Expert: Dan Griffin - 10/15/2009

Question
QUESTION: Hello Dan,

Thank you in advance for your time.  I am building an outside patio cover with exposed 2x12 rafters.  Since we will be able to see the roof sheeting I would like to use something that would look nice with a burnt hickory oil stain on it.  I was thinking about some kind of plywood or maybe some 1x6 t/g pine or cedar.  the pine is much cheaper but I've been told I shouldn't use pine outside it would require alot of up keep. I also saw some 2x6 t/g spf that didn't look too bad.  What do you think?  I live in WA and plan on putting a metal roof on the patio cover.  Also my rafters span 22' on one side and 18' on the other (flat roof with the gutter edge at an angle matching the deck)  I was thinking I should block the 2x12s a mid span but am not sure how to do this and have it look nice, any ideas?

Thank You

Billy

ANSWER: Billy, this sounds like quite an undertaking.  Have you honestly looked at stained wafer board? - it may give you a look to consider.  Old tradition would install a bead board ceiling on the bottom of the joists.  A true glue lam and exposed t&g deck job would turn the boards perpendicular to the eaves which would be a major redesign of you plan.  Check at your local lumber yard for rough sawn ship lap roof decking.

Another approach, especially with the metal roof, would be to run purlins across the joists, perhaps 1x or 2x4 on 2' or more centers depending on the gauge of metal used, leaving the underside of the metal deck exposed.

Solid ply sheathing will help wind shear in the structure.  It may still require some form of x bracing in the roof diaphragm and especially in the shear wall carrying it.  I would bridge these joists at least twice.  Solid bridging is perhaps the easiest to install, alternating each side of the bridging line.  X bridging requires accurate lengths and angles to work well.  I have used heavy galvanized steel banding  run from the top of one joist to the bottom of the next.

I hope this has given you some ideas.

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

QUESTION: Thanks Dan,

I am a beginner when it comes to this stuff and this is my first build so you lost me a little.  "Alternating each side of the bridge line"  Is that the like this  |_|-|_|-| instead of |-|-|-|?  When you state "Shear Wall" is that referring to the posts and beam I put up or at the house where I attached it to my roof?  One last thing you mentioned "x bracing in the roof diaphragm"  What is roof diaphragm? and would x bracing it be bracing from one corner of the roof to the opposite corner?  Thank you for being patient with me.  

Billy

ANSWER: Billy, you got most of it.  It is really hard to nail solid bridging in a continuous line and leaves you with lots of toenailing.  By staggering the blocks you can straight nail both ends.  

Once the rafters, roofing, sheathing, etc are all put together, you have a roof diaphragm.
You are creating quite a sail that will catch a lot of wind, snow, and rain water. Snow,rain, dead weight, and live loads try to push down on the diaphragm and make it sag, break, rip loose from the house, or crush the columns.  The wind can cause uplift and shear - two different forces that must be overcome.  Uplift is dealt with by rafter tie downs, column anchors, joist hangers, etc.  Shear is trying to push your structure over.  This force tries to push over your posts and beams in line with the eave.  You can counteract this problem with gussets or knee braces at the columns or moment frames. Shear in the roof diaphragm tries to take the aerial view of your square or rectangle ( I know you plan an uneven end, but I hope this gives you the concept)  and twist it into a parallelogram.  You can counteract this force with full nailed sheathing and/or flat strap or tie rod x bracing in the plane of the roof.

I don't tell you any of this to scare you away from the project, only to make you aware of the issues.  Only you or your design profesional can determine if you have dealt with them.

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

QUESTION: Thanks Dan,

If we decide to go with T/G I would like to do what you mentioned below.

You mentioned this when talking about "shear"
"You can counteract this force with full nailed sheathing and/or flat strap or tie rod x bracing in the plane of the roof"

I'm picturing this as if I would put down  an x on the aerial view of the roof the entire span of the roof is that correct?  

My columns supporting the roof are 3 6x6 with a 6x12 header.  The columns are anchored with 3' 12" diameter concrete pillars and blocked to the framing of my deck.  Here is another question that I just thought of.  I attached my rafters to the header and the ledger on the roof with 2 90 degree angle brackets per rafter do you think that would be ok or should I use hurricane brackets as well?  My neighbor who does some framing that the two 90's might lift.  Sorry that this is fragmented I'm on break at work.

Thanks for your help and I look forward to your reply.

Billy

Answer
Billy, lean on your buddy that is a framer for plenty of advice.  Will there be a local inspector involved?  If so, it would be much better to work out issues with him/her long before execution and inspection.  I would expect that they would require a detail of all connections and even require an engineer's stamp on the submission drawings.

The X banding of the roof can be done as one huge X or two smaller X's.

Have you looked at Simpson Joist connectors and choices?  I would think you could have found every type of connector imagineable.

http://www.strongtie.com/products/Category_list.html
scroll down to Miscellaneous and look at ridge rafter connectors and bridging.

I assume your columns are buried in concrete below frost line and are fastened to the header with some type of plates..