DBUG> OT parapet wall vents

[Paul Rabinowitz]

Hi Guys,

I have a parapet wall that's about 4'-6" high.  Wood roof trusses are 24" o.c. with trusses being almost parallel chords, top and bottom.  The vertical leg on the truss, which forms the parapet, is a 2x6.  I am trying to provide roof venting on the back of the parapet wall.  Anyone have a good way to do this?  I need 1/150 of the attic area in venting, which equates to 78sf of vent space.  The only vent I found on line has 44sq. inches per vent.  That's right, 255 vents!

There's gotta be a better way.

Cheers,

Paul

[John]

What about using  a return air grill like you would use on an interior wall. You might need to repaint it on both sides for weather, case the openings in such a way that you would not have water infiltration.
You could use a width up to 20".   You would need about 30 to 40 vents @ 20 x 20.

John

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[Roger Donaldson AIA]

Power Ventilation?

On 12/6/2013 10:08 AM, Paul Rabinowitz wrote:
Hi Guys,

I have a parapet wall that's about 4'-6" high. Wood roof trusses are
24" o.c. with trusses being almost parallel chords, top and bottom.
The vertical leg on the truss, which forms the parapet, is a 2x6. I
am trying to provide roof venting on the back of the parapet wall.
Anyone have a good way to do this? I need 1/150 of the attic area in
venting, which equates to 78sf of vent space. The only vent I found
on line has 44sq. inches per vent. That's right, 255 vents!

There's gotta be a better way.

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[James Horecka]

Paul,

Do the studs at the end of the truss extend upward above the top chord forming the parapet above the roof deck? Remember that you cannot link the spaces between studs in wall with an attic of a roof. That juncture must be firestopped, per code, with solid wood blocking or other acceptable means.

You'll need to instead use roof vents for a flat roof. Those look like mushrooms. I used them often.=0A=0AAnother option might be doghouses every so often, ringed with louvered vents. I can't imagine you'd want to do that.

Sidebar: Keep OSHA in mind. Most architects do not. Minimum height for fall protection, like a PARAPET WALL ON A ROOF where thereis equipment requiring frequent service, is 42" above the walking surface behind it. See OSHA for details, conditions, & understand how this may affect your design. If nothing up there but roofing, most likely a non-issue.


James Horecka, AIA

[Paul Rabinowitz]

Hi James,

See red answers below.

Cheers,
Paul




On Dec 6, 2013, at 12:26 PM, James Horecka <jhor...@verizon.net> wrote:

///Do the studs at the end of the truss extend upward above the top chord forming the parapet above the roof deck?

The truss has a 2x6 vertical leg that extends about 4'-6" above the top of the top chord. The top chord of the truss has a 1/4" foot slope with the bottom chord being =
parallel to the floor below.

///Remember that you cannot link the spaces between studs in wall with an =attic of a roof. That juncture must be firestopped, per code, with solid wood blocking or other acceptable means.

The trusses sit on a 6" metal stud below, so there is no connection between the space in the wall studs and roof truss vertical legs.

///You'll need to instead use roof vents for a flat roof. Those look like =mushrooms. I used them often. So, there's no larger wall vents to use?

The vertical leg of the truss which forms the parapet does not need to be vented?

///Another option might be doghouses every so often, ringed with louvered =vents. I can't imagine you'd want to do that. Don't think it will work in this case. :-)
//:Sidebar: Keep OSHA in mind. Most architects do not. Minimum height for =fall protection, like a PARAPET WALL ON A ROOF where there is equipment requiring frequent service, is 42" above the walking surface behind it. See OSHA for details, conditions, & understand how this may affect your design. If nothing up there but roofing, most likely a non-issue. ////

Around here, if the rooftop equipment is at least 10'-0" from the =
parapet wall, you don't need the 42" protection. The closest any =
equipment is to the parapet (after telling the hvac engineer to move it yesterday) is 20'-0" :-)

[djk...@att.net]

Hello Paul,

A couple ideas come to mind. Separate the roof ventilation from the parapet wall and use gravity (turbine or witch's hat) or solar powered flat roof ventilators. Vent the parapet wall separately with a strip vent similar to to detail 1 or 3 here
http://blog.buildllc.com/2010/12/smart-flat-roofs-the-craft-of-parapet-detailing/.
Detail 3 is another idea.

Second, use this vent which can be mounted vertically on the inside face of the wall http://www.roofvents.com/universal_dormer.html. You would "only" need 27 of the longest unit.

Dan

On 12/6/2013 10:08 AM, Paul Rabinowitz wrote:
Hi Guys,

I have a parapet wall that's about 4'-6" high. Wood roof trusses are 24" o.c. with trusses being almost parallel chords, top and bottom. The vertical leg on the truss, which forms the parapet, is a 2x6. I am trying to provide roof venting on the back of the parapet wall. Anyone have a good way to do this? I need 1/150 of the attic area in venting, which equates to 78sf of vent space. The only vent I found on line has 44sq. inches per vent. That's right, 255 vents!

[Neil Blanchard]

I'm no expert, but I have heard of situations where snow drifts being a challenge; with melted snow leaking into the structure and insulation, etc.

Sincerely, Neil

Dec 6, 2013 12:11:18 PM, P...@pbrdesigninc.com wrote:

Hi Guys,

I have a parapet wall that's about 4'-6" high.  Wood roof trusses are 24" o.c. with trusses being almost parallel chords, top and bottom.  The vertical leg on the truss, which forms the parapet, is a 2x6.  I am trying to provide roof venting on the back of the parapet wall.  Anyone have a good way to do this?  I need 1/150 of the attic area in venting, which equates to 78sf of vent space.  The only vent I found on line has 44sq. inches per vent.  That's right, 255 vents!

There's gotta be a better way.

Cheers,

Paul

[Paul Rabinowitz]

Hi John,

I'd like to do what you're saying, but I'm trying to find a pre-made product for exterior use.

Cheers,

Paul

[Paul Rabinowitz]

Thanks Dan.  This seems the closest idea to what I'm looking for (your second idea below).

Cheers,

Paul

[James Horecka]

Keep in mind that ventilation of spaces in wood framed *walls* is not a code requirement. Ventilation of wood framed *attics* is.

Venting back of parapet wall should not be necessary.

Attic, you bet.

Fire-stop that juncture. Often combined with the necessity to transmit lateral loads from horizontal diaphragm of main roof deck to vertical diaphragm of boundary walls (shear transfer blocking).

BTW: Mushrooms would be no-bueno if snow. No snow here.

See screen capture below for some 'shroom vents I did awhile back, part of a three-acre-under-roof factory. A bunch of baby shrooms over fire-stopped shallow chambers in the outdoor covered patio at the employee break area, which has a plastered ceiling. Over a far larger attic, a row of large shrooms.

These vents have proven effective. The main facility is now over 25 years old. Patio about 20 years.

James Horecka, AIA

Architect

[James Horecka]

One more screen capture.

This is an auto body shop I did. The roofed porte cochere has a closed attic between the low-slope roof and the finished plaster ceiling. Wood framed, so that attic in there must be ventilated per code. All the parapet walls are fire-stopped at juncture, per code. I used mushroom ventilators. Half of the required quantity I placed low, and the other half high. So there is a vertical delta. Takes advantage of the natural chimney effect for constant air circulation. All are screened, of course, to deter flying insects and embers.

The next zone has an attic that is vented differently. The third zone has no attic. But there are double-louvered storm-resistant louvers at the skylights. These work well.

Notes on this low-slope roof:

1/4" per foot is the minimum, per code. As I account for deflection, sag over time, and heave, there are no areas designed to 1/4" per foot, aside from ledger lines at fixed walls. All other zones exceed that. The roof over the porte cochere spans about 45 feet; a tilted plane dropping at consistent slope exceeding 2%, to account for deflection of long-span wide-flange steel beams within. The other zones slope 2% at ledger lines, but 4% to 6% at purlins that are at roof drains. That is, each roof zone is not planar: Rather, each distinct catchment area takes the form of a hyperbolic paraboloid. A double-ruled double-curved surface. There is no line anywhere on the roof, even crosswise, that exhibits a flow slope less than 2%; nearly all areas exceed the minimum. After many years of learning "What NOT to do," I try to evolve stuff that, I sincerely hope, will perform better. Low slope roofs are tough. We're always between a rock and a hard place, eh? Somewhere between height limits, usable floor-ceiling heights desired, interstitial floor-ceiling and roof-ceiling mechanical spaces, and parapets sufficient for screening mechanical equipment and providing compliant fall protection.

Have fun!

And remember: Fire sprinklers save lives!

 

James Horecka, AIA
Architect

[James Horecka]

Crap. I hit Send instead of Attach. Here is the image discussed.

 

James Horecka, AIA
Architect

[James Horecka]

Paul,

Red helped clarify. Thanks.

That vertical parapet wall, which extends above the roof deck, will have spaces between each "stud" (wood = truss element). You cannot link those spaces with the attic space. Not legal under IBC. Must be separated by fire blocks.

Therefore you cannot vent an attic by venting any of those spaces in that parapet wall. I've done scads and scads of low-slope roofs with parapets. Zero had parapet spaces linked to attic spaces. The mushroom vents work fine. Turbine vents are also fine. I did a whole row of those a few years back, on an attic 48 feet wide and 300 feet long. Worked great. Saved big bucks in air conditioning, too, by the way.

By the way: Code minimum for roof pitch is 1/4" per foot. Note that that is the MINIMUM. Best practice indicates that one pitches a low-slope roof more like 1/2" per foot, to account for normal deflection, sag with age, seasonal variations and truss heave. Design of 1/4" per foot will not meet code minimum in real life. Much of my expert witness type consulting has involved inadequately sloped low-slope roof. Happily, that's like shooting fish in a barrel. Ya bring out a Smart Level and walk roof with a camera. Any spot less than code minimum, BOOM! Fail.

Low-slope trusses should be at a minimum 1/2" per foot. If crickets are involved, you'll need much more than that. Typically 1" per foot. And the crickets must be VERY large to obtain minimum 1/4" per foot fall along the r= esulting flow lines.

Sidebar: Not long ago, I was researching old real= blueprints for a facility. Sixty years old. "Flat" roof, ringed by 33:12 pitch mansard roofs. I was pleased to see that the original architect called= out 1" per foot slopes. Nicely detailed crickets. Decking was all diagonal= 1x6 back then. The crickets were wide wood boards (no plywood). Crickets v= ery deep, resulting in flow lines behind each of 1/4" per foot. Sweet. I pu= rred a little, as I admired the fellow's hand-drafted work and excellent lettering.

James Horecka, AIA=0AArchitect

[Paul Rabinowitz]

Hi James,

Once again, you're a fountain of info.  I changed the slope to 1/2" per foot which got me out of another issue as well.  It only made a minor change to the look of the building.  Regarding parapet heights, is there a minimum requirement if the structure is not fire rated?  I can only find a requirement of 30" but only is a fire rating is involved.  I don't want to create a tripping hazard with too short a parapet.  What if the parapet was omitted altogether and I have the top chord of the truss cantilever out to form a roof overhang?

Cheers,

Paul

<FQAB-ventilation.jpg>

[James Horecka]

Paul,

Roofers often won't warrant a roof with turn-ups of less than 8". I won't do a parapet less than 12", at the highest point of the roof. That's the 8" turn-up, plus the two-piece reglet flashing, plus the coping: BARELY. More is better.

Federal OSHA dictates fall protection requirements. In my four decades plus, I pretty much have seen that nearly zero architects know OSHA.

Here's the popular saying in my circles: Architects have no clue what OSHA requirements are: Until they get sued after an incident.

I'll leave it to you to study OSHA and determine what perimeter fall protection might be necessary on your specific rooftop. Many factors come into play.

Where I did most of my work, Planning Commission Design Guidelines REQUIRED that all rooftop mechanical equipment be fully screened from view. As rooftop duopacks are usually about 32" tall on a 14" curb, plus leveling shims, my parapets trended toward 48" above that location on the roof. More if necessary.

Remember: If you use crickets to divert water to roof drains behind a parapet, 1/2" per foot is still not sufficient!!! If you run the math, you will see that the plan view angle of such a cricket would have to be 30 degrees off the parapet. That is the minimum to achieve 1/4" per foot slope along the flow line behind a cricket where the main deck slopes at 1/2" per foot. Gigantic crickets! And if all is not perfect, and it never is, you'll not meet the 1/4" per foot. Water will pond. Damage will ensue.

Strive for steeper main pitch. Seasoned wisdom indicates 1" per foot, if you can. Accounting for all factors, if you're lucky you'll maintain 1/4" per foot minimum behind a cricket that is not insanely huge in plan.

By the way: For fun, you can fly above a town in Google Maps, with 45 degree view toggled on, and look for crickets with insufficient slope behind them. They are EVERYWHERE! Here is what you look for: Dirt stains! Yep. Behind most crickets, you'll see dirt stains left behind as the ponded water evaporated. Behind most every cricket. The worse the problem (lower roof slope) the bigger the dirt stain. This applies to SoCal, where there is a lot of airborne dirt and not much rain. Areas with more rain may not exhibit as much staining. But there, you'd want even better roof slope! Enjoy your fly-over! You will be entertained at all the dirt stains behind crickets. Every one evidence of poor planning by a person with a pencil (or mouse).

For your amusement, I've attached a screen shot of some random building in the middle of Ontario, California. I annotated the issues, which are quite clear. I chose Ontario because I fly out of there often, so I can see those roofs often.

Have fun.

 

James Horecka, AIA
Architect

[Paul Rabinowitz]

Hi Dan,

Your drawing is very similar to what I have going.  I've started speaking with a truss company yesterday.  I sent them my parapet section, they modified it, I modified their sketch, and so on.  Thanks for your's and everyone else's input on this!

Cheers,

Paul

On Dec 7, 2013, at 12:21 PM, "djk...@att.net" <djk...@att.net> wrote:

Jim -

I think you may be misinterpreting Paul's design (I may be also). The end legs of his almost parallel roof truss are extended upward to create the parapet. They become part of the combustible, concealed roof space and included in the ventilation area. I see these parapet designs a lot in this area (northern Ohio, Paul is in southern Ohio, Cincy). With this high of a parapet I would be concerned about wind pressures, but, that's a different discussion. 

I drew (in DataCAD for the 1st time in 2 yrs. since I retired from design work) what I think Paul had in mind -- see attached. I currently work as state certified plans examiner for 4 cities (retiring from this also next year), and, this design is acceptable in the IBC and the Ohio-ized version of the IBC (717)(IMO).  If 'shrooms are used, they are usually placed on a pedestal, open ended to the roof space. See more turbine or witch's hat vents, many times ugly without a high enough parapet because the vent opening has to be high enough for snow and drifting snow.

Dan

On 12/7/2013 1:04 AM, James Horecka wrote:

Keep in mind that ventilation of spaces in wood framed *walls* is not a code requirement. Ventilation of wood framed *attics* is.

Venting back of parapet wall should not be necessary.

Attic, you bet.

Fire-stop that juncture. Often combined with the necessity to transmit lateral loads from horizontal diaphragm of main roof deck to vertical diaphragm of boundary walls (shear transfer blocking).

<paul.jpg>

[James Horecka]

No, I understood correctly.

There is no way that configuration would pass in California.

At the juncture of parapet wall to attic, there must be fire stopping. That necessary blocking also serves as shear transfer blocking.

In the detail that was attached, there is zero shear transfer from the horizontal diaphragm of the roof structure (roof deck) into the vertical diaphragm at the outer face of the bearing wall below. That is: The lateral load path is incomplete. A structure of that nature will go down like a house of cards.

Also, chambers in wood-framed attics over a certain height are fire-sprinklered differently. I trust you fire-sprinkler all your commercial buildings. Please nod "Yes." If those stud bays were opened into the attic, that would screw up fire sprinkler coverage. Checking NFPA, you'd have to stuff all those stud bays with fire stopping. Down here, for built-out plant-ons and such, the local AHJ will insist that we stuff such cavities full with non-combustible material (we usually use fiberglass or rock wool, properly supported to avoid settling over time).

I'm with you on the raised things needed for deck vents if you have snow. We have no snow, so that's never an issue here. I've used 'shrooms that have cowls just an inch above the deck; no problem. Usually more like 6", with an 8" tall internal cylinder. I've had standard 14" turbines on the roof of my house for 25 years: Never an issue, not a single leak. Simply cut into the comp shingles, held in place with four roofing nails.

Most of the parapet walls I've done are precast concrete, 6" to a foot thick. Sometimes I do wood. Always firestopped, and with proper shear transfer blocking, nailing and clips for lateral load path continuity. Also huge here are wall ties: Specialty hardware securing the parapet wall and building wall to the horizontal diaphragm of the roof structure. Sometimes these are friggin' huge! I did a pair of drag ties a few years ago, each of which had thirty one-inch diameter machine bolts in double-shear, linking roof to wall at a re-entrant corner.

 

James Horecka, AIA
Architect

[djk...@att.net]

The detail was mine not Paul's and only for concept. There are other
issues that need to be addressed, but, beyond Paul's original question.

Sprinklers? No - not all buildings and not required for residences either.

I'm sure Paul has a better handle on all the issues than any of us.

Dan

[James Horecka]

Yep. Varies by locality.

Except OSHA. That's federal. Nationwide. Well, except for California. Here we suffer under Cal-OSHA, which is even tougher than Fed OSHA.

James Horecka, AIA

Architect

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[Roger Donaldson AIA]

And Michigan has MIOSHA, but contractors have another name for IT.

On 12/7/2013 4:31 PM, James Horecka wrote:

Yep. Varies by locality.

Except OSHA. That's federal. Nationwide. Well, except for California. Here we suffer under Cal-OSHA, which is even tougher than Fed OSHA.

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Sincerely,

Roger Donaldson, AIA, CSI, CDT, Leed ap bd+c NCARB

Roger L. Donaldson, AIA P.L.C.

 

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