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Old 10-17-2012, 12:52 AM   #1
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Question about flat roof joist spans


Hi there,
I'm a long time reader, first time poster. Thanks in advance for your advice.

I'm building a 20' x 20' garage. This is as basic a box as it gets.
I have the walls up - 2x6 8' studs spaced 16" oc.

I want to build a relatively flat roof for simplicity's sake and to keep the overall building height low.

I've been reading (googling) about spans and from what I can tell, I should be able to run No 2 Douglas Fir 2x12 "joists" the full 20 feet span of the ceiling without any problems, preferrably spaced at 12" oc.

There will be a slight pitch to the roof as I plan on having an extra 2 or 3 top plates on one supporting wall, to make it around 5" taller than the other. Enough to get some water run off.

I'll run two rows of blocking to keep twisting down and glue and screw t&g 3/4 plywood on the roof.

The roof will be treated like a deck - I plan on fiberglassing it and maybe have the odd BBQ on it with 5 people.

Does this sound ok? The inspectors in our city seem to let anything go through in respect to sheds and garages. I just want something that will last and won't collapse if we get a foot of snow (which happens every couple of years).

Thanks a bunch!

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Old 10-17-2012, 01:36 AM   #2
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Question about flat roof joist spans


You can't just pick a joist size (in your case 2x12) and assume this will be OK unless you know the required loading on your roof. The live load component (eg snow) will be stipulated in your local Code.
If the roof will be a venue for BBQs, then additional loading requirements will apply.


Last edited by tony.g; 10-17-2012 at 01:39 AM.
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Old 10-17-2012, 01:43 AM   #3
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Question about flat roof joist spans


"Does this sound ok? " Not to me. I am not an engineer, but that is a long, long span for dimensional lumber. One source I have shows each joist will carry approximately 1000 lbs, uniformly loaded. By the time you take out the weight of the board, roofing and the plywood, you won't have a whole lot left for tea parties w/ Lady Gaga. For loads in the center, you cut the uniform load about in 1/2. Then there are the bending moments you have to worry about. Again, this is all approximate and rough; I am not an engineer, so I'd see one before I tried dimensional lumber on that long of a span. Maybe BCI's would do it, and possibly even taller ones. What loads will you have on it? Snow, wind, etc.
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Old 10-17-2012, 07:04 AM   #4
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Question about flat roof joist spans


you could do 12" centers and double the joists at every other one...
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Old 10-17-2012, 07:59 AM   #5
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Question about flat roof joist spans


as tony.g said you have to know what your loads are. This would require a call to your local building department to determine what your ground snow load requirements are, whether you need a building permit, and what drawings you would need to submit.

depending on your roofing materials a typical low-slope membrane and framing will be around 10 pounds per square foot for dead load.

Live loads are based upon loads that may change, such as snow and people. Typically at a minimum live load is 20 pounds per square foot, unless your ground snow load exceeds it.

Also, should you plan on using the "flat roof" as an elevated deck you would need to account for the live loads associated with this use.

this link will provide you with the Prescriptive Rafter Spans from the 2009 International Residential Code. See Tables 802.5.1. You will need to know what species and grade of lumber you will use for the rafters, and again your ground snow load. Also the type of ceiling to be attached, if any. See Table R301.7 for Deflection Limits.

For example in my area our ground snow load is 30 psf. for a rafter without an attached ceiling (deflection L/180) to span 20' using #2 grade spruce-pine-fir (SPF) lumber (typical for my area) I would be allowed to span 21'-5" at 16" o.c. with 2x12.

Of course should this roof be adjacent to the house, with a wall that extends above the garage roof then you would have to take into account snow drift (where snow piles up against the wall increasing load).

Also with a low-slope roof you can have a build-up of snow which can melt and turn into ice, which weighs more than snow. It takes experience and knowledge to evaluate anticipated loading properly. A couple of winters back we had numerous flat roofs collapse from unusual snow loading.

With all that said would I frame my low-slope roof (your example in my area) with 2x12 #2 SPF at 16" o.c. as allowed by the code? NO!

Remember, codes are MINIMUMS

Should you have snow in your area I would recommend consulting a local professional engineer to evaluate your roof to properly size the framing members.

why must you have a low-slope roof? I think you'd be better served with a higher pitched roof that would be able to shed snow. Simple to accomplish with a truss roof, or a properly stick framed assembly. Membrane roofing is not a typical DIY item (in my opinion).

Good luck!
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Old 10-17-2012, 08:13 AM   #6
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Question about flat roof joist spans


Also your going to need railings. A flat roof is far more prone to leaking , then add to that having railing sticking up though it adds to the issue.
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Old 10-17-2012, 08:42 AM   #7
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Quote:
Originally Posted by BlackVan View Post
The inspectors in our city seem to let anything go through in respect to sheds and garages. I just want something that will last and won't collapse if we get a foot of snow (which happens every couple of years).
doesn't matter what he let's go through, it's your and your family's safety and investment that count. I would not do anything that is less than code (remember that MINIMUM part) just because it may be allowed.

I did miss the part on your OP about the BBQ and people, even though I did mention about the possible use of the roof as a deck.

you may want to contact a local truss manufacturer and discuss with them using floor trusses for your project. the top chord could be sloped to provide your 1/4" per foot slope for drainage. your walls would be built level all around with the trusses on top. I'd recommend going 16" o.c. maximum, though most truss companies prefer 24" o.c. have to account for the deflection between the trusses and one layer of subflooring typically won't get it done. Your deck would be stronger in my opinion going this route. Just let the truss manufacturer know you are using it as a deck.

Joe is correct, if it is a surface that people normally and routinely walk on it will require guard rails for fall prevention. Penetrations can be troublesome.
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Old 10-17-2012, 10:22 AM   #8
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Thanks for your input everyone.
Just to repeat, I have checked span tables based on my type of lumber and size. They're ok with it at a load of 40psf, up to 21' span, some even more.

I will be using #2 Douglas Fir 2x12 beams, which is stronger than spruce.
Even though most span tables suggest 16" oc spacing, I'm going with 12" oc for extra strength.
Instead of a heavy tar or asphalt based roof, I'm going with a light fiberglass, which also adds strength. (I've done fiberglass before, it's every easy).

I'm basically building the roof like a deck. Everything I've checked says that I should be able to build a deck like this safely. No difference between a deck and a roof on such a short building, in my opinion.

I am doing this with a permit, and yes, my drawings were passed by the city with the 16" oc, although I'm actually going with 12" oc now.

Was kind of hoping for someone that had tried something like this to see what kind of results they got - sag over time, bounce, etc.
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Old 10-17-2012, 12:48 PM   #9
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Question about flat roof joist spans


Presumably you've read the load tables for floors and not roofs, if you are using it for the occasional BBQ.
The tables will ensure that the timber is not stressed beyond an acceptable limit for the relevant species.
However, I would suspect you might get a little bounce on that span, and the best way to counter this is to include solid blocking between the joists, suggest 2 rows at third span.
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Old 10-17-2012, 04:00 PM   #10
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Hi out there, this site could help you http://www.blocklayer.com/Default.aspx it gives you calculators for general construction.
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Old 10-17-2012, 10:50 PM   #11
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Quote:
Originally Posted by BlackVan View Post
Hi there,
I plan on fiberglassing it

Thanks a bunch!
What is this method? I have not heard of a fiberglass house roof, well other then shingles
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Old 10-17-2012, 11:54 PM   #12
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Question about flat roof joist spans


I think his roof is going to be a deck in reality.

I have never seen it but apparently there is a fiberglass decking material, interesting ides. I have a friend here in So. Cal. that might benefit from just such a beast.

His spans seem to be acceptable for a roof as stated with a light roof material system.

But roofs are not usually habitable decks used for grilling either. Completely different animals.

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Old 10-20-2012, 06:20 PM   #13
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Thanks everyone for your replies and comments/concerns.
Believe me, I'm all about safety and over-building for strength. Oh and figerglassing is just that - like a boat deck. You just lay your fiberglass and resin it over the plywood. Lasts like 20 years without any maintenance.

That being said, I've decided on my original set up. Here's why:

I looked at getting custom made triangulated trusses with a built in slope, but the taller end would wind up being 18" tall, sloping to around 14" at the narrower end. Very expensive, and too frail on their side for one guy to work with.

I then looked into the TJI floor joist system. Talked to a couple of engineers and they recommended this route. With the 11.98" tall joist in the 560 series, I could have them placed at 16" oc and achieve a deflection of L480 with a roof load of 50 psf and a dead load of 20 psf. Sounds great, but again, you have to be very careful with transporting these beams - they can only be carried and rested upright, not on their side. Worse than that, they can't get wet. Further more, they were about 30% more expensive than the Douglas Fir, even with buying more DF joists for the 12" oc placement.

So again, after reconfirming with some other span tables, I'm going with Douglas Fir #2, 2x12s, 12" oc, and that gives me a deflection of L360 with a live load of 50psf and dead load of 15psf. Not as stiff as the TJI Joists, but definitely more than what I need. Our local code calls for 35psf snow load in any case.

My actual open span will be about 19' 2", so I'm beyond the minimum requirements.

Love this tool from the American Wood Council by the way.

I'm also partial to proper lumber. Not a big fan of anything made up of woodchips. Douglas Fir beams can get wet, and they last twice as long in a fire. Call me old fashioned.

Thanks again!

Last edited by BlackVan; 10-20-2012 at 06:23 PM.
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Old 10-21-2012, 03:12 AM   #14
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Question about flat roof joist spans


BlackVan; when I read your first post I was sceptical about using timber on such a wide span, but you've obviously thought this through. The only thing I would add is that the joists are quite tall in relation to thickness, and some good deep blocking (bridging?) would prevent sideways buckling and add considerably to overall stiffness.
(that calc. is great - wish they did it in metric!)
Good luck with it.
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Old 10-21-2012, 08:13 AM   #15
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Quote:
Originally Posted by BlackVan View Post
Thanks everyone for your replies and comments/concerns.
Believe me, I'm all about safety and over-building for strength. Oh and figerglassing is just that - like a boat deck. You just lay your fiberglass and resin it over the plywood. Lasts like 20 years without any maintenance.

That being said, I've decided on my original set up. Here's why:

I looked at getting custom made triangulated trusses with a built in slope, but the taller end would wind up being 18" tall, sloping to around 14" at the narrower end. Very expensive, and too frail on their side for one guy to work with.

I then looked into the TJI floor joist system. Talked to a couple of engineers and they recommended this route. With the 11.98" tall joist in the 560 series, I could have them placed at 16" oc and achieve a deflection of L480 with a roof load of 50 psf and a dead load of 20 psf. Sounds great, but again, you have to be very careful with transporting these beams - they can only be carried and rested upright, not on their side. Worse than that, they can't get wet. Further more, they were about 30% more expensive than the Douglas Fir, even with buying more DF joists for the 12" oc placement.

So again, after reconfirming with some other span tables, I'm going with Douglas Fir #2, 2x12s, 12" oc, and that gives me a deflection of L360 with a live load of 50psf and dead load of 15psf. Not as stiff as the TJI Joists, but definitely more than what I need. Our local code calls for 35psf snow load in any case.

My actual open span will be about 19' 2", so I'm beyond the minimum requirements.

Love this tool from the American Wood Council by the way.

I'm also partial to proper lumber. Not a big fan of anything made up of woodchips. Douglas Fir beams can get wet, and they last twice as long in a fire. Call me old fashioned.

Thanks again!
I have doubled up(sistered) many engineer stamped floor systems at every other joist to cover a long span. If done you would have a double joist at every 2'. It may not be needed in your case but something being over done is not to bad.

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