# Does a flat roof need to be ventilated per code in CA?



## SJP (Feb 20, 2014)

Hi all - I am working on a residential project in San Diego, CA. It is new construction and they're going to be two units side by side with a common wall. They are each two stories with a roof deck. The portion of the 2nd story roof not covered with the deck is a flat roof w/ a parapet. The contractor is wondering whether or not the flat roof should be vented. I couldn't find anything in the code that requires this (he is doing a TPO cool roof). I did some research on the general idea of venting a flat roof and it seems there are arguments in both directions on whether the roof should or should not be vented. So my question is: is there anywhere in the building code that requires a flat (low-slope) roof to be vented? Note: It is not called out to be vented anywhere on the permitted set of drawings. Thank you for any help on this!

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## Badeeba (Feb 20, 2014)

It is required in WA.  I believe you are using the same code amended to CA standards.  IRC section 806 requires enclosed rafter spaces with a ceiling directly attached to the underside of roof rafters to be cross ventilated.  There is an exception allowing the building official to allow for unvented rafter assemblies where atmospheric or climatic conditions permit.  There is also an entire code section for unvented attics R806.5 which has a list of conditions to be met.


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## mark handler (Feb 20, 2014)

Yes, unless BO official waives the req.


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## SJP (Feb 20, 2014)

Thank you, this section below was helpful. Interesting to see what will qualify with the building official as "_not necessary by the code official due to atmospheric or climatic conditions_". I believe San Diego is in climate zone #7.

*SECTION R806 ROOF VENTILATION *

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. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air.

Exception: Attic ventilation shall not be required when determined not necessary by the code official due to atmospheric or climatic conditions.

R806.2 Minimum vent area.

The minimum net free ventilating area shall be 1/150 of the area of the vented space.

Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:

1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

2. At least 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located no more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by eave or cornice vents. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

R806.3 Vent and insulation clearance.

Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.

R806.4 Installation and weather protection.

Ventilators shall be installed in accordance with manufacturer’s installation instructions. Installation of ventilators in roof systems shall be in accordance with the requirements of Section R903. Installation of ventilators in wall systems shall be in accordance with the requirements of Section R703.1.

R806.5 Unvented attic and unvented enclosed rafter assemblies.

Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted if all the following conditions are met:

1. The unvented attic space is completely contained within the building thermal envelope.

2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly.

3. Where wood shingles or shakes are used, a minimum 1/4-inch (6 mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing.

4. In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class III vapor retarder coating or covering in direct contact with the underside of the insulation.

5. Either Items 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.

5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.

5.2. Air-permeable insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control.

5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

5.4. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.

TABLE R806.5 INSULATION FOR CONDENSATION CONTROL

CLIMATE ZONE	MINIMUM RIGID BOARD ON AIR-IMPERMEABLE INSULATION R-VALUEa

2B and 3B tile roof only	 0 (none required)

1, 2A, 2B, 3A, 3B, 3C	 R-5

4C	 R-10

4A, 4B	 R-15

5	 R-20

6	 R-25

7	 R-30

8	 R-35

a. Contributes to but does not supersede the requirements in Section N1103.2.1.


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