Flat roofs, parapets, & head-scratching
Bruce
| Posted in Energy, Heating & Insulation on
Ok, here’s my puzzle.
About to start a new residential project … 3-story walk-out tucked into a steep hillside. Flat roofs w/ a 16-18″ parapet. So how do you ventilate the roof(s) and the parapet? And will the framing (platform vs. balloon) of roof/wall intersection make a difference? The roof structure is also the third floor ceiling (9′). Additionally, some of the rooms will have a perimeter soffit carrying both ductwork and indirect lighting. Amy creative ideas out there?
Replies
Where is it?
I would definitely balloon frame the parapets, but then, I'm a known eccentric.
Where is it?
If you mean climate, it's SW Colorado. Wide daily temp swings, but very low humidity. I like the balloon idea myself too. Doing the attached garage that way, which will allow me to pitch the framing one direction to facilitate draining. Not too concerned if it looks a bit wierd from inside the garage.
I would platform frame and run the roof joist across the top of the wall, then build the parapet on top, depending on type of parawall. That way, you can use the parapet wall to detail a vent in the back side of it.
Alternatively, you can drill lots of holes to connect the joist spaces (or use web trusses) and then install vents in the flat roof surface as in commercial.
Excellence is its own reward!
I'm wondering if I could foam the parapet solid and vent the joist spaces as you mention. Gonna take $$ one way or the other, and I don't think the foam cost is going to be as much as the continuous vent detail.
Why do you feel this need to ventilate?
What is it you think needs to be ventilated?
Gabe
By code we're required to ventilate certain types of flat roofs. We've got a detail that works real well, very inexpensive.
I don't know how to attach a drawing, so I'll try to describe it.
If you go with regular platform framing, i.e. your joists sit on a double top plate, this is real easy.
You nail the parapet studs right to the joists, so if you have an 18" parapet on 11-7/8 I-joists with 5/8 CDX over it you cut the parapet studs to 29 inch.
Then sheathe the deck right up to the parapet studs.
When you sheathe inside the parapet, hold the sheathing short enough at the top to install the same metal strip you use for gable vent.
Then use a commercially available flashing profile to cover the vent from the top, but hold it off of the vent (lateral or horizontal seperation) so you don't restrict airflow, but you do keep out wind-driven rain and snow.
That's all there is to it. Works great.
DRC
Very slick!
That sounds like a good solution ... functional and not a budget buster.
Thanks.
You're welcome.
If I were to give a good idea back for all the good ones I've received here I'd wear my keyboard out.
Gabe, Bob, as regards vewnting or not venting, with all due respect, it doesn't matter what we think. We have these inspectors here who insist on it, and there is no point arguing.
Maybe someday the authorities will catch up with the experts. Meanwhile, I just do what they tell me to. It's easier.
DRC
Hmmm, something just struck me about your parapet framing method ...
If you 're sheeting the roof right up to the parapet, framed as you suggest, it seems like you've got nothing to nail off the edge of the roof to. Is that right? I have to build to a 50-lb. snow load here, so not only do I not know off the top of my head if 5/8" OSB will handle that on a flat surface (16" cewnters), I might be real concerned if the edge of the sheet isn't landing on a nailer. I could envision the OSB sagging along the edge and pulling away from the EPDM. What do you think?
Also, I'm wondering if my friendly inspector is going to require fire blocking between the roof and the parapet (we build to UBC '97 in the city limits).
As regards the edge nailing, I haven't had a problem, but I also don't have anywhere near your snow loads. If there's no other way to address this, maybe you could use TJI blocking, knock out a few of the pre-punched web holes, and call it vented. I just use a fairly dense cant strip over top of that intersection between the roof and parapet, and it seems to be working fine.
As for fire blocking, if that's how your inspector is going to act, I wouldn't know what to say. I just don't see how you could have a vented roof that was also fireblocked. I guess at that point I'd ask him to tell me what he wants. <G> So far all my inspectors think it's a pretty cool detail, but then again I think they're amazed that anyone would take that much care to vent a roof.
Please let me know what you think about this. Those are good questions, and this is the best I can answer them so far.
DRC
Thanks for your insight. This all helps me. I have (1) not done a flat roof before, and (2) never dealt with the city inspector, vs. previous experience in the county. All new to me. I do have the services of a roofer who does almost nothing but flat roofs, and he knows the EPDM drill inside out. Thank goodness for that!
Indeed, I'm pretty grateful for my good roofer, too.
He's the one who showed me this detail we've been discussing.
Man, you find good subs and you really appreciate them, que no?
DRC
OK, I'm thinkin' about this whole deal too much ...
Building the parapets with your system, it just occurred to me that the outside edge of the parapet studs would have to be behind the rim joist, would they not? Or am I missing something here?
Bemw, ah , my two sense worth, go with a web truss, as Piffin mentioned. At the Truss Plant, have them Pitch the web truss, I went 1/2 inch in 12. At both ends of the web truss, spec out a 2 by 6 verticle "tail", just a little longer( taller ) than the top of parapet. With the 2x6 you can taper the exterior side, nail your double 2x4 plates.Note; you would then have to double overlay for your crickets, headed towards your scuppers. Don't put any scuppers draining on the North side. I would also talk with your Stucco Contractor and the Roofer, to make certain everbody likes the Details, if you are venting this roof. I put my vents on the horizontal face of the parapet's, and yes I'am still concerned about wind blown rain infiltration, though, it's been 3 years, and not a leak/water problem so far . Jim J. Prescott AZ
BEMW,
1) You sheathe the outside face of the parapet, the stucco guys lathe over everything, and it all works out.
2) 44 is correct about the ease with which a truss plant can solve this for you. Unless I'm misunderstanding his reply, you can make this even easier. Describe the parapet detail to the truss manufacturer, they will build it for you as part of the truss. I didn't suggest this because flat roof trusses are ususally only economical after the building hits a certain size and shape, and I didn't get the impression you were there yet. It's a good idea, though, and worth looking in to.
DRC
I agree with Gabe. What are you worried about ventilating? I guess this comes from the supposed "need" to ventilate an attic, which is being challenged as of late by the building science gurus. I've built 100's of 1,000's of SF of commercial space with flat roofs, and never had to ventilate them just for the sake of ventilating them.
As far as the parapet framing goes, it would be easier to platform frame and then frame the parapet above the joists (avoids putting in a ribbon). Just run the sheathing past the joint to help stiffen the parapet wall. If you're really worried about it, you could run the parapet studs down next to the joists, notching them around the rim joist, to lock them in better.
Rather than pitching the joists and making your framing more difficuly, just use tapered insulation on the roof. You only need an 1/8 to 1/4" per foot of pitch, so unless you've got a really big area, the overall taper isn't a whole lot- especially if you pitch to centered roof drains rather than sending all the water off the edge.
If you've never done a flat roof before, just pay attention to the details at the roof/parapet joint, and at any flashing and counter-flashing points. It wouldn't hurt to use a rubber or EPDM roof either, rather than the old-fashion mop-down felt system.
Bob
I don't care to have a 14" joist space sealed on the top w/ 60 mil EPDM and no ventilation. There are two bathrooms directly below this roof, not to mention all of the other moisture in the house. I find trying to put up an overhead vapor barrier that actually seals is a whole lot harder than doing the vent work. If I went to whole house, full-time powered ventilation, maybe I'd consider it.
My place has a flat roof with parapet wall. The real virtue of the parapet is that it hides the whole roof for you, so you can do what you want with the slope. Go for at least 1/2" per foot, 1" is better. The better it drains, the longer it lasts.
The bane of parapets is blockage of the scupper(s). I had the good fortune to need to deal with the TV dish shortly before the last rain, and noticed a beer bottle that some a-hole next door had tossed up there. In a rapid downpour, that could have been big trouble. So make your scuppers big. If possible, put in secondary overflow scuppers to save you from collapse if the mains get plugged. From now on, I'll be taking a look-see up there from time to time throughout the rainy season.
-- J.S.
Decided to dispense with the scuppers altogether and go w/ six 3" internal drains. Here's the thinking (from the architect/engineer who has done a number of houses in the same location).
Right now we have 2 feet of snow on the ground. Winter nights are ususally in the single digits, and days in the 30's-40's. North side scuppers mean the sun hits them from the roof side, but you have a frozen waterfall down the wall. South side scuppers mean they get sun from the outside, but the parapet shades the roof side, which then freezes up. There's actually another architect in the same area, who did the same deal on his flat roof 12 years ago, and has had no problems.
Additionally, there aren't and won't be any tall trees, so there isn't much to clog them. The house's heat should keep them clear. Now, if someone leaves for 2 weeks in the winter, and the heat goes out, it could mean deep doo-doo.
I hear you, and others, on the more-is-better pitch idea. I plan to give it all I can.
I did a similar project in e.c. Colorado a couple years back. 2x6 balloon walls with TJI roof framing. The roof was calcted out for heavy snow load also. I had to let in micro-lam into 2x6 bearing walls and install hangers to carry TJI's. Lots of work..... roof had 1/4 " to foot run. I cut down on high side parapet height to avoid deeper snow trap. Balloon frame is more difficult but avoids hinge point if parapets are framed on top. I did n't have the vent problem to deal with, but as you said , lots of head scratching. We used 5/8" CDX with rubber roofing . Roofers did build -ups in corners for drainage to scuppers. As far as I know , no problems but Iwas just framer. O, yeah, fireblocking is required where roof and walls intersect .MAC-CO
Oy, veh. A flat roof in snow country.....
-- J.S.
I honestly don't think that's a real issue. How many commercial buidings do you see, snow country or not, that have pitched roofs? I think the issue is doing the job right.
You a skier at all? I used to live in Salt Lake City, right outside the mouth of Little Cottonwood Canyon. Ten miles up the canyon to Snowbird, one of the world's premier ski areas. The entire resort is flat roofs and internal drains. Average snowfall there is about 550 " a year. No shedding problems, no frozen gutters or ice dams, snowmelt is controlled and directed where you want it.
Interesting anecdote about flat roofs in snow country. Right up the road from Snowbird, at Alta, there is a house with a flat roof (all concrete) that sets just above the floor of the canyon, and right in the path of a notorious avalanche chute. The house is tucked tight into the hillside (OK, mountainside) so there is literally no exposed exterior wall on the uphill side.
Got an avalance? No problem. The slide roars down the mountain and skids across the flat roof of the house!
Wow, I guess no parpet wall on that avalanche proof house, right? ;-)
With sufficient engineering, the flat roof can be made to work, though traditionally it might not be the most cost effective solution. We don't get snow here, but with the first big rains every year there'll be TV news stories about a flat roof/parapet buildings that had a drain plugged up and a partial collapse.
-- J.S.
parapet with tji's
this is great, simplifies everything. I like balloon framing on parapets to stiffen the upper wall but your attaching the parapet all the way down the sides of the joists will work well. tji's really don't work well with balloon framing-- couldn't find a way to attach them decently.
this is an old answer from you, but it keeps on working.