PDA

View Full Version : Shed roof addition


texanncincy
07-07-2006, 12:09 PM
I was looking to add a 10' x 12' shed roof to my existing 10' x 12' Gambrel roofed yard barn. Having never built a roof of any kind, outside of bird and dog houses, I did some research on the net and in books. That's when things got complicated. My idea was to pry up the first row of shingles that cover the first ridge (9' from ground level) down from the top ridge, slide a 2x6 underneath and screw it to the barn's existing rafters through the shingles . Then cut the angles on the rafters and screw them to the 2x6 on the roof and to the 2x6 I'd placed on
2 - 4"x4"x8' support posts I'd set in the ground 10' out from side of the barn. I was ready to start, but then I found out about 'snow load' and 'roof pitch'!! I was planning to use 2x4x 12's for the rafters at 24" centers, but waiting to hear from a local building inspector for the snow load in my area (Cincinnati, Ohio). My biggest concern is that by setting the support posts at an 8' high level I will only have a roof pitch of 1/12. Even if I lower it to 7', that still only gives me a 2/12 pitch. Everything I've read says you don't really want anything less than a 4/12 pitch. Any thoughts? Am I being too cautious? I would really like to have a roof, but don't want to be at risk either.

Roy J.
07-07-2006, 03:56 PM
Below a 4/12, roll roofing or a hot mop with gravel will work and passes most codes.

texanncincy
07-10-2006, 01:47 PM
Thanks Roy J. Do you think I'm OK with the 2x4x10' rafters with either of these two methods? I'm not overly concerned about leakage due to the fact that this roof is to keep the sun off of me while I'm in the garden. I won't be storing a lot of stuff under it. My main conern is that with the pitch the roof will weaken from 'snowload' and collapse on my ass.

Roy J.
07-10-2006, 03:37 PM
2x4x10's would not hold that load. Even when framing a roof with 2x6's it's recommended that there is some kind of bracing every 11 feet or so. I don't know much about snow-loads but I'd use at least 2x6x10's on that roof.

texanncincy
07-14-2006, 10:28 AM
Thanks Roy. I'll go with 2x6's.

David
07-14-2006, 11:32 AM
Texanncincy --

I'm not liking the sound of this design! Hmmm.... I'm an amateur, so maybe I look to overdo things because of my lack of experience, but I grew up in the midwest with snowstorms. You need to run your plans by your inspector for sure. I hope others good with framing experience will jump in here.

The first thing that catches my eye is that you are screwing into the rafters. I think you should be nailing into the top plate and into adjacent rafters.

Second, even though this is a canopy attached to a shed, you are adding more weight to a load bearing wall, but not adding any more support to the wall. In Texas, not too big of an issue. We get rain. In Cincy, you should reconsider snow weight, IMO.

You're putting a $hitload of snow load weight on the side of your shed. Snow loads can be huge. A 1/12 slope is nominal in that snow will accumulate just as fast on it as a flat deck. Perhaps a 2/12 roof provides enough relief for your design, but I doubt it.

Think about a house design. 2x4's are okay for trusses, but they are (1) well reinforced, and (2) support a higher pitched roof.

A little wind and some snow and you could have a big problem. You've got a 10' span with a 4x4 in each outter corner supporting a header which is also 10' from the supporting wall? And, your load will be greatest on the lower end, which is the two 4x4 posts. At 24" oc, what kind of sheathing will you use to support this snow load? And 2x6's may be fine for a roof load, but they are not sufficient to hold the snow load, even if the sheathing can. Not to mention, you have an adjacent steeper roof providing more potential for snow runoff to build up on your new canopy.

Think about it this way. Take a 24" square of OSB (or whatever your roof sheathing is) and support it with 2x6's along 2 sides (not all four). This gets you 4 square feet. Now place 4 times the snow load on it. If the snowload is 50 psf, then place 200lbs of weight on it. Imagine it laying there for a week. I'm not sure even 1" ply will hold that without warping.

So a 10' x 10' canopy will hold 100sf. So at 50psf snowload, you've got 5,000 lbs, or 2.5 tons. Add in weight of shingles, sheathing, and rafters, and I think you've got over 6,000lbs. That's about the weight of 3 midsize cars. A litte flex in one of those 4x4 posts, and poof!

I wouldn't risk my shed for this, even if an inspector approved. Don't underestimate a good snow storm Tex. Your design may pass inspection and it may last a decade. But a bummer of a snowstorm will not only take down your canopy, but could pull the roof off of your shed, too.

Hope I didn't scare you away from the project. I only meant to strongly caution you!

- Dave

Edit - 50psf may be high for a 2/12 roof, but at 30 or 40, the total load would be 30/50*5000 (=3000lbs + materials) or 40/50*5000 (=4,000lbs + materials). But, 50psf may be closer to reality due to the adjacent steeper roof.

Edit2 - spelling and better math.

Don_P
07-14-2006, 02:38 PM
David,
Ummm, I hope you don't mind,
Snow load doesn't care about pitch, and trusses can be flat. I know you're thinking of some type of slope reduction, not so. Here's the reasoning. A 1' run of flat surface is 1' long. A 1' long level run at 12/12 projects the same 1' to the sky, and it measures 1.4 feet long on the angle. Snow load is calculated on the horizontal span, no pitch reduction. I have had customers with 12/12 and metal call with 3' of consolidated snow on the roof, they don't always dump. Half the load is calculated on the shed, half on the posts.

You brought up a good observation talking about the post, creep deflection. A chronically overloaded member bows a little out of plumb, increases the bending stress, which causes it to bow a little more out of plumb, which further increases bending stress... you get the idea. One day, might be 20 years later, it crosses the line, poof. Ever hear guys who do things obviously wrong and defend it with words like "I've been doing it this way 19 years"? They've quite possibly laid a string of deadfall traps with time delay.

If this is a cheapo yard shed, I'd look inside at the gambrel framing, I doubt you'll want to load it if it looks as light as most I've seen. If its a sun shelter, a seasonal sun awning might be a better bet. Short of that you probably need a purlin beam under the pitch break in the shed sized to hold that end of your roof up.

David
07-14-2006, 03:55 PM
David,
Ummm, I hope you don't mind,
Not at all.

Snow load doesn't care about pitch, and trusses can be flat.
Flat trusses are still reinfornced with diagonal bracing, correct?

I know you're thinking of some type of slope reduction, not so. Here's the reasoning. A 1' run of flat surface is 1' long. A 1' long level run at 12/12 projects the same 1' to the sky, and it measures 1.4 feet long on the angle. Snow load is calculated on the horizontal span, no pitch reduction. I have had customers with 12/12 and metal call with 3' of consolidated snow on the roof, they don't always dump.
I can agree to that. Thanks for pointing that out. And you've made my case even stronger by indicating that a 2/12 surface will gather as much snow as a 0/12 surface.

Something is gonna give. Either the sheathing because it's supported at 24", or the canopy rafters (since 2x6's will not hold that snow load), or the 4x4 posts will eventually give.

Just for the record, I'd make it like a pergola, so it supports (almost) no snow at all.

Don_P
07-14-2006, 05:24 PM
Good, I'm glad.

Actually the diagonals are part of the truss. Think of a truss as a panel like a sheet of plywood with the fluff cut out. One of the most dramatic uses of trusses to me is to look at one of those old camelback truss bridges and a modern I beam bridge that replaces them. Back in the old days they couldn't transport or erect the large I beams we use nowadays. To get the bending stresses down to a low enough level so the small steel that they could move could handle, they made the "beam" 20' or more deep by riveting together pieces of angle iron to make a deep truss. We now use a heavy I beam to cross the same span that might only be 3' deep, but much heavier in section than anything they could have set. A truss is an efficient use of materials.

For span checks, this is a real handy calc;
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

I think Ohio is mostly 20 psf snow/10psf dead load
A number 2 spruce/pine/fir 2x6 will span 10'3" as a rafter under those conditions at 24" on center.

If we're calling the roof 120sf it has 3600 pounds up there on a bad day. Half of that is bearing on the existing shed. Half, or 1800 lbs is bearing on whatever beam is atop the 4x4 posts. The posts support 900 lbs each.

We have a workable rafter. I wonder what size will be needed for the beam (I'd retrofit another one inside the existing shed and put studs under it with support to the ground). The beam will have a 12' span with a uniform load of 1800 lbs. A double 2x10 will handle that safely.

The posts then are the next thing. The 900 pound load at that unbraced length, 96 inches is only halfway up the column stability limit, the same in its other restrictions, the post is only doing about half the allowable work.

I would build the roof if I assured myself the existing structure could either handle it or be retrofitted to handle it.

Span rated sheathing is designed to handle these loads. That's one reason I was a little tough on the salesman here the other day. We must be assured that structural materials have been proven capable of safely handling the loads we expect them to. My friends have babies in houses I've built.

I got a little longwinded, sorry 'bout that :).

David
07-14-2006, 05:42 PM
Snow doesn't accumulate evenly, unless it's a no-wind snowfall. The potential for drifting is still there, though the grambel roof probably isn't all that high, maybe 5' or so? So I think (which means it's worth about 2 pennies) 20psf may be right, but may be low of a load.

Also, are you saying that a 4x4 can support 1800 pounds (900 lbs being halfway up the chart). I just find that hard to believe...a 4x4 can hold up the weight of my Honda Accord.

Don_P
07-14-2006, 07:41 PM
I way oversimplified snow load, there are books on how to apply different scenarios. The way I described is the check most used though.

Try this calc and see if it makes sense,
I'll give you the variables to enter in this case;

http://www.ls.net/~windyhill/Calcs/columncalc.htm

96"
3.5"
3.5"
1280 psi
.51 million psi
1800 lbs

It'll handle the weight just fine, however you are probably visualizing the Honda on a flagpole. For that condition you need to multiply the actual column length by 2.1 and enter 202" in the "unbraced column length" box.
Honda hits the ground, sorry bout that. Your gut call was right, engineering just tries to quantify common sense so it can be repeated... hopefully.
End conditions on the post make a huge difference. Notice in the real scenario that the post can handle a 2404 lb roof load though.

All that proof aside, 4x4's are for mailboxes in my book.

David
07-15-2006, 05:53 AM
I take it that the 4x4 has standard and known psi ratings (1280 psi for compression and .51 million psi for elasticity). Where do you find those, from the APA?

Don_P
07-15-2006, 05:32 PM
All structural wood has published allowable design values that are used in figuring spans and loads.

The AF&PA's NDS, National Design Specification for Wood Construction, has hundreds of species and grades allowable strength properties listed. The joist and rafter calc above is one of theirs. If you play with it you'll see it giving adjusted design values for the conditions you input.

The Southern Pine Council has span tables for southern yellow pine on their website.

NeLma has a number of tables of base design values on their website as well as some good lumber grading info.

Normally when you get beyond those basic tables an engineer is a good idea. You can find the methods the engineer is using in the NDS and basic engineering texts.

(edit) This page might be a keeper, check out the online span tables and span calcs links, there's lots of good info on that site.
http://www.awc.org/technical/spantables/index.html

texanncincy
07-20-2006, 12:31 PM
OK guys, now my head hurts. First I'd like to thank you both for all of your input. And no David, you didn't scare me off the project. This is exactly why I posted in the first place. And by the way if you're an amateur on roofs then there isn't a noun in the English language for what I am (although I'm sure the wife could volunteer a few).

Without going into everything both you and Don P wrote, tell me what you think of my revised plan. Don P's right about the structure of my barn - all
2x4's and I was really worried about adding weight to the existing rafters without bracing. But running supports to the ground would cost me 1/4 of the barn's internal space. I had intended on adding a purlin beam Don P, just didn't know the name for it and that it belongs on the 'inside' of the barn. Anyway, how about I run 2x6x10's 24" OC to 2 - 4x6x7' posts on the outside with a 4x4x7' post inbetween at 6' center. I would attach the 2x6's to the top plate supported by a purlin beam and nailed (why not screwed David?)into the barn's existing rafters. Then I would add 3 - 4x4x7' posts that are spread out evenly set right next to the barn and run to a 2x10x12' double beam that spans all of the 2x6 rafters. This would put the posts and beam about 24" from where I'd tie into the existing roof. My intention is to relieve the barn's rafters of virtually any weight. I had intended on using 7/16" sheathing. I would even lower the outside edge of the roof to 6'6" to gain a little more pitch.
Would it be better to place the 4x6x7' posts next to the barn instead or leave them at the outside edges?
Beyond this I don't see the addition being worth the cost or the risk. If I can't build it right, I'm not going to build it wrong and leave it, as Don P said, as a deadfall trap for someone later on. I've got babies myself.

Thanks again guys!

Don_P
07-21-2006, 05:48 PM
Sorry for the headache :D

"I had intended on adding a purlin beam, just didn't know the name for it and that it belongs on the 'inside' of the barn."

I made up the name trying to be descriptive of where it was and what it was doing, another person would probably call it something else. The beam supporting that end of the roof does not have to be inside the barn. Putting that beam on posts secured to the outside back wall, the overhang on the upper end bumping the barn would work. Free standing but securely attached to the barn