lower 1/3 or lower 1/4 could be considered "the bottom".

The best ventilation is, of course, with half of the ventilation "at the ridge" and half of the ventilation "at the eave", but that is not what it says.

[Ted Menelly]

First time post!

1000 sq ft home, hip roof 5/12, truss span approx 30'. Perforated soffit isntalled along the entire eave of the home. The problem potentially is that the roof vents (two) are installed about 1/3 down the top chord of the truss and i worry that they may not be high enough, to accomodate the 25% venting at the top rule. What i am unsure of exactly is what categorizes the top portion of the roof.

My gut tells me that the hip helps, wind from all direction, and that the volume of attic space is minimal and that what is installed may work.

Any ideas?

Aaron

If you are looking for real world opinion and not book opinion, or should I say code, Any roof vent a third of the way down the roof is useless and does not do a proper job of venting attic space at all. As I see it , the lower the pitch roof the vent should be even higher. Ridge vents have always been popular but not used nearly enough and they have gone thru design changes over the years. There is nothing wrong with expressing opinion even if it does not fit to a specific code.

[Aaron Hayward]

Ted,

i think ridge venting is a great idea, minimal interference with existing, cap removal and maybe the last course on either side pretty minor tear up no where near as risky as trying to install new vents a little farther up the roof.

Aaron