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ADVICE NEEDED: Rafter spreading

JustThink's picture

ADVICE NEEDED: Rafter spreading (post #206961)

Hi, I have been wanting to cover my back patio for a while now, and I want to do it right. My goal is to leave the patio as open to the yard as possible without ruining the integrity of the structure. The concrete pad is 30' wide and 8' deep, and I will setting new footings. I would like to frame a cross gable and tie into the existing roof, and shingle it to match.

Here is my plan:

I would like to use only a 6x6 post on each side with a 4x6 beam running back to the brick and top plate behind the eaves. Then run 2x6 rafters 16" OC. On the end gable I plan on adding a rafter tie somewhere high withing the bottom 1/3 of the rafters and adding supports. With the same idea as a truss. I don't know what that is called but I have it in the drawing. 

1. Will 6x6 pine be enough for posts?

2. Are 4x6 beams overkill or not enough? 

3. Will the "gable truss" keep the posts from spreading. I am thinking about doing one on the end and another a few rafters back, in the center. 

There are many more questions but these are the most important. Does anyone see any mistakes, please, let me know what you think. Any addtional advice will be welcomed.

 

Thanks in advance,

Taylor 

Ofhand I'd say no way in hell (post #206961, reply #1 of 19)

Ofhand I'd say no way in hell will that work. A 30' span will have a heck of a lot of compression in the rafters. No way will a couple of rafter ties resist those kinds of forces.

Trusses will span it easily. If you want it more open use scissor truses.

Well, as I read it he's only (post #206961, reply #2 of 19)

Well, as I read it he's only going out 8 feet, so a "couple" of ties amounts to one about every third rafter.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I am worried that in the end (post #206961, reply #5 of 19)

I am worried that in the end I will agree with you. Trusses are not out of the question. I just really wanted to accomplish this with as little outside help as possible. An ego thing I suppose. Do you think rafter ties and supports every/every other rafter would help my cause. I know I'm being stubborn but I just had that original idea of rafters only.

For the rafter question, What (post #206961, reply #3 of 19)

For the rafter question, What is the snow load (# per sq. ft.) for your location?

Regardless of that, 6 x 6 posts will be more than adequate for anywhere in the country.

4 x 6 will be plenty for an 8' span.

Also, if you will need a permit for this, don't settle on any design until you check with your inspector. He will also know the snow load, if any, for your location.

A quick search shows the snow (post #206961, reply #4 of 19)

A quick search shows the snow load in my area to be 5-10.

First off, you've given no (post #206961, reply #6 of 19)

First off, you've given no indication where this is located. Snow, wind, earthquakes....?

Now, what you have sketched looks like a truss with grossly extended top chord. Not only does this thing look precarious I'm betting it actually would be if you were to follow through and build it. You need an engineer or architects help. 

Central Mississippi. I will (post #206961, reply #7 of 19)

Central Mississippi. I will get in touch with someone who can help make this safe.

 

If I dropped the bottom chord down would it look any less precarious to you?

For Example: http://www.houzz.com/projects/18463/Lang-ranch-style-patio

In that design they're using (post #206961, reply #8 of 19)

In that design they're using a beam across the top to carry half the load on the rafters, and the beam is in turn supported by trusses on each end.  This is different from a more conventional roof where there may be a ridge board, but it doesn't really carry any load, and so the rafters must be tied together near the bottom to resist outward thrust.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Snow load wouldn't be an (post #206961, reply #9 of 19)

Snow load wouldn't be an issue in that area.  But you'd still have live load for construction.

How about rafters with a ridge beam? If the top end of the rafters is supported you have no worries about horizontal thrust.

After lot of reading on roof (post #206961, reply #19 of 19)

After a lot of reading on roof design I found that what I descrided is a legitimate type of truss. There are people out there using them. It has a few names: raised collar truss, raised tie truss, and the most official I think, modified king post truss. I think I will do further research in order to follow through with my original plan successfully. Thanks for your efforts.

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Thanks guys! That's a good (post #206961, reply #10 of 19)

Thanks guys! That's a good point. It looks like I will still need to run a beam across each post in order to support the ridge beam. I suppose it wouldn't be that bad. Safety wins over a clear view of the sky. I wouldn't want to be looking at the clouds with this thing laying on top of me :-P. So now it seems that I must decide on scissor trusses or a ridge beam. I plan on building this spring. I will post pictures.

Just (post #206961, reply #11 of 19)

Remember that a ridge beam needs constant support down to foundation or a header calc'd to carry the load.

A post in the middle of the gable to footer would suffice-as would  continuous posting down to the foundation at the house outter wall.  Building over the existing roof would not mean a post to foundation at the end of the ridge beam.  If you didn't post down at the outside house wall-all bets are off.

The big span is across, you have little in the 8' depth.  So no post in the middle of that long span of 4x6 is way too much in a framed "truss".

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Thanks Calvin. If I decide to (post #206961, reply #12 of 19)

Thanks Calvin! To avoid the issue of putting load on the existing roofing deck I'll build it as a solid free standing stucture, then, attach to the house with a cricket. To be honest, I really think I'm going with scissor trusses. They offer what I'm really after. Which is no middle support and a high ceiling. Now, I just need to calc the post and beams for the ends of the trusses. Which leads me to my next question.

What did you mean here: "So no post in the middle of that long span of 4x6 is way too much in a framed "truss"."     

Are you talking about the 4x6 beams from post to house not being enough to support the roof? Do I need and post under the 8' span? Or do you mean along the bottom chord of the "truss"?

Just (post #206961, reply #13 of 19)

Reading your original description and looking at the sketch-with a 30 foot span across (8ft out from the house), a home made truss would not have the engineering to span the 30 ft without sagging.  Most thinking of a truss might assume the bottom member does all the holding up.  But it's much more than that (and I'm not BossHog, he used to design trusses for a manufacturer), the whole thing together and the members and connection points give it it's strength.

Further, to connect wall to wall with a 30 ft -4x6 out front or even back in as a tie from wall to wall, will be prone to sagging of it's own weight.

Think of barns-post and beam.  You don't find many 30 ft unsupported spans with that small a framing member.

 

But since you're going with scissors, all this is mute.

Best of luck.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


"I'm not BossHog, he used to (post #206961, reply #14 of 19)

"I'm not BossHog, he used to design trusses for a manufacturer"


I still do.

See........ (post #206961, reply #15 of 19)

did I get the rest right?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


("Moot", not "mute".) (post #206961, reply #16 of 19)

("Moot", not "mute".)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

For an 8-foot eave length, (post #206961, reply #17 of 19)

For an 8-foot eave length, another option is to use thrust beams along the sides.  Would take a little calculating to figure their size, and there would need to be a tie (could be cable or rod) across the gable end, but not rocket science for such a short length.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I got ya. (post #206961, reply #18 of 19)

I got ya.