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Expert: - 7/16/2011


Question
hip roof patio beam question
hip roof patio beam qu
Mr. Johnson,

We have an outdoor patio here in Houston, TX (outside city limits so no building code limitations per se).  I think I remember that the company who built the wall-less patio/hip roof structure said that one column may not be necessary structurally.

The patio only has 2 roof sections that are not supported directly by the walls of the home.  The walls of the home are 2x4 studs on 16" centers with a double plate at the roof line (if that info is pertinent?)

In the drawing, the walls with the thick black lines are connected to the home.

The patio hip-roof structure is on center depicted in red.  The beam in question is in brown on the bottom of the photo and covers a span of 22.5'.

The stick built roof is made 100% of #2 southern yellow pine.  2X6's on 16" centers were used for the slanted pieces under the roof decking and 2X8's (also on 16" centers) were used for the ceiling rafters.  The roof decking is standard OSB 1/2" at best, 20lb felt paper and a 30 year composit Owens shingle.

The 2 posts (1 marked with a black square and the other on center with a red "X") are 4X4 pressure treated pine.  No mounting hardware such as hurricane clips was used in this project.  The 4X4 posts were cemented into the ground during the pouring of the concrete patio below.  I hope that gives enough background for the question.

The beam that covers from the black square post over to the right house wall is a sandwiched double 2X12 (also #2 yellow pine) with no reinforcement other than a ton of nails holding them together.  They may have put construction adhesive between them but I do not know this to be true so let's assume it does not.  They did not use a plywood center between the 2X12's but I do believe that these 2X12's were NOT pieced together.  They were 2 single, long boards.

If I was to knock out the center 4X4 support post, is this thing going to "smile" on me after a few months/years baking in the 100+ degree Houston, TX sun or is this light dead load sufficiently supported with just the end post?

IF I find out that they did use a 1/2" plywood center on the beam sandwich, does this change your answer?  Thank you very much for your time on this in advance!

Bob

Answer
Hi bob, the best I can tell you is that a composite 4x12 beam such as you describe can span maybe 16 feet on its own.  23 feet is a little too far for this type of beam to span and hold up its own weight without  sagging.  My rule of span for supporting roof beams is one foot of span per inch of thickness..in other words a 4x8 spans eight feet etc.  It appears your post placement is correct as it stands..I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
    Questioner's Rating
    Rating(1-10)Knowledgeability = 10Clarity of Response = 10Politeness = 10
    CommentThank you very much for the rule of thumb on the foot per inch span. If I was to bolt an LVL onto this 4X12 (a 11.5X? LVL) how thick do I need to be on the LVL to span the 22 to 23'? I assume an LVL is quite a bit stronger and thus does not need to be as thick. Also, could I do 2 separate LVLs end to end supported at the post in order to make my install easier...lighter ha!


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Bruce E. Johnson

Expertise

I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.

Experience

Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.

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