Modern House with Flat Roof - Thoughts?

[reedie2000]

So here's my first modern desert house with flat roof. I'd like to get a structural involved but I don't want to be totally embarrassed. I think I'm ok with things like shear walls but the roof is bananas.

 

I've attached:

1. a render of the model (the roof profile shown is too thin, but that's the idea). Note the wide semi-supported gull-wing overhangs. These have grown and will likely need some steel.

2. A roof-ceiling plan showing the joist layout. Trying to keep spans under 23' or so; that seems to be the limit for 11 7/8" I-joists.

3. The draft roof CAD detail. I'd like to get integral headers within the ceiling/roof so the windows can be right up to the ceiling - note the joist extenders.

 

Any and all thoughts welcome. Am I overbuilding? Do I not understand joists? 

 

Thanks guys!

 

 

 

 

[JiAngelo]

The 32' span is your biggest hurdle.  Deflection will likely require steel to be greater than 12" tall. I know glulams would be greater than 16" based on another job we just sized.  ForteWeb.com will allow you to run some calculations yourself.

 

Maybe (4) columns spaced 6'-24'-6' would work.  That would keep the posts out of the triple door openings line of sight.  Headers in the ceiling over patio doors negates cantilevering (which would lessen the load actually on the steel beam.)  Uplift would still be concern.

 

Give it to an engineer and see what they have to say.  Why be embarrassed? You're at the point where you need his input to refine your plan.

[GeneDavis]

This company makes open web joists and there might be hope at a joist depth of 20 inches.  http://www.openjoist.com/spans.html#20

 

Consider doing a simple set of construction docs drawn with roof trusses drawn in a 20 inch cavity space for the large span roof, and 16 for all the smaller, then get it to a roof truss supplier locally and see what they say.

 

I didn't look at your plan.  Maybe only the big-span roof (is it really one big huge space under with no bearing walls at all) needs the trusses.  The rest can be framed in various i-joist sizes.

 

Download and learn to use Trus Joist's structural design app, ForteWeb.  You will be able to design all the flat roofed houses you can dream up.

 

Edit:  I spoke too soon.  Just opened up my ForteWeb page, opened a new file named Maricopa after checking to see if there was a snow load (it is zero) and set the clear span at 32' then let the app do some work for me.  As you can see, one of their 16" deep models does it just fine at 24" spacing.

[reedie2000]

Thanks John and Gene. I really appreciate the information. I'd like to keep the narrow roof profile if possible. I don't need an open web system for HVAC, so would prefer to stick to I-Joists if possible. I'll definitely check out ForteWeb!

 

My previous contacts with Structural Engineers have been less than enjoyable. If you work with any in California you like, let me know. 

[GeneDavis]

Give us some iterior camera views of the spaces with 32 feet of clearspan roof overhead.

[reedie2000]

Sure Gene - whatcha think? Ten foot ceilings - well maybe 9' 9" with the joist extenders bringing the ceiling down just under the double top plate.

 

I know I can get the structure to work if I had 2 million bucks but can I do it with more standard materials? Look at this one: https://www.contemporist.com/house-has-a-thin-floating-roof-that-extends-out-15-feet/

 

Sharing the floor plan with you as well. I have a bubble marked where I think I'll have to align a single bearing wall. This plan doesn't exactly match the earlier joist roof/ceiling plan, since that was a test and the framing is not correct in my model yet. You have to force Chief away from the standard way of framing I find.

[JiAngelo]

Take a look at these P3 I-joist span charts.  On page 30 the PJI 90 series 11-7/8" spans 32' @ 12"oc.

 

You could run the rafters left to right on the porch only and eliminate the steel beam.  Your (2) columns would be supporting heavier 8' beams back to the house with rafters hangered on each side.  Lesser series will work elsewhere.

 

On a side note, minimum flat roof slope is 1/4" per foot, so your 2" tapered iso is only good for about 6' (min. 1/2" req.)  Your going to need ~4-3/4"  to ridge front to back, then hip the sides.

 

02-P3-Joist-User-Guide-US.pdf

[JiAngelo]

Additionally, ForteWeb shows a single 32' TJI 560 11-7/8" @ 12"oc should work as well.  Another option would be (2) 32' TJI 560's 9-1/2" at either 12" or 16"oc.  Costs more, but is 2" thinner in roof height.

For the 8' beams supporting ~12' tributary span one side and ~16' tributary span other side, requires (2) 11 7/8" 2.0E ML's using top hangers.

 

Using your framing image, here's your iso tapered ridges if you want to maintain the thinnest perimeter profile everywhere.

 

And here's an alternate iso tapered ridge layout that requires building up the inset area noted.

 

Chief should readily show you the first if you set your roof to 1/4" /12 slope.

The second requires a bit more work

I also think you could 14" I-joists on the interior 32'x23' room and save some tapered iso. (those would be flush with the bottom of the 11-7/8" and rise above 2-1/8")

ForteWeb actually says (1) 14" TJI 560's @ 19.2"oc can span 32', which would then have all your rafters running left/right and make it easier to run MEPs.

 

Hope this helps.

[GeneDavis]

These guys, they're in San Antonio, do a lot of flat thin roofs with big overhangs.  Money is no object.  https://www.lakeflato.com/projects/houses

 

And this summer house, built up in Wisconsin has thin overhangs, done using a combination of metal roof deck and douglas fir lookouts.  https://residentialdesignmagazine.com/case-study-box-camp-by-sala-architects/

 

Here is another look at the Box Camp house.  

https://salaarc.com/project-types/featured/box-camp-bunkhouse/

 

Since Box House was built in a location with ground snow loads somewhat similar to the locale where I do work, I did a study of the structure using Sketchup to understand the details.  Here is the whole thing (they have since added a guesthouse wing that is very cool).

 

The structure begins with steel bents, the steel all exposed in the interior finish.  I think it was CorTen but maybe just painted. 

 

The steel bents are there to support the I-joist roof structure over the conditioned spaces.

Then comes the part that does the overhangs.  First the 4x6 doug fir lookouts.  Get out your wallet.

Atop this goes the metal roof decking.  Over all this is the tapered polyiso buildup that gives the membrane roofing the needed drainage pitch.

 

The same Sala Architects partner that did the Box House project, did this little three-floor tower, one bedroom one bath mini kitchen with adjacent sitting area, also in Wisconsin.  Note the same arrangement of timber lookouts and steel decking.  https://salaarc.com/project-types/featured/metal-lark/

 

 

 

[reedie2000]

Wow. Thanks guys, this is great. A lot to take in.  

 

Let me go through everything and think it out. Didn't process how thick the foam would need to be. 

[Gawdzira]

I think they are getting the thin roof overhang profile but hiding the deeper structural beams/rafters. Either way, their work is outstanding.

[mtldesigns]

11 hours ago, Gawdzira said:

Either way, their work is outstanding.

if your talking about SALA, I def agree...  I have their link saved as a favorite.