# The vanity and excess cost for thin flat roofs and all-glass walls



## Willin (Aug 20, 2012)

If you crawl around through the trade mags that cover the residential architecture happenings, you are seeing a lot of flat roofs, big tall glass walls, and for the roofs, thin profiles and large overhangs.

I'm a structural engineer and not an architect, and draw plans for builders and clients who do mostly second homes, the locations are in northern snowy climates. A lot of what I see in the trade press and on website galleries are thin-roofed houses built where it rarely if ever snows. So not as much stucture required, for sure.

Here is a model of an as-built thing, a lakefront weekend house, this one built not far south of Duluth, where it does snow. I did the 3D using Sketchup and the process of modeling lets me think through details as I build and place members, to see how I want to apply the arrangements and details to my own custom designs.

Roof overhangs are achieved using two key elements. Type N commercial steel decking which can span and cantilever big distances, and doug fir purlins which if sized and spaced properly, can handle overhangs in the other direction. The structure underneath is I-joist rafters supported by a steel superstructure, the joists running 90 to the doug fir purlins above. The steel deck atop the purlins is again another 90 rotation and the runs are parallel to the building's long axis.

With some walls having large areas of glass, bracing is needed, and two sides of the building have wood framed walls which function as shear walls. The roof, which has 5/8" plywood atop the 3" decking, is one large shear membrane, the ply fixed with 4.5" screws which run through the steel deck webs and into the full-size 3x6 doug fir purlins which are spaced at 16". 

The reinforced concrete fireplace stack, an 8" rear wall and 12" side buttresses, is a massive piece of shear support on one end of the building, and locks the steel framing both ways. 

But look at all this expense, to get this look. The steel frame, the steel deck, the doug fir timber purlins. Insulation is spray foam, and there is way more than enough cavity in which to spray to get an average 7.5 inch thickness under the deck and R-50. Atop the deck sheathed with ply, there is commercial TPO membrane roofing glued down on tapered rigid foam insulation, all done to give a hipped-roof drainage effect with 1/8 in 12 pitch (common on commercial) with the roof draining to grade on all four sides. That continuous tapered foam board insulation on top adds another R-10 to the energy wrap.


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## Rio (Oct 13, 2009)

Very nice! I'd be concerned about ponding with a .125:12 pitch, it has to be perfect to not have some with that flat of a roof.
I was wondering with all of the glass how do you get the wall insulation to work?


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## Willin (Aug 20, 2012)

I did not build this nor do I know who did. Do a little research on commercial tapered roof insulation, and you'll see 1/8 to 12 pitch roofing happening everywhere.

As for glass, I'm gonna guess triplepane was used, with coatings and xenon fill to yield a pretty good wrap.

Here is a fairly good video of one done by same architect, near same location, same roof scheme. You can sort of see the pitches on the TPO roof, which in the right lighting looks like a hipped roof, diagonal hips running out to each corner.


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## Rio (Oct 13, 2009)

They do look really interesting with the incredibly thin roof and all the glass.


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## Joe Fairplay (Aug 26, 2021)

Rio said:


> They do look really interesting with the incredibly thin roof and all the glass.


It's just a half wrapped steel frame with a lot of commercial glass.
They can be quite striking.
I think we could replicate this cheaper than most people think.
If anyone watched the video, did they say what it cost to build?


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## Willin (Aug 20, 2012)

Reported at near $800 / sf. Ka-ching.


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## Rio (Oct 13, 2009)

Willin said:


> Reported at near $800 / sf. Ka-ching.


That's for the interior finishes also or just for the shell?


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## Willin (Aug 20, 2012)

Complete build. Read about the one story build here. Case Study: Box Camp by SALA Architects - Residential Design

And the two story one bedroom. Case Study: Metal Lark by SALA Architects - Residential Design

You can see the same themes of flat roof, heavy steel skeleton all exposed (the tower has it as an exoskeleton), two-way roof framing with 3" type-N steel deck, windows going right up to the underside of the purlins to give the appearance of thin roof structure, and corrugated exterior steel siding.

The one-floor one called Box Camp has more steel in the connector going from the center large box to the primary bedroom with guest bed, three bents like seen in the central big box, and shorter, but both "bed box" ends don't have steel. Roofs are the crisscross of I-joist rafters, doug fir purlins, steel deck, and built up TPO roofing.


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## Joe Fairplay (Aug 26, 2021)

Am I the only one that thinks they could build something similar to that for under 300 sq/ ft?


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## Willin (Aug 20, 2012)

Something like that means clad wood windows, not aluminum storefront, no matter if it is thermally broken. Read the blog link above and look at all the photos. Begin with the slab on grade for the center box, the screened porch, and the primary bedroom box end. It is all dead flat and sawcut and polished, probably with a terrazo grinder, then stained. It might be a lot of metal out, but inside it's all meticulously detailed sheetrock, trimless (which is quite costly with all that tearaway bead), and vg doug fir. And remember, that the steel needs to be finished. I don't think they used corten. Price out the fir purlins (I'm guessing $20/lf), the type-N deck at 18 gage, the window package (likely Leowen again), ten feet of NanaWall, and let us know.

Some quantities for you to get started. Here is the steel. About 9,300 pounds of it, not including the plate weldments for bases, knife plates at the moment connections, and clips for wood ledgers for the rafters.









Then add the principal load-bearing elements of the roof, the series 360 9.5" I-joists and their LVL ledgers. Figure 675 lf of joists, add for the LVL.









Then come the FOHC doug fir 3x6 purlins, 36 of them at 34' each, total 1224 lf.









And the 18 gage type-N 3" roof deck, 1836 sf.









Note that this is only the "center box" of the multi-box build. See the blog for more info. There is more steel, more purlins, more rafters and LVLs, and more deck.


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## Fourthgeneration (Jul 25, 2021)

I look for a relatively cheap automatic power window and door insulated shutter system to make these silly windows affordable energy waste wise. 
Currently most energy losses in today's home are throught the low R windows and doors, and air changes through entry ways and lack of make up air for many home appliances.
Even a ribbon of wall under each window wall would lower costs of building and make the space much friendlier to changes over time/remodeling utilities etc....

Love to see the year around energy bills.....


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## Willin (Aug 20, 2012)

I don't think they care. It's a weekend house an 8 hour drive from home, likely visited only between July 1 and October 15, all of it open-window time. Probably drain the lines, antifreeze the traps and wet appliances, and lock the house and gate for the winter.


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## JenniferTemple (Oct 30, 2011)

Joe Fairplay said:


> Am I the only one that thinks they could build something similar to that for under 300 sq/ ft?


Only if you forgo the architect & engineer.


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## Rio (Oct 13, 2009)

Willin said:


> I don't think they care. It's a weekend house an 8 hour drive from home, likely visited only between July 1 and October 15, all of it open-window time. Probably drain the lines, antifreeze the traps and wet appliances, and lock the house and gate for the winter.


An interesting way to get large glass fenestrations to be a lot more energy efficient is to use 'bead walls' where beads of styrofoam are blown in the space between the panes when the sun goes down. Heard they can have issues with particles of the beads clouding the inside pane surfaces.






On the construction end, out here it wouldn't matter if they cared or not, they'd have to comply with the Title 24 energy requirements in order to get a building permit, looks like it would definitely be a challenge. There's companies that do glass wall structures, at least up to the last building code cycle so they were getting some good R values probably using the methods Willin outlined along with other measures then using other high efficiency systems to do a trade off in order to comply with performance software.
The problem now is California is in the hands of zealots so it doesn't matter how well a building complies with energy usage, it has to also comply with building efficiency. This can be a big problem, used to be able to just go with a bunch of solar panels, high efficiency heat pumps and the like but now that won't cut the mustard.


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## Willin (Aug 20, 2012)

Rio said:


> An interesting way to get large glass fenestrations to be a lot more energy efficient is to use 'bead walls' where beads of styrofoam are blown in the space between the panes when the sun goes down. Heard they can have issues with particles of the beads clouding the inside pane surfaces.
> 
> 
> 
> ...


My condolences to you for having to live and work in such a regulated place as California. If you can move, do so.

But how did this one near Sonoma pass muster?









The whole case study is found here. Case Study: Split Rock Springs Ranch by Nielsen:Schuh Architects - Residential Design


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## Rio (Oct 13, 2009)

They found a way, probably big overhangs, like you mentioned up the thread special glazing, used the performance approach and were able to eek it out.
Was skim reading the article and think the radiant heat might have helped a lot along with the rolling shades, there's a big credit for permanent shading fixtures on certain orientations on glazing, also sometimes glazing gives a project credit. East facing glazing will bring heat into a structure in the morning and this can help with the calculations.

It does suck to have all the regulations but on the other hand it generates revenue. Part of our business is doing Title 24 calculations and we get paid well for them. I'd prefer not to have them but it's a case of if you can't beat 'em join 'em. California has a lot of issues but it's also a very nice place in many places, it's long overdue for the pendulum to start swinging back, not going anywhere.


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## Willin (Aug 20, 2012)

Here is a view of the prototype build showing what I believe to be all the steel. Where there is no steel, the roof loads are carried by conventional wood framing.


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## JBH (Feb 17, 2017)

What gets me about houses like this is that they tend to be built and owned by people who profess to be concerned about energy efficiency, climate change etc., all the while ignoring the fact that even top-of-the-line triple paned windows achieve R-7/R-8 at best...where even a plain old, cheap 2x4 stud wall with fibreglass batts can easily achieve twice that. Standard residential wall construction in Ontario, which now mandates continuous insulation to reduce thermal bridging, typically achieves R-24 at a minimum. All this glass area is a complete joke if you actually care about efficiency.

Not to mention that, in urban areas where I see these types of glass-walled houses being built (I'm in Toronto), what's the first thing the homeowners do when confronted with all this wonderful visibility? Cover it all up with blinds, curtains, etc. to restore a modicum of privacy. So, um, why did you bother with all that glass in the first place?


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## templein (1 mo ago)

Willin said:


> If you crawl around through the trade mags that cover the residential architecture happenings, you are seeing a lot of flat roofs, big tall glass walls, and for the roofs, thin profiles and large overhangs.
> 
> I'm a structural engineer and not an architect, and draw plans for builders and clients who do mostly second homes, the locations are in northern snowy climates. A lot of what I see in the trade press and on website galleries are thin-roofed houses built where it rarely if ever snows. So not as much stucture required, for sure.
> 
> ...


Clean and wonderful model.


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## Joe Fairplay (Aug 26, 2021)

JBH said:


> So, um, why did you bother with all that glass in the first place?


So they could see outside from inside.


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## 3rd4thGEN (2 mo ago)

One More thing for your consideration when the HO wants Glass wall buildings, is where are the shear wall panels/ seismic structure elements?

Love to see how the magazine style summer homes survived the last 10 Years if California's fire mismanagement?

Glass melts, not burns?


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## Rio (Oct 13, 2009)

3rd4thGEN said:


> One More thing for your consideration when the HO wants Glass wall buildings, is where are the shear wall panels/ seismic structure elements?
> 
> Love to see how the magazine style summer homes survived the last 10 Years if California's fire mismanagement?
> 
> Glass melts, not burns?


The steel frame is designed as a moment frame.


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## 3rd4thGEN (2 mo ago)

Thanks, that would work with a few more pounds of steel....Grade beams between columns?


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