Author Topic: SFR Engineering Advice Needed!  (Read 1402 times)

PR Mustachian

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SFR Engineering Advice Needed!
« on: April 04, 2017, 05:18:12 PM »
I have a CAD mockup of an SFR I designed and am looking to build but need some advice on load diagrams when it comes to my 2nd floor deck over 1st floor conditioned space.  I've been using Forte by Weyerhauser to size beams and headers but it is slow and I would really just like an experienced opinion.  I'm not looking for a wet stamp or any liability whatsoever, just an expert looking to teach a novice a thing or two. 
« Last Edit: April 04, 2017, 11:32:06 PM by PR Mustachian »

paddedhat

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Re: SFR Engineering Advice Needed!
« Reply #1 on: April 05, 2017, 05:08:36 AM »
Once our rural area adopted the IRC code, a dozen years back, design work (assuming we are discussing dimensional lumber) became pretty painless. Get a copy of the book (I have found digital versions online for free) and use their span charts for whatever your live and dead load requirements are. When using engineered material, most manufacturers have online resources that are well illustrated and easy to use.

And now for an unsolicited opinion. It appears that you CAD drawing shows finished living space below an exterior second floor deck. JMHO, but having built dozens of new homes, that would be a nope, as in hell no, never. A flat roof over occupied space WILL leak. Not "if it's done correctly, maintained correctly, and inspected on a regular basis it will be fine", but more like, "It developed a hidden pin hole in the flashing where it meets the wall, I didn't find it until water staining appeared on the ceiling, and now the entire roof/floor structure is a rotted, moldy mess". Might not leak in a year, or ten, but at some point in the life of the vast majority of these things, somebody will pull the ceiling finish off underneath and say, "holy shit, what a mess".

PR Mustachian

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Re: SFR Engineering Advice Needed!
« Reply #2 on: April 05, 2017, 08:56:09 AM »
Thank you for the reply padded.  I'm very familiar with the IRC and both my state and county amendments but they don't help a lot with multiple spans or with the shear forces that will act on the deck beam/rim joist connection for the deck.  I am using the online resource provided by the manufacturer as I stated (Forte by Weyerhauser) and it's just not very user friendly.  As far as your opinion,  I do agree it's a critical part of the build.  The design for the deck over conditioned space is from Green Building Advisor and written by my idol Joseph Lstiburek.  Not to take away from anything you wrote because I know you are backed by building history and your personal building experience that surely surpasses mine.  Seems to me that most of them leak because of poor flashing or because of using air permeable ceiling insulation.  Bear in mind that I am building this entire thing myself, flashing included, so I'll be able to take my time and really ensure it's done right.  Most houses are built by contractors that are motivated by speed and cost savings and have no vested interest in the long term performance of the house.  That having said, you could very well be correct and five years from now I'll be back here telling you so ;)

paddedhat

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Re: SFR Engineering Advice Needed!
« Reply #3 on: April 05, 2017, 11:00:47 AM »
Yes, I too have a huge respect for Joe, dating back decades. A lot of my experience comes not only from residential work, but from larger projects, especially institutional type projects (schools, hospitals, hotels, etc)  In that realm, most work had flat roofs, ad anybody in the game will tell you they all leak. If you have convinced yourself that it's a good idea, and there is no turning back, see if you can find a contractor who does fiberglass roof/deck installations. I have talked to a builder on the coast who is plagued by flat rood/deck situations and  the fact that they are in demand on beach homes. He claims that a properly installed fiberglass roof is the only thing that will have a chance of decades of performance. These things are done much like a boat with glass matt hand laid into a thick waterproof surface.


As for your deck rim concerns, if they are limited to the screen shot posted, I do not see an issue.  There are virtually no second floor loads, since the wall above the rim is essentially a curtain wall running with the roof trusses. If you are looking to create a rim that's flush with the plane of the wall framing, for a deck that is a few inches lower than the second floor,  inset a  single laminate of LVL into the first floor framing. As for shear, the Simpson catalog will have everything you need. One final though, I assume that you are going with engineered lumber on the as much as possible? I would continue that out to the deck/roof assembly, as PT in this application will shrink ridiculous amounts, with a SYP treated 2x10 often losing 3/8 to 1/2" in total height once it reaches moisture equilibrium in a dried out structure. That kind of shrinkage can be a major factor in roof leaks, literally pulling flashing details and caulk joints apart. Good luck.

EDIT: Now you have me overthinking this too...LOL. The rim should be a lot simpler than I described, or whatever answer you are looking for  from your CAD program. I would simply cut an LVL rim the length and width you need.  Then install it with Ledger Lock hardened screws, hitting each stud and other framing member you can. Fastenmaster is the manufacturer and they will have all the load charts and ICC approval letters as required. If you do this, remember to offset your deck framing by 8' from the stud wall framing so that the joist hangers do not bury the Ledger Lock fasteners. Inspectors are no too fond of approving fastening that they cannot see.
« Last Edit: April 05, 2017, 11:11:34 AM by paddedhat »

trollwithamustache

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Re: SFR Engineering Advice Needed!
« Reply #4 on: April 05, 2017, 11:52:50 AM »
...use their span charts for whatever your live and dead load requirements are.

This. If you use the same span chart your building inspector will be looking at, it makes life very easy when it gets to the passing inspection part of building it. You may be able to calc it out and use a cheaper beam, but you may not save enough to make dealing with the inspector worth it. (assuming you aren't doing all the work yourself and during delays you are paying people)

PR Mustachian

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Re: SFR Engineering Advice Needed!
« Reply #5 on: April 05, 2017, 01:19:03 PM »
Thanks guys.  Padded if you notice on the pic, the deck joists and sleepers are running perpendicular to the main floor joists so no worries on hiding the fasteners (I can easily stagger them 24OC with hangers) but there are no studs to fasten to either because there is no first floor wall below the main floor rim joist that buts up against the deck rim joist.  I could throw some blocking in there but otherwise it's just the 4x6 deck header bolting to the 2x10 main rim joist, the top plate and the 4x6 header below it.  As you said, the first floor below the deck carries all the loads but there are still some shear forces there.  Interesting info on the PT.  I'll bear that in mind for the sleepers that will go above the flat roof and carry the deck
« Last Edit: April 05, 2017, 01:42:00 PM by PR Mustachian »

paddedhat

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Re: SFR Engineering Advice Needed!
« Reply #6 on: April 05, 2017, 02:13:05 PM »
Can you generate a cross section drawing through the deck rim where it meets the house?