Thoughts on This Low Slope Roof Design
leon_g
| Posted in General Questions on
We just received the proposed design details for our our new home in Portland OR, CZ 4C. I would like to get some feedback on the proposed roof design and insulation.
As the pictures show, the roof has pitches of 3:12, 1.75:12, and 0.75:12. So we’re on “flat roof” category.
I’m concerned about standing seam metal roofing at such a low slope, and also want to make sure that the insulation strategy makes sense. They call it a vented roof in the drawing package, and they do show a continuous soffit vent, but I don’t see any high vents (ridge, cupola, etc).
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Replies
leon_g
There are two issues:
The first is whether a low slope roof can be successfully vented, even with outlets at the peak. This article suggests they can under certain circumstances, but those appear not to be present in your roof design. https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs.
The second is using metal panels on such roofs, which brings up a few things. Both our building code and most manufacturers limit their products to slopes above 2/12 without modifications (usually things like sealing panel ribs, and using closure strips). My feeling is that even if you can find detailing which will allow them to be used on such low slopes, you are putting on an assembly close to the limits of its use, which makes it risky. Metal roofing details (especially for things like skylights) typically work based on laps, which rely on the slope to drain and stop water moving into them by capillary action. Low slopes defeat that mechanism and make them a lot more vulnerable to leaks, especially if any debris accumulates.
There are couple of other things on your drawings that caught my attention.
- One skylight looks much too close to the ridge to be detailed successfully, and it is oriented so that it's long side blocks the slope, making it's installation a lot more problematic, both for the trusses and roofing.
- The section through the roof at the eaves seem to be missing a few things. There is no rough fascia on the end of the trusses to fasten anything to, or framing shown at the exterior wall to support the soffits.
- Are they really suggesting the ends of the top chords will be tapered toward the fascia? If so, what are the dimensions?
- Hipped roofs with different slopes, end up with the fascias and soffits at different heights. Do the drawings reflect that?
Hi Malcolm,
Thank you for the reply. I agree, there are two areas to consider - venting/insulation, and the roof panels themselves.
For the metal roof panels, the architects are recommending Taylor Metal panels. They didn't specify the panel, but browsing the Taylor website they do make butyl sealed panels that are approved for 1/2:12 slopes - https://taylormetal.com/residential/ms200/. So that should be good for our 3/4:12 slope? Would that also address the skylight concerns? I realize that higher pitch is better, but we're at the point where the design is mostly done, so we'd prefer to not make major changes, unless we felt the current approach is simply unworkable.
For the venting, I have read Martin's article that you linked. As I read it (and specifically Joe Lstiburek's comments), venting can be accomplished successfully if the following is done:
1. airtight ceiling,
2. air gap of at least 6 inches between the top of the insulation and the roof deck
3. perimeter air coming in at vents at the soffit or fascia above the insulation
4. ventilation openings near the center of the roof through some kind of cupola or doghouse
1 is subjective, but let's assume for now that they do a good job air-sealing the ceiling;
2 appears to be done per detail R1 above;
3 appears to be done with the continuous soffit vent
4 is the part that I don't see in the drawings, and will have to ask the architects about.
Would you agree that that is the only part that's missing?
To your other questions, I'm attaching a closeup of the fascia. It looks like there is a 1x fascia attached to the truss ends, and then a metal fascia is wrapped around it under the roof panels and over the soffit.
I do see your point about lack of framing at exterior wall to support the soffit, I'll ask them about that.
And from my crude scaling, it appears that the top chord tapers down to about 3" at the fascia/gutter.
They don't show the truss details in the drawings, so I can't decipher all the details, but I do believe they show all soffits to be at the same height.
Leon-g,
I agree. What you need is some mechanism (a cupola or similar) to create air movement. The rest is doable with diligence.
That leaves...
The metal roof:
It's a bit late in your situation where you are this far along, but at the design stage it's really useful to to include a list of not only things you want to include, but elements you should exclude for practical reasons. In my PNW climate they are:
- No flat or low-sloped roofs.
- Roofs with no overhangs.
- Decks over conditioned living spaces.
- Decks or roofs that rely on roof drains.
- No roof assemblies that require spray foam.
By deciding with these things before ever putting pencil to paper you avoid designing in problematic things so they don't need dealing with later.
The eaves detailing:
- That unidentified material used as a rough fascia is inadequate both to stay straight between rafters and as a substrate to fasten to. it should be a 2"x4".
- Why is there what appears to be 1/2" plywood on the soffit, and what supports it? At one end it could be fastened to the rough fascia (if it is increased in size), but at the continuous vent opening it just floats?
- What is a "1/2" batten", what are they for, and again what supports them?
With the differing pitches, and assuming a consistent overhang, how are the heights of the fascias kept consistent? The difference may be accounted for by the heel heights of the trusses, but it's something to inquire into.
The skylights:
- Without seeing the proposed truss layout I can't be sure, but it l0oks like both skylights are oriented so that they will not fit within the trusses and will require a break in the layout and maybe girder trusses.
- The smaller one needs to be moved away for the ridge to allow it to be flashed.
Martin,
I'll admit to not fully understanding the structural drawing that was included. As I read it, there are premanufactured roof trusses 24" o.c. oriented north-south (up-down in plan), with girder trusses on each side of the skylight openings. The truss pattern appears to stop about 15'-4" from the east and west ends, with some other members overhanging in that direction (I can't decipher what they are).
I'd like to assume (always dangerous) that if a licensed structural engineer designed this and the state approves, then it works, structurally speaking (?).
I agree with you about the fascia - maybe since the trusses are 24" o.c., they nail the 1x to the truss ends, and the gutters are screwed into the truss ends as well?
I also don't fully understand the battens and the plywood, I'll ask. Probably attached to truss underside on the outer edge, but not clear what they attach to near the wall.
There's obviously lots of detail to work through, and I appreciate this feedback. At this point I'm primarily trying to see if there are any major issues that may require significant redesign now, since we are just submitting for permits. The cupolas (or lack thereof) is a good example.
leon_g,
I think you are right. If you can resolve the roof venting issue the rest are fairly incidental.
The comment on the skylight orientation is more one of surprise. You don't typically see them located so that they run against the direction of the slope, or necessitate girder trusses to install, but both are certainly doable.
The comments about the build-ability of the eaves comes from the perspective of the poor guy who finds himself up a ladder trying to find backing to fasten the various elements to. There are simply too many unidentified bits and pieces with no obvious means of support. Your builder will thank you for cleaning the detailing up now.
Good luck with your build!
Thanks Malcolm! I completely understand your point about getting the details right and communicating them on the drawings, instead of the person doing the work later down the line having to guess or assume - that never ends well :). So I definitely appreciate your feedback!
Taylor should have a vented ridge detail for their low slope panel. I would include that as part of your drawing package and make sure it is detailed exactly as per drawing. The one problem is you don't have a lot of length there but better than nothing.
Deck mount skylights are generally limited to 3:12 slope and the flashing kit won't work for low slope. This means curb mount. I haven't looked at the low slope metal roof details for a curb but they are not easy and unforgiving, so make sure these are done properly. The last thing you want is chasing endless leaks by laying beads of caulk over other beads of caulk down the road.
As Malcolm suggest, nudge the skylight away from the ridge. You are already looking at a challenging roof, don't make the job even harder.
P.S. Skylights on older homes seem to always leak, so spending some extra time to get the details right is well worth the effort.
Thanks for the tip Akos! I didn't find a vented ridge detail on Taylor Metals site, but they do have a download for VersaVent ridge vent, so maybe that's their recommendation. I'll ask the architects about that.
The drawing package does not show much detail on the skylights, other than the picture I'm attaching. They have another view with a callout to a closeup view on another sheet - but lo and behold, that detail does not exist on the referenced sheet. Ugh.
I'll try to nudge the skylight away from the ridge, unfortunately they have it centered over an interior feature, so I'm sure they'll push back against moving it. I don't worry about fighting with them, but I suspect my wife might be in the same camp, and I don't want to argue with my wife :).
In fact, I think that some of these concerns can be addressed by increasing the roof slopes, like I'm showing with the red lines below - but again, the asymmetrical low slope roof was something that was very appealing to my wife, so it's a function vs form discussion - we may be on opposite sides of it :).
Couple of things.
The VersaVent doesn't look like a low slope product. I would stick to the Taylor vent, make sure the Z is sealed to the panel with mastic and caulked against the ribs as shown in the drawing. You want to make sure that blown rain can't get into the ridge vent.
Skylights should have a flared tunnel. The narrow tunnel as shown in the drawing takes a lot of the light away. If you flare the tunnel, you can nudge the skylight over but keep the same opening location on the bottom, any offset will be hidden in the flare. A flared tunnel also lets you use a smaller skylight so you get the same amount of light but much less solar gain.
Skylight tunnels are huge air leaks. Make sure these are detailed properly. I prefer to wrap the interior with peel and stick and tie it into the ceiling air barrier and the skylight with flex flashing tape for air barrier continuity. It is also better to insulate with rigid or spray foam instead of batts, this way there is no chance of the batts falling down overtime.
Does your roof need to be asymmetric? Looking at that elevation, it seems to me that the building would look perfectly handsome with a symmetrical hip roof where all the slopes are the same. Perhaps a low, but not extremely low, slope like 2:12 would ease some of the detailing while still getting you the look you want.
I'm with Brendan, house would look just as good if not better with a symmetrical roof.
If you want asymmetrical, you should embrace it and make it noticeable, something like this:
https://www.dezeen.com/2017/01/08/timber-fins-asymmetric-glazed-gable-added-melbourne-house-warc-studio-australia/
Instead of dealing with the low slope, I think your design dollars would be better spent in making the living space taller with a cathedral ceiling.
For some reason I can't reply to individual posts, so I'll combine the responses here.
Akos, which "Taylor Vent" are you referring to? I couldn't find on on their website.
I completely agree with you about the skylight light shaft, we had quite a bit of back and forth on that with the architects. They keep wanting to "focus" the light on the area directly below, rather than light up the whole area. I finally decided to pick my battles, and just go with their recommendation on this one.
My bigger concern is that the three most important rooms to us (living room, kitchen, and master bedroom) are all facing the south covered patio - so there is a 14 foot deep overhang over all the windows and doors. Granted, the overhang is 11 feet above the floor level, but I still am concerned that these rooms will always feel dark and will never see any sun, even in the winter.
The roof obviously does not have to be asymmetrical, but my wife really liked it when they presented it to us. So if I have to choose between an unhappy wife or having to do repairs due to leaks in 10 years, I'll choose the latter. Ideally I can avoid both with some clever choices with the roof.
What do you guys think of the marked up roof design I posted above, with the reduced asymmetry and higher slope for the right side of the roof? It also gets the skylight further away from the ridge.
Leon_g,
I think your suggested revision is a big improvement that will solve a lot of your difficulties.
Thanks Malcolm. Now to sell this approach to my wife and the architects....
Leon_G
Hopefully a bit of back and forth will result in a shape that meets both their aesthetic wishes, and isn't so risky. Good luck!
Leon,
Your proposed roof is better in many ways, I would definitely consider switching to it. It eliminates the low slope section, gets the skylights away from the ridge and gives you a long enough ridge to have a proper vent there. Since it is not low slope, you can go with standard snap lock or lower profile non-gasketed seamed panels which should be cheaper material and install.
I also personally think it is better looking roofline.
Natural light is scarce up here in the winter time, so what is bright enough might be different. I have an 9' deep porch and can tell you even with a pretty large window the room next to it is dark. For a new build, I would want to be able to see sky sitting in the living room.
The best way to bring daylight is by stepping down the porch roof from the house roof and having a row of clerestory windows in the step. The other option is to install a large skylight right by the house in the porch roof. This doesn't bring as much light into the house but a simpler install.
Vented ridge detail:
https://taylormetal.com/wp-content/uploads/2021/04/MS200-Installation-Guide-08_31_22.indd_.pdf
P14.
Thanks for the thumbs up on my proposed roofline change. That's very helpful, and I think it'd be an easy sell to my wife and the architects.
The bigger issue that I'm struggling with now is the deep roof overhang I mentioned. It's 12 to 15 feet deep (depending on location), and extends over the kitchen, living room, and master bedroom - our most important locations (see attached picture). I think this overhang is going to make all those rooms feel like caves, and we'll never have any view of the sky from those rooms and will never get any sun into them. I think that in an overcast area like Portland, this will feel very dark and sad. But I don't know how to fix this easily.
I'll try to sketch out the idea of stepping down the porch roof and having clerestory windows above it. I also like the idea of the skylights in the porch roof, but like you said, it's not as helpful unless we put in a ton of skylights into the porch roof, which will get quite expensive - but maybe less so than some of the other options.
I'm also thinking possibly eliminating the overhang over the kitchen and living room, and just having the overhang over the master bedroom suite.. or eliminating the overhang altogether and having an open deck and a separate gazebo.. Any other suggestions/ideas ?
This is keeping me up at night these days (literally).
Love the sauna! I'm always trying to convince our clients that they really want a sauna ;)
You could keep the footprint of the roof the same, but punch some large holes in the overhang in strategic locations to get some more light in. Having a roof with a big hole in it, with a cute tree growing below the hole is a a move that might get you into a design magazine ;) There are some logistical things to sort out with making sure the tree always stays exactly the right size, but it would be pretty fun!
Alternatively, putting a whole bunch of skylights in the overhang as mentioned is an option as well.
If I'm understanding correctly, it seems like a big part of the concept of the house is that you have a giant rectangular roof, with an irregular building underneath it. Is that correct? If so, I think maintaining the visual effect of the simple rectangle of the roof probably makes sense aesthetically. If you do reduce the overhang in some locations, you could explore running a beam along the perimeter (that supports nothing) in the location where you eliminate the overhang, to still define the boundary of the roof.
While here in Portland, you'll definitely feel the darkness in the winter, you're also going to really appreciate having a large covered outdoor area to be outside in the rain in the spring and fall. It's a balancing act for sure.
Leon_G,
As both Akos and Brendan have said, covered outdoor space is a must have in our wet but mild PNW climate, but only when coupled with some way of bringing light to the rooms behind. That's a task you should give to your architects to sort out, not sweat it yourself.
Brendan, yes the sauna was a "must have" for us, I'm glad we found a good spot for it.
You are correct in your understanding of the design intent, the architects suggested the large rectangular roof for aesthetics but also for cost control and ease of construction. We're using a modular house design, so most of the rectangles you see in the floor plan are 15'-6" wide by 30'-40' long modules. They're built and assembled in the builder's factory, then taken to the site and placed on the foundation - at that point the roof gets site-built. So the idea of the simple roof was to speed up the process and reduce exposure to weather - hence using simple long trusses.
So if we got rid of the overhangs in some areas (e.g. over kitchen and living room), I suppose we'd have to go with more complex roof framing, assuming we want to maintain the hip roof? Or is that what you mean by running a large beam at the perimeter that supports nothing - basically just the trusses, but without a soffit and roof? I'm not quite clear on how the sides and back of the house would look, I guess we'd just bring the siding higher up to meet the roof?
I also like the "big opening" idea, though if we put a tree in there it'd make things dark again :). Or maybe the lots of skylights approach.
Malcolm, of course you are correct, this should be the architects' job. But, our relationship is quickly souring, and given that we're at the permit stage, they just want to be done (as do we). So I suspect that they'll try to argue that this is fine as is. Obviously I can insist on changes, and maybe I'll be pleasantly surprised and they'll want to explore all these other options. But if not, I would like to have as many ideas ahead of time, rather than counting on them to come up with some of their own.
The best news is that my wife is now agreeing that this house will be too dark as-is, and we need to make changes - that's the most important step in my book.
What I would do is south of the living room where you plan says "covered porch" put a large cutout in the roof. Start from say 2' from the living room wall and all the way to about one foot from the roof edge. Fill in the missing sections of the roof with angled wood slats. This maintains the look of the house and simple roofline but gives some shading and brings natural light into the place.
Framing should be pretty simple, similar idea as the girder trusses for the holes for the existing skylight just a larger opening.
At this stage of the game your simplest option is a set of skylights that are narrow enough to fit between the trusses. This doesn't require major structural changes and relatively simple to add on.
AKOS has done a better job at describing what I meant with my perimeter beam idea. the wood slats are a nice idea as well.
OK, so maybe like some of the views below? I sketched an elevation view of the living room wall and the roof, then the same view and an isometric with a series of 8' x 4' skylights, and then a view with a large roof cutout. I understand the idea of added wood slats, that may look pretty cool.
This seems like the right path to be going down. The next step is probably to get input from your architect and contractor (and engineer and truss manufacturer, but presumably the architect and contractor handle those consultants) to talk about details, costs, and all the other complexities!
While changing things late in the game is a pain, and your architect may grumble, it's also a fact of life, and it's almost always doable. Sometimes it costs more money. Usually it's worth it to get the design absolutely right, even if everyone's a little grumpy about it. This is easy for me to say of course since I neither have to make the 11th hour changes, nor pay for them! ;)
I'm not (too) afraid of grumpy architects, they are already grumpy with us. I just want to make sure I'm fighting a smart fight before I start it - and I'm not sure how to decide on what's the "right" overhang depth.
I've read a bunch of GBA articles about overhangs, and played with the Overhang Design tool. First two attachments are the living room, the other two are the kitchen. Unfortunately the tool does not go to 14ft depth (stops at 10ft). Is there a way to properly model the 14ft overhang?
With the 10ft overhang, for the living room window I have full shading through mid-Sept at noon, then start getting some sun until I get about 3/4 full sun in Dec (again, this is with a 10ft overhang, not 14ft). In the kitchen, I have full shade through mid-Oct, and about half-full sun in Dec - so I am guessing that with the 14ft overhang, I'll never get any sun in the kitchen near noon.
I also drew scaled elevations of the living room and kitchen overhangs - for some reason they don't look all that bad.
I guess I'm not sure how to decide what is "too dark", which I'd like to be able to do in order to talk to talk to the architects (who I suspect will say "it's fine as-is").
Here are the elevation views.
You can search around your neighborhood for places for sale that are running open houses and have a similar sized porch. I would be surprised to hear the amount of light is enough. A room behind a fully covered porch is always dark.
Looking at your place, while talking to your truss people, I would ask them about plenum trusses. If the place is slab on grade, with the mechanical room as is you'll have a very hard time running ducts along the house unless you put them into the attic which a bad idea. A plenum truss would have a long chase and space for most of the ducting which would keep it inside conditioned space.