Ultra thick roofs for shed-roofed modern houses


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This is more about architecture and aesthetics, but what the heck, I'll put it here rather than in chat.


I am doing one of these with what I call slab roofs.  Low pitched, large overhangs, no ridges, all shed-type roof planes.  A look through the plans-sellers images, and in galleries at realtor shops, and also in the Chief sample files such as "Austin," and others with roofs like this, I see very thick roof edges.


And this style demands thickness.  The roofs must look like appropriately thick slabs.


Austin has 1.5" x 16" subfascia, for example.


The one I am doing does not have the mass in its elements to go 16, but still, it takes 12 to make it look right to me.  My question to you, is how are you framing these type roof edges?  Depths like this take a lot of 2x12 lumber, or composite rimboard lumber, or a buildup of OSB and sticks.  Do you just say I give up and sock the timber to it?


I attached a snip I took from one of the plans-selling websites.  The one I am doing has about this scale.



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SIPs are an option that will give you insulation, structure, and thickness all-in-one; they are often our go-to for this type of project. Or, sloping parallel-chord trusses with a cosmetic eave attachment built in to the truss design.

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I joists for the roof framing - with a dropped ceiling (for HVAC) 


What we typically use is anywhere from 14" to 16" flat roof joists with a minimum of a 2 x 6 depth for the interior "drop" The ideal flat roof aesthetic is when the exterior flows into the finished space - and to Alan's point it does indeed appear like a continuous slab. 


In the image of the first post... that sloped roof appear to have a rather large rake - which would probably be made from a Versatek or Azek material - if not a build up of T & G siding. 



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So lots of ways to do it.   I have done a bunch of modern homes where the builder still wants to keep conventional roofing.  By code we can do that here with 2:12 and double underlayment.

For those projects we have used I joist or roof trusses.  Roof trusses work best as HVAC runs are easy.   Spray foam is pretty much a must.


If going high budget, you can just make roofs flat w/ floor truss/ i joist and then install commercial built up roofs.   Rally depends on slopes, and overall spans.   Also if window headers go right under soffit makes a big structural difference.   So very design dependent.


I try to 'cheat' that look a lot while still staying in 100% conventional construction.   



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Mine's nowhere near as sophisticated as Mr Hampton's beauty.  As it is looking now, I have the high roof, which is not "modern" flat, but a 4:12 pitch, framed with series 230 14" i-joists, and a pieced-up overhang as shown in the detail, attached.  Why I did not go with just extending the whole joist out, I don't know.  Probably because I was thinking about a much thinner edge than what this has become.  


The roof arrangement is a hybrid, the high part being the thick-edge 4:12 "slab," the front a low 2:12 arrangement with a hipped piece.  Roofing will be standing seam steel.


I gave the hipped roof part a smaller fascia, thinking the heavy slab look for the edge was only right for the shed pitches.1615285417_Screenshot2022-04-15152504.thumb.jpg.5c196e1f34a793c69a95ee52af3b21ae.jpg

Living room view with figure.jpg

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Thank you for the kind words. Modern has become more of a trend these days, and it's always good to have some diversity. 


My findings on shallow (or flat roofing) however, is that the snow loading requires the structure to be in the I joist variety - or as in Alan's project, some kind of truss work. In colder climate regions, there's almost no way around not using engineered lumber unless there were some steel involved to reduce the spans.


To that end, we always use TJI's in the floors to avoid any kind of shrinkage or reverberation (Jurassic Park effect). There's nothing's worse than seeing a cup of water jiggle when someone steps across a room - particularly in a new home.  ...Per the attached progress image, the only Douglas fir that we use is in the wall framing. 


BTW... I was looking at your elevations Gene, and the rakes could definitely be made more delicate (if you wanted them to) by simply keeping the rafters tight to the siding. With this approach, the rake flyers and sub-fascia can be done with conventional material (like 2 x 6's). Then the finished trim won't become excessively bulk if you'd like to steer away from that appearance.


If gable end overhangs need to remain heavy however, then the appearance can always be softened by using different trim colors. My guess is that 9 times out of 10, the roof framing is going to overhang beyond the outside wall - just as you have it shown - and therefore, it's inevitable that the fascias and rakes are going to be a tad heavier. 


That's a very nice design by the way. If you do a search on "skillion" roof styles there are many examples. 





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What would the detailing for 14" IJoists would look like? Why can't you use 12"?


Technically 3" of closed cell = R21 and 3x 3" mineral wool @R23= 69.


So what is the point of going 14" I joists?  Unless you have some really large spans.

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