# Many-layered (and quite expensive) flat roof



## Willin (Aug 20, 2012)

Flat roofs and shallow-pitch shed roofs are all the rage now in residential architecture, and I've been studying one a client would like a version of on his own little camp in the woods.

The subject of the study is a 20-foot square two story arrangement in structural steel, glass, and rusty corrugated steel siding, that got build a couple years ago in the upper midwest, and got a lot of publicity, which is how my client came across it. It's an air bnb and thus has a lot of good film footage showing it off.

It is meant to have one of those flat roofs with large overhangs, with quite thin structure, different from the thick chunky look seen in a lot of today's "modern" marvels. There is a well-known firm down near San Antonio, Lake-Flato, whose signature look is these paper-thin flat roofs. But Lake-Flato is down where snow load is just a fantasy. This one in WI, done by the also-well-know firm SALA, from MPLS, has to deal with something like maybe 60 psf gsl, resolving to maybe 40 or so for roof snow load.

So let's take a look. First, from SALA's project writeup, here is the kit of parts, in a nice exploded view.








You have to look close and study what pictures are available, and this one shows what can be seen of the roof's underside. Looks like 1.75 x 11.25 LVLs on about 19" centers, and there is a plywood skin on top. Or maybe not on top. More on that later.








The LVL joists are all that is needed to handle the roof load, but the archy wants that Lake-Flato big-overhangs and a thin-edge look from outside, so the next layer is there to give the overhangs at front and back. What appears to be maybe rough sawn, maybe planed, but I really think full-size 2x8s, crisscrossed atop the LVLs is done in an array on top, with big six-plus-feet overhangs front and back. Spacing again at about 19". This shot taken during construction shows the 2x purlins.








Then atop that, we get what I believe is rollformed steel roof decking, painted white on underside, running crosswise the purlins, and doing 4-foot overhangs at the sides. See if this pic is saying metal roof deck. Various makers do a 3" deep 8" centers product, called type-N deck, in gages as heavy as 16, which can cantilever.








Then over the type-N decking, we get insulation, likely for the subject job something thin, and then a membrane. Like this.






But I am thinking they used tapered iso foam deck insulation at maybe 1% slope (1/8" per foot) to eliminate ponding. Such an arrangement would have thicknesses (if you are going minimal) at 1/2" at the perimeter, rising to 2-1/4" at center. So figure the 20x20 center area, over the heated space, has an average thickness of iso board continuous at 1.5 inches. So R-8.

So how did they insulate the roof? They certainly did not put much on top of the N-deck. Look at this pic. They got the Texas hill country thin roof look, which is why I said the board starts at 1/2" at edges.








Remember there is a 7 or 8 inch deep cavity between those purlins. I think they put it there. It is not continuous, but at least it is something. With closed cell foam it could be as much as R-48, to add to that continuous R-8 on top. So not too bad.

But all those layers add up to an expensive roof.

Back to that plywood, though. The plywood may be above (maybe not) the LVL joists. Go back and have a good look at that inside photo. Look at the visible seams of the plywood. Looks to me like they ripped plywood to go joist to joist. Why else would a seam not go continuous across two bays? Look also at that pic of the frame-up, and note all the light coming through the joists and purlins above. I'll bet the plywood came after. They left that cavity open so they could sprayfoam from under, between the purlins up against the roof deck underside. And the plywood came after.

Furthermore, consider how the 3" deck adds a lot of cavity depth to that offered between the 7" (or 8") purlins. So maybe R-60 for this roof? Who knows. When you are spending this kind of cash on a glass and steel jewel box and you want to feel all net-zero in your virtuous self, what's another thousand board feet of sprayfoam?

What do you think?

Are they LVLs, the rafters?

Are the purlins 2x8? Full size or dressed?

Is the white part steel decking?

Pretty pricey, right? The whole thing?


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## META (Apr 9, 2015)

The plywood looks to have been ripped in 2'x8' lengths and then installed parralel with the joists, with butt joints alternating every other bay. That was likely installed prior to above roof layers. 

TPO over foam could be a reasonable roof system that could integrate with a steel decorative fascia line.

Yes this is pricey. Have a builder price one for you so you can decide whether you want to invest in this type of real estate. Location will influence build costs.



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## Willin (Aug 20, 2012)

I thought that, too, at first, the part about the plywood atop the joists before the 2x8 purlins went on. But have another look at this pic and notice how much daylight is coming through the roof framing, consisting of the 2x12 LVL joists and the 2x8 purlins laying on top. Contrast that, with the floor deck below, which is surely decked atop the joists, producing deep shade below.

All the press on this building portrays the job as one for which energy was a prime concern. Triple-glazed windows, and ultra-thick double-framed walls (where there _are _walls, half of the building is glass). I'm guessing they would want at least R-50 for the roof, and to get that with iso board atop the metal deck, there would be almost 8 inches of it. That roof ain't that thick in the pics. And it would be wasting a lot to insulate the whole thing, including its mammoth overhangs with nothing underneath but air. I'm now even more certain it's insulated from below, and that plywood all went up after, carefully ripped and fit, then fixed to the bottom of the purlins with construction adhesive and some finish nails. Those joints we see are centered on a purlin.


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## META (Apr 9, 2015)

Fair point. However, I don't think we're looking at 2x8 perlins cantilevered off that steel beam on the front. Those look thicker and potentially taller. 

It is a difficult one to determine how far into the insulation area the perlins (or maybe we should call them rafters) that we can see, go.

They may have stopped those on the doubled up LVLs to gain support and strength from drop... from their cantilevered load.


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## Willin (Aug 20, 2012)

OK, so maybe not 2x8, but remember I said full 2x8 not 1.5 x 7.25. But whatever size, they have to be engineered to handle however much cantilever is out there and relative to the gsl (ground snow load) as it resolves to roof snow load. For the WI location of the prototype build, number 1 doug fir in full 2x8 would make it. For my clients location, with 100 psf gsl, we're gonna need full 2x10.

Look at the images attached. I modeled the arrangement in Sketchup to study things. The tapered roof insulation is the top cap, and under are the full-length purlins at 2" x 10" cross section, then the type-N 3" 18 gage roof deck (we may need it in 16 gage), and finally, holding it all up, the 20x20-foot array of 1.75 x 11.875 LVLs.

We can special order the doug fir at the required length of 34 feet, which can be special-ordered from a number of western sources. I did not show doubled LVLs in the two outermost runs, because I believe the steel picks up the cantilever load.


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## META (Apr 9, 2015)

So my hunch is that they doubled up those LVLs to counter uplift on them, from the significant cantilever load out front. If this is the case, we may be able to utilize more economical perlins in the burried area/thermal envelope..instead of full length members. 

I'm still thinking they sheeted the majority of that plywood before the upper level. I just can't see how else one would go about it, save for some sort of small trim detail along the LVLs were the plywood installed afterward. 

Do you need to make this work exactly as the original model/design, or more just capture the majority of the visual features? 



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## Willin (Aug 20, 2012)

The client wants the huge overhang over the view side, and one way to get it is as done with the prototype. Those purlins are there strictly to support the overhang. The N-deck roof is running the other way. There is a rule of thumb about cantilevers, which says you want twice as much inboard as is sticking out, so if the stick-out is 6 feet, you want 12 in. And since there is an overhang in back, we have something like 5 out and 10 in back there. This, plus the roof deck is spaced up by the purlin depth at front and back and needs the same space-up in the middle. Thus continuous purlins.

Note that the sheeting, the plywood, is there to decorate what you see when you look up. It is not there to make a structural membrane or to provide a surface for the roof. The roof is decked atop the purlins. They wanted the look of a wood ceiling. You have purlins up there at 19.2 inch centers for fasting up the sliced ply sheets.

I think the photo makes it clear the purlins bear on the roof joists, with no decking between.


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## META (Apr 9, 2015)

I noted the plywood was decoration specifically because of product choice and parralel with purlens. 

The 2/3:1/3 rule can be overcome with proper uplift protection and connections. The load and member stress at pivot point obviously needs to be factored.

Why the two double LVLs?

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## META (Apr 9, 2015)

How about they just cut the plywood really well and fastened it to the perlins later...

That is a difficult way to go about it though. 

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## Djea3 (Jul 6, 2020)

I like the design. Since there is no fascia on the overhang I expect that there will be water running back along the joists there? Hell it is hard enough to keep water rolling down from a fascia without wicking back up into a soffit and rotting wood. Exposed...it may dry quickly, but what of build up of ice there in cold weather? could that be an issue if point loaded at the end of the cantilever?

A hell of a lot of clips needed between the two sets of joists, but damn that roof should be pretty strong built that way. did you ladder the unsupported sides after all?

Well, what was decided? seems this is an older thread.


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