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Author Message Colin Gilboy Senior Member Username: colin Post Number: 351 Registered: 09-2005 Posted on Monday, October 07, 2013 - 04:27 pm:    Quention emailed to me: I am building multiple SFDs in the Los Angeles area. The plan checker, using the CBC, is requiring the flat roof to be vented. None of my previous projects have ever had this requirement. I am using solid roof joists and typical insulation. He suggested I create holes in the joists to achieve the venting, but also use a wire mesh to squish the insulation enough to clear the holes. Is there a “loophole” or any opposing CBC documentation that could help me argue my case not to insulate? Also, I can’t find the true definition of Roof Rafter vs. Roof Joists in the CBC. Any help would be appreciated. Colin Gilboy Publisher, 4specs.com 435.654.5775 - Utah 800.369.8008 Colin Gilboy Senior Member Username: colin Post Number: 353 Registered: 09-2005 Posted on Monday, October 07, 2013 - 04:29 pm:    not-to-vent - mistake - not insulate. Colin Gilboy Publisher, 4specs.com 435.654.5775 - Utah 800.369.8008 Wayne Yancey Senior Member Username: wayne_yancey Post Number: 612 Registered: 01-2008 Posted on Monday, October 07, 2013 - 05:14 pm:    Insulate above the roof deck. If the insulation is under the roof deck, between the solid roof joists, the plan checker is correct based on my experience in the PNW. What is typical insulation? Alan Mays, AIA Senior Member Username: amays Post Number: 141 Registered: 02-2003 Posted on Monday, October 07, 2013 - 05:17 pm:    Colin, I wondered when someone was going to get called on this section. Below is the section taken from the CBC. Yes, you have to provide ventilation. He just hasn't been caught prior to now. 1203.2 Attic spaces. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing members shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain and snow. Blocking and bridging shall be arranged so as not to interfere with the movement of air. A minimum of 1 inch (25 mm) of airspace shall be provided between the insulation and the roof sheathing. The net free ventilating area shall not be less than 1/300 of the area of the space ventilated, with 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. [BSC] Exception: Attic ventilation shall not be required when determined not necessary by the building official due to atmospheric or climatic conditions. 1203.2.1 Openings into attic. Exterior openings into the attic space of any building intended for human occupancy shall be protected to prevent the entry of birds, squirrels, rodents, snakes and other similar creatures. Openings for ventilation having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum shall be permitted. Openings for ventilation having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl 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. Where combustion air is obtained from an attic area, it shall be in accordance with Chapter 7 of the California Mechanical Code. HBarbosa (Unregistered Guest) Unregistered guest Posted on Monday, October 07, 2013 - 05:49 pm:    Alan- According to code, if we are to vent our ceiling joists at least 3 feet above eaves or cornices that dont exist, being a flat roof, the code is obviously focusing this section on sloped roof conditions. Roof I-joists need to be addressed differently than "rafters" to address flat roof conditions. What is the difference with roof joists vs walls? At what pitch does a wall become a roof and vice versa; we are not required to vent walls that are insulated. This is also not an attic space (1203.2), right? Alan Mays, AIA Senior Member Username: amays Post Number: 142 Registered: 02-2003 Posted on Monday, October 07, 2013 - 08:24 pm:    @HBarbosa: The code official should have told what section the code he was referring to when they commented on the plan check. The plan checker may feel that the space that you are considering not attic space as attic space. They may consider the rest of the sentence as why you need the ventilation. "...where ceilings are applied directly to the underside of roof framing members..." Why not set up a meeting with your plan checker and discuss his position with him? It is best to have that discussion with him. They usually are willing to explain their position and listen to your position. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 609 Registered: 10-2007 Posted on Tuesday, October 08, 2013 - 01:30 pm:    The need for venting exists for flat as well as sloped roofs. The exception does not apply to residential construction. There may be situations where venting is not needed but you will not be able to find them unless you consult with an expert who knows about moisture flow in buildings. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1537 Registered: 03-2002 Posted on Tuesday, October 08, 2013 - 02:56 pm:    Well, I know this is in the code and it has been for decades. Many may realize that the quantity of ventilation indicated was the result of a study many years ago - I believe in the '30s - that has never been revisited and confirmed - in fact, quite the opposite. It was also done on sloped roofs. There has been research that indicates that, except for continuous venting of a sloped roof with no interuptions (how often does that occur?), it does not actually provide even close to complete ventilation of the space. This is do to the dynamics of air flow and movement. Some parts of the space will have fresh air, others will have none at all. Other research has been done that disputes the need for venting of the sort described, and it can actually bring in outside moisture where you don't want it. What if your roof membrane is also your air and vapor barrier? You wouldn't want to vent below that. (Crawl spaces have similar issues, maybe worse.) In addition, for very large areas of roof, it is nearly impossible to provide the amount of venting required. Okay, that all may be so, but the code still says you must vent. One strategy therefor would be a careful analysis of moisture transport and air movement, thorough enough to convince the code official that you have provided a compliance alternative. Short of that, you may be forced to design a building that will not perform well. Ron Beard CCS Senior Member Username: rm_beard_ccs Post Number: 408 Registered: 10-2002 Posted on Wednesday, October 09, 2013 - 02:55 am:    Venting a Flat Roof: Does this mean a plenum between the ceiling and an overhead flat roof needs to be ventilated? Where is the "attic" in vaulted ceiling construction? <<1203.2 Attic spaces. Enclosed attics and enclosed rafter spaces...>> If a rafter is filled with insulation, it is no longer a "space" IMHO. The practice I have always followed is to ventilate any spaces between the thermal envelope and the roof construction above. "Fast is good, but accurate is better." .............Wyatt Earp Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 611 Registered: 10-2007 Posted on Wednesday, October 09, 2013 - 04:20 am:    Venting is needed when there is a potential for condensation and there is no air flow. Plenums solve this problem since they are provided to accommodate air flow. Insulation if it fully fills the space will inhibit air flow and thus make the problem worse.