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Old 04-10-2014, 08:54 AM   #1
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Need help sizing roof beam


I'm looking at installing a covered deck off my house and need help sizing the roof beam. Roof will be a simple "lean to" roof, attached to the house. I'm hoping to have only 2 posts, rather than have a 3rd in the middle for extra support. What size of beam do I need?

Location: Ottawa, ON
Posts: 6x6 on concrete pier/footing
Roof Pitch: 3/12
Rafters: 2x8 16oc. (possibly 24oc.) - still debating
Distance between house to center of beam: 12'
Roof overhang: ~16"
Span: 16'

Considering tripled up 2x?, LVL, or steel beam. I'm looking to get the beam as "low profile" as possible for clearance considerations. Depending on the height, I may have to decrease the roof pitch or decrease the length of the roof from the wall.

Let me know if you need any more details!

Thanks!

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Old 04-10-2014, 08:55 AM   #2
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Need help sizing roof beam


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Considering tripled up 2x?, LVL, or steel beam.
Yes to all four

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Old 04-12-2014, 09:39 PM   #3
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Need help sizing roof beam


not sure what you meant by all 4? I need help sizing the beam to span 14' now, not 16' as mentioned in the original post. Thanks everyone.
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Old 04-13-2014, 05:20 AM   #4
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Need help sizing roof beam


Single story house?
Got a picture of where you plan on building this?
Reason I'm asking is if it's single story it's almost never a good idea to just attach the roof to the wall.
Far better to run it up onto the house roof.
More slope for less chance of leaks, gives you more ceiling height.
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Old 04-13-2014, 07:42 AM   #5
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Need help sizing roof beam


just a Joe says about running new roof up onto old roof. you can put a knee wall on top of the exterior wall that your new roof notches ( bird mouth) to. you will likely have to have a rafter break on the wall as well so that the rafters are able to reach up onto old roof.

edit, do you have tall ceilings in your house that allow you to run the roof at or below overhang?
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Old 04-13-2014, 01:51 PM   #6
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Need help sizing roof beam


Sorry everyone, it is a 2 storey house. I've attached a picture of where the roof/deck will be. I've also attached a few drawings from sketchup of the way it may look. I allowed for about 8" beneath the 2nd storey window for flashing before starting the new roof. At a 3/12 pitch, the bottom of my roof beam will end up more or less level with the top of the patio door. I could have a 3rd post in the middle of the roof, but wanted to explore the idea of 2 posts only, especially since it's right in line with the patio door (it might not look nice from inside or outside having a post in the middle). With a post in the middle, I'd also be left with about 7' between posts, which is pretty far for a railing span. Putting a post for the railings between the roofs posts would then result in about a 3.5' span, which may look too tight. If built similar to how it's drawn, there would be a railing on the outer edge of the stairs, and likely a potted plant so you don't step into the window well. The upper middle downspout would drop into the new roof's eaves trough, flow to the left, and some how combine with the left downspout to get to the water barrel. I'll figure all this out later. The roof would be attached to the house using a ledger board lagged into each exterior wall stud.

Thanks everyone!








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Old 04-13-2014, 05:04 PM   #7
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Need help sizing roof beam


What's your local snow load?
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Old 04-14-2014, 07:44 AM   #8
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Need help sizing roof beam


you will have to be careful where your steps land on the window well. probably want to put a landing there and turn the steps out into the yard or else make the steps narrower and rail up the right side before the window well. the idea is to not direct the steps to the window well. you are going to need sideways lateral bracing on your posts too... and if you are using asphalt shingles for a 3 pitch go with 3" exposure on the shingles. figure lvl for your beam, you can span the furthest with the least beam height with those beams.
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Old 04-14-2014, 08:16 AM   #9
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Need help sizing roof beam


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What's your local snow load?
Based on my 'limited' understand of loads, here's my best take at it using an online calculator. I read up on this section of the Ontario Building Code as well and cross-referenced some snow loads from other sources as well. I don't typically get un-even snow distribution in this corner of the house, so I don't think the parameter Ca needs to be increased. Hope this makes sense:

Specified Snow Load

S = Is[Ss(CbCwCsCa)+Sr] [4.1.6.2]

Factors

Location: Ottawa, Ontario
Ss = 2.5 kPa / Sr = 0.4 kPa
Importance Factor, ULS: Is = 1.0 / SLS: Is = 0.9
Roof slope = 14 degrees
Slope Factor
For non-slippery roof:
Slope <= 30 degrees.
Cs = 1
Cb = 0.8
Cw = 1.0
Ca = 1.0



ULS:

S = 1.0[2.5(0.8*1.0*1*1.0)+0.4] = 2.4kPa

S = 2.4 kPa

S = 50.1 psf



SLS:

S = 0.9[2.5(0.8*1.0*1*1.0)+0.4] = 2.16kPa

S = 2.16 kPa

S = 45.1 psf
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Old 04-14-2014, 08:34 AM   #10
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Need help sizing roof beam


In dimensioned timber, and depending on species and acceptable deflection under snow load, you would be looking at something like 4" x 12".
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Old 04-14-2014, 08:51 AM   #11
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Need help sizing roof beam


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you will have to be careful where your steps land on the window well. probably want to put a landing there and turn the steps out into the yard or else make the steps narrower and rail up the right side before the window well. the idea is to not direct the steps to the window well. you are going to need sideways lateral bracing on your posts too... and if you are using asphalt shingles for a 3 pitch go with 3" exposure on the shingles. figure lvl for your beam, you can span the furthest with the least beam height with those beams.
Yeah, I agree with the stair analysis. I drew up that concept without the right hand railing hoping it would work, but likely won't. I'll do something to prevent the window well from being a hazard.

For sideways lateral bracing, are you referring to post-to-beam bracing? Will I also need bracing on the side of the post facing the house? I would be concerned about where the stairs are coming off if I need to do this.

Yes to asphalt shingles. Is it not advisable to do the standard 5" exposure on the shingles with a 3/12 pitch?

I'm liking the idea of an LVL over a steel beam. I get the feeling it's going to be easier to work with than a steel I beam.
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Old 04-14-2014, 08:58 AM   #12
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Need help sizing roof beam


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In dimensioned timber, and depending on species and acceptable deflection under snow load, you would be looking at something like 4" x 12".
Thanks tony.g. As species and acceptable deflection could vary, when ordering an LVL, what information do I need to tell them on my order? Can anyone else also confirm this sizing (always good to get a second opinion). I also know that adding width to the beam usually doesn't save you in height, but if I went with something closer to 5-1/2" to match the post width (depending on LVL available widths when I order), would the height be and shorter?

How do you size an LVL anyways? Are there any calculators on-line to do so?
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Old 04-14-2014, 09:50 AM   #13
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Need help sizing roof beam


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Yeah, I agree with the stair analysis. I drew up that concept without the right hand railing hoping it would work, but likely won't. I'll do something to prevent the window well from being a hazard.

For sideways lateral bracing, are you referring to post-to-beam bracing? Will I also need bracing on the side of the post facing the house? I would be concerned about where the stairs are coming off if I need to do this.

Yes to asphalt shingles. Is it not advisable to do the standard 5" exposure on the shingles with a 3/12 pitch?

I'm liking the idea of an LVL over a steel beam. I get the feeling it's going to be easier to work with than a steel I beam.
it will depend on how tall deck is and/or free standing to determine lateral bracing under deck The lateral bracing for the roof will be on the roof beam to post connection fixing side to side sway. Reduce shingle exposure by 1" increments anything below standard 4 pitch roofs, you should not use shingles on anything less than 3 pitch and at 3 pitch expect to use 3" exposure for shingles. lvl will be way better than steel and you can get exterior grade lvl but if you wrap the beam tight from exposure to the elements it will not be needed imo, just standard lvl will work.
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Old 04-14-2014, 10:02 AM   #14
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Need help sizing roof beam


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it will depend on how tall deck is and/or free standing to determine lateral bracing under deck The lateral bracing for the roof will be on the roof beam to post connection fixing side to side sway. Reduce shingle exposure by 1" increments anything below standard 4 pitch roofs, you should not use shingles on anything less than 3 pitch and at 3 pitch expect to use 3" exposure for shingles. lvl will be way better than steel and you can get exterior grade lvl but if you wrap the beam tight from exposure to the elements it will not be needed imo, just standard lvl will work.
Thanks for this input, I hadn't thought about the exposure before now. As for the LVL, it will be wrapped in aluminum, along with the posts. The goal is to hide all exposed wood for the roof, and use composite/PVC decking and railings.
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Old 04-14-2014, 11:31 AM   #15
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Need help sizing roof beam


there you go, looks like a plan. one of the most important parts after it is built is flashing correctly around new roof. flashing detail is the hard one to figure out sometimes and it is harder to flash vinyl siding.

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