Expert: Bruce E. Johnson - 8/7/2009

Question
QUESTION: Hello, I recently began building an enclosed porch/sunroom off of the side of my house.  There was originally a screened in porch on a concrete slab there but I tore it down as it was in bad shape and not big enough for the jacuzzi I want to put inside.  I extended the concrete slab and put in a proper footer 36" deep around the perimeter.  The outside dimensions of the new room are 18'-8" Long by 9'-8" Deep.  It is standard 2x4 wall framing.  It has a "shed" roof style, 5/8 OSB, asphalt shingles.  The new room top plate is standard 8' high, and top of the ridge where the roof connects to the house is about 11' high, so it is just about a 4:12 pitch.  I used 2x6 rafters (which all of the tables I looked at said this was sufficient for my span - 9'8"), 16" OC which are joist hangered into a 2x8 PT header board which is lagged into the house studs & house brick every 16" or so.  I used 1/2 x 5-1/2" galvanized lags.  At the sunroom wall the rafters have birdsmouth's cut, are toenailed into the top plate and I also used a Simpson H-1 Hurricane tie on each rafter.  Now for my question:  Originally I was going to run ceiling joists from my top plate of the new wall to the house however I REALLY screwed up.  There are 3 windows on the house wall which will now be inside the sunroom and they are slightly higher than 8', so if I were to run ceiling joists they would go right into the glass if you get what I'm saying.  I am wondering if It is OK to have an open ceiling and just drywall over the rafters.  Do I need ceiling joists for structural purposes since this is a shed roof and not a gable?  I am worried about the extra stress of not having ceiling joists and extra dead load of the drywall on the rafters and am wondering if I undersized them for this application?  The other thing I was thinking about doing was putting collar ties about 2/3 the way up the rafters and attach them to the house with joist hangers.  That way I would have a sloped and then flat area on the ceiling.  I was planning on using 2x4's for this as it's only a 3' span.  I don't know if this would help structurally or not, and then there's the downside of adding the extra dead load of the 2x4's onto the rafters (although I know it would be minimal).  The only other solution I have is to tear out the 3 windows and lower them so that I can run the ceiling joists, although I'd really like that tall ceiling.  I think I covered all of the info you might need.  Any help would be greatly appreciated as I'm at a dead hault until I figure out what to do.

Thanks!

ANSWER: Hi Mike, you should be okay attaching your drywall directly to the bottom of the rafters.  If the rafters are slightly uneven you can cross them with z metal called resilient channel or 1x4 stripping that will help take up any discrepencies along the bottoms of the rafters.  Don't forget to insulate between the rafters and nowadays most roofs of this type are built without any attic ventilation which means no eave vents or roof vents are necessary as long as the whole system is sealed well at the outside.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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QUESTION: Bruce, Thanks for your response.  I was planning on using R-13 which would give me a 2" gap for airflow between the top of the insulation and Roof sheathing.  Was planning on having open soffit.  The rafters are pretty darn even as I'm anal-rentantive and picked through home depot's finest lumber (lol) for an hour to find 15 straight pieces of 2x6 and cut each birdsmouth individually to fit.  Just to reiterate though, you're saying the ceiling joists aren't necessary from a structural standpoint?  I didn't think they were since it's a "shed" roof and there's no way for the roof to "do the split" as there would be in a gable roof (unless it ripped the side of my house that my ridgeplate is lagged to off - and the chances of that are 0).  Also, is there any harm adding the collar ties I mentioned earlier so I will have a flat area on the ceiling?  I would think this would strengthen it and add to the holding strength of the rafters to the house.  Thanks!

ANSWER: Hi again Mike, no you don't need the collar tie in this situation and when ceiling joists are used in conventional rafter framing they are suspended from the rafters anyhow so really you can do whatever you want.  If you want a flat spot in your room's roof then install the collar ties.  If you want the volume of a straight vaulted ceiling then attach your drywall to the rafters.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com


---------- FOLLOW-UP ----------

QUESTION: Bruce, thanks for your response.  Yea, I've seen so many questions where they say something like "I built a deck, do I have enough wood to support it?", and expect some kind of answer.....haha

The next thing that has me worried is if I have enough fastener strength on my ledger and joist hangers.  The roof area 180sq/ft times 40psf (10 dead, 30 snow load) gives me 7200lb's of roof.  I have the ledger (which is 18 feet long) laged into the house studs with qty 16, 1/2"x5" lags.  According to awc.org's calculator, the shear strength of each lag is only 347lb's.  That would give me only 5552 lb's of shear strength.  As far as the joist hangers, they are each nailed in with approx 8 nails each, 15 rafters total, 13 joist hangers total, (the end wall rafters are toenailed and l-bracketed into the ledger).  Again, according to awc, shear strength of each nail is 77lb, so 13x8x77 gives me 8008lb's of shear strength for the joist hanger nails.  Granted the weight of the roof isn't solely depending on those lags & nails, it also distributes the weight onto the front and side walls, and none of the fasteners are in direct shear mode, so is there any cause for alarm?  What gives me a warm feeling is the original porch was 8'x16' and the ledger for that was a 1x6 which was nailed into the house with literally 10 nails (and some of them weren't even into the studs, just the exterior sheathing).  Rafters (which were actual size 2x4's) were tonailed in and had no birdsmouth's cut and no hurricane ties.

I was also planning on studding the house wall so I can drywall over it like in this pick:

[IMG]http://i629.photobucket.com/albums/uu14/mpildis/ledgersupport-1.jpg[/IMG]

Unless I'm mistaken, that would put a solid wall below the ledger and take much of the load off of the lags (provided those cripple studs are snug).  Then the only question is if my joist hangers are sufficient.

Answer
Hi again Mike, first of all your outside bearing wall is holding more than half your roof load.  The shed of the roof pitch transfers a good part of the load to the lowest point.  In the wall you are dealing with the compressive strength of the wall studs which is significant.  Even if your ledger is holding half the roof load you are still good by several thousand pounds. I am not an engineer but from field experience it looks to me like your structure is adequate. A local P.E. can look at it if you still have concerns.  Sincerely bruce e johnson