Expert: Bruce E. Johnson - 4/1/2009

Question
QUESTION: Thanks for taking the time to look at my question. Ok so here’s the scenario…I extended my current concrete pad to be about 20’W x 18’D. I want to build a fairly large patio cover covering this pad. My homes roof slopes down along the length of where the patio cover is to be. I attached a make shift diagram to better illustrate what I am trying to explain. Anyway I was planning on having a total of 5 posts around the perimeter and one a couple feet away from the house in the center, out from the house as far as the fireplace, to help bear the large span. (figures I could frame it that small area in as storage). I plan on using pine for all posts, beams and rafters. I want them all to be seen from underneath. On top of the rafters I was thinking of using 1”x whatever to cover the top. Shingles are what i currently have on my roof so want to continue that on to the patio cover. So my first question is on size of timbers. Here’s what I was thinking:

Posts: (6) 8” x”8” x whatever height they need to be.

Perimeter beams: (4) 8”x 10” x 9’ and (2) 8” x 10” x 10’

Rafters 4” x 8” x however long they need to be.

Center beam: (1) ?????? was told around 8” x 12” x 24’ or 10” x 12” x 24’ What do you think?

So I need to know if these sizes are adequate given my design? Is it overkill or do I need even larger? Remember I was going to use pine (for cost reasons) but is Douglus fur a better choice? Maybe just for the large center beam?
 
Thanks for any help you can give thanks.


ANSWER: Hi Eric, this is a pretty beefy structure for sure.  Your perimeter beams and columns can be 6 by's unless you are going for the massive beam size for looks.  You didn't mention what spacing on your rafters.  Three foot spacing will accommodate 1" tongue and groove sheathing, anything over that up to six feet needs to be 2" tongue and groove.  Your center beam size can be reduced if you use an LVL laminated beam.  However if you want the big timber look and you can get the beams sized to your needs then I really can't fault the big timber look.  Ridge beams are normally sized to relate to the rafter size when it is cut to pitch.  In other words a 2x8 rafter cut at a 5/12 pitch is over ten inches long at the cut end so your beam would need to be larger than that or step up to a 12" deep beam.  A 6x16 should handle this ridge span.  Believe it or not but southern yellow pine has a higher structural rating than doug fir.  So you are on the right track for lumber choice.  However the difference isn't that much and if you prefer the look of the douglas fir then by all means it is a wonderful wood as well.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: Hey thanks a lot your response has really helped me. I had a couple questions though.  I was talking to a guy that is building a house for a family member and he said if he was doing building it he would use a continuous beam along the sides. I was wondering what you thought about this? As i am sure you know price on those massive beams is not cheap so if not needed it would be easier for me and cheaper to use two.

And my other question has to do with the area over the existing roof. (I kind of outlined it in the drawing attached, the red lines). So i figured i would run a 6", 8", 10" by (what size?) from the ridge beam to the roof and two ledger boards from the the outside beams to the point where the three meet. Then run rafters (what size, spacing?) along the length. The two outside ledger board would be under the outside beams as well. so the beams are not just attaching to the roof itself. How does that sound?

Second when attaching that area should i cut and remove the asphalt shingles in that triangle? I know i will need to slip W flashing under the shingles along those lines so figured easier to cut them out there. Also thought better to attach directly to the plywood of the roof rather than over the shingles.

Any help is greatly appreciated.  Thanks again for your help.

Eric



ANSWER: Hi again Eric, it looks like you are using a truss system for the main body of the roof?  Or are you planning on hand framing the rafters?  A front beam will be needed if you are hand framing unless you add a tall center post to support the ridge beam at the outside.  Otherwise you will have to figure out a way to keep the sides from spreading out  as the rafters have a tendency to push out at the bottom.  With a well supported ridge beam the ridge beam supports the rafters and keeps the side beams in place.  As far as the valley framing goes: tear the triangle down to the plywood and install sleepers along these valley angles.  The sleepers are wide enough to handle the rafter cut.  In other words.  I suggest 2x6 rafters spaced at 24" in this area.  So you will need 2x8 sleepers laid flat and nailed to the existing rafters.  Then when you frame your valleys you have a solid support for your rafters to attach to.  As far as the roofing tie in: I like to tear back the existing shingles far enough to put down a layer of peel and stick Ice and water shield over the existing felt and the new felt.  I use this product on all my overhangs, ridges, hips and valleys.  Then I do the aluminum valley flashing and complete my shingles.   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: hey again i had another question. Well first off I finally started building this thing! got all the posts up and the side beams. So anyway i have another size question. I ended up using 8" x 8" for the posts and 8" x 10" for all the side beams.  I was able to find locally and at a decent price a 8" x 12" for the ridge beam.  So since I am kinda going for the big timber look, I was going to use 6" x 8" or 6" x 10" for the rafters spaced around 4 to 5 feet o.c. what do you think? would the 10" be better to match the 12" ridge beam when the rafter angle is cut? Second, I know you had mentioned 3 foot spacing on the rafters and i could use 1" x for the sheathing and anything more need to use 2" x. Would it be a option if i used 1"x with 4 to 5 ft spacing and used a layer of osb on top of that? I'm doubting it but seeing if i could save a little money.  thanks

Answer
Hi again Eric, I suppose you could do the 1x with the osb cap with a 4 foot spacing if you are using tongue and groove 1x.  The tongue and groove locks the wood together and helps keep it from sagging under its own weight. As far as the rafter sizes go.  Try to size it so that when the rafter is cut at its appropriate angle the angle cut is not larger than the ridge beam. I hope this information helps feel free to write again regarding this or other matters, sincerely, bruce e johnson..bejohnsonconsulting.com