12x15 Shed - Ridge Board vs. Ridge Beam

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sonofacarpenter
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12x15 Shed - Ridge Board vs. Ridge Beam

Post by sonofacarpenter » Mon Aug 26, 2019 5:12 am

Hey Shannon,

First, thanks for making your videos. I've been watching them over the last year or so and this is my first post on your board. They've been super helpful to me and like how you take the time to go into the details.

I'm writing this post in hopes that you can help me get some clarity on a shed design I'm working on. The footprint is 12x15 and I've been back and forth in my head on whether to go with a ridge board or a ridge beam.

The difference is (as I understand it, please correct me if I'm wrong):

1) A ridge board isn't necessarily supported at either end of the gable roof and can be one size larger than your rafter lumber (ie. 2x4 rafters can use a 2x6 ridge board). They also require the use of rafter ties as well as potentially ceiling joists?

2) A ridge beam is supported at either end of the gable roof, and if built up to the proper specification, no rafter ties are required as the supported beam becomes a loaded member (supporting 1/2 of the total roof span).

My question is in reading my local building code to determine the size of the beam required. I'm looking at Table A-12 ("Maximum Spans for Built-up Ridge Beams") of the Ontario building code and in the footnotes it says "Beam and lintel spans are calculated based on a maximum supported length of 4.9 m" which I think is 16'. What is the difference between "Maximum span" and "Supported Length" when it comes to ridge beams? I have about 2 kPa of snow load in my area and I'm having trouble determining what size of ridge beam I'd require if I'd like to avoid rafter ties.

Thanks so much

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Shannon
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Re: 12x15 Shed - Ridge Board vs. Ridge Beam

Post by Shannon » Mon Aug 26, 2019 9:25 am

So the supported length is the length of the the rafter on each side of the beam not the length of the beam. So in your case you are well under the 4.9m and can increase the total beam span(length) by certain percentages that should be layed out in the foot notes.
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sonofacarpenter
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Re: 12x15 Shed - Ridge Board vs. Ridge Beam

Post by sonofacarpenter » Wed Aug 28, 2019 2:02 pm

Thanks so much for your response Shannon. Just so I'm clear, when you say " the length of the the rafter on each side of the beam", in my case with a 12' shed width, the supported length is 6'? I think I've heard this called the tributary width as well?

Thanks again

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Re: 12x15 Shed - Ridge Board vs. Ridge Beam

Post by Shannon » Wed Aug 28, 2019 4:08 pm

yes it is 6' (1.83M) in your case. So you can be 10%.
Looking at the chart I do not see a viable option for you to span the entire 15' with a beam.You would need a support half way to make things work .
What about going with a steeper pitch and using a ridge board and collar ties? Idealy you want rafter ties to be as long as half the width of the building to be most effective.
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sonofacarpenter
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Re: 12x15 Shed - Ridge Board vs. Ridge Beam

Post by sonofacarpenter » Fri Aug 30, 2019 8:41 pm

Hey Shannon, thanks again for your reply.
I had a good look at the sentencing and I think it says I can increase the spans by 25% if my supported length is not more than 2.4m. Since 6' is under that I found that I can use 5 2x12s together to make a beam. I haven't quite figured out my LVL options. Supporting the span halfway doesn't sound like a bad idea either.

If I've done my math correctly my slope is 7.7:12 currently. I can't make it much steeper while keeping under the required height of 4m. I have 7' of run to 4'6" of rise. I'm not sure how this affects possible beam selection though I know shallow roofs require more support.

I'm not against the use of a ridge board and collar ties. Looking over the building code it seems that collar ties and rafter ties (or ceiling joists) would be required which would eliminate some of my overhead storage.

Regarding the use of a beam. Do you know if the rafters need to top load on the beam (birds mouth at the ridge), or are hangers enough to pass code? Thanks so much.

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Shannon
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Re: 12x15 Shed - Ridge Board vs. Ridge Beam

Post by Shannon » Sat Aug 31, 2019 9:09 am

Ok so I did a little more digging and I don't see the reference of the 25% increase you are referring to but my book is a couple years old. What I did find though is that 9.23.13.8 (4) " when the roof slope is 4/12 or more, ridge support need not be provided when the lower ends of rafters are adequately tied to prevent outward movement". So to me this means ceiling joists? It does not specify.

Now a 12'x15' shed with a 8/12 roof and 8' walls sitting on a 2x6 framed floor should come in around 13' (3.96M) so if you had ceiling joists you would not need a beam. You keep mentioning "over head storage" and i'm not sure what you are getting at? Are you planning on suspending a canoe or something from the underside of the rafters? If not the ceiling joists provide a space to slip some things up onto of them.

I would run the joists on top of the beam (no birds mouth) butt them together and apply a plywood gusset to each side to tie them together over the beam.The double birds mouth is a lot more work and I really don't think your getting any better support then using the gussets on both sides glued and well screwed.You can use hangers and mount them to the side but then you will need make up a bunch of caps to sit on the beam and make up the slope over the beam to prevent having a flat top peak or even run a piece of 2x4 cut to right height down the center of the top of beam to fasten the top of the plywood to ,helping form the peak.

The 5 ply 2x12 just seems like so much over kill to me in this situation even though that's what the code seems to say.
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Re: 12x15 Shed - Ridge Board vs. Ridge Beam

Post by sonofacarpenter » Wed Sep 04, 2019 8:38 pm

Hey Shannon,
Thanks again for the response.
To be honest, I'm new to the building code and was just looking online. But I think that the statement you quoted does mean ceiling joists.

When I mentioned overhead storage I was thinking in my head that I would hang things like patio furniture from the ceiling to save some space on the floor. I think I'll be taking the ridge board approach with some ceiling joists and as you mentioned, try to slip some things up in overtop.

Thanks for the tip on the plywood gusset. I haven't thought of that, but it's a good option. I suppose if I go with a ridge board I can simply butt the rafters up to it and secure it with nails.

I totally agree that a 5-ply 2x12 beam seems like so much overkill. I really appreciated your inputs here. Perhaps I'll pop back with some pics as things get moving.

Thanks again!

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Re: 12x15 Shed - Ridge Board vs. Ridge Beam

Post by Shannon » Wed Sep 04, 2019 8:52 pm

sounds good.
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