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SubArcticGuy
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# Posted: 14 Jan 2013 15:17
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I have just started hauling materials for a lakeside cabin and I am second guessing my roof design and am seeking opinions.
The cabin will be a simple 16'x16' with 8' walls (2x4, 16" OC). I am planning a vaulted ceiling with a 24' roof (18" off the back and 6.5' extending over the front where there will be a deck). I was planning on making three 24' beams from doubled 16' 2x8's. One beam would be a ridge beam and the other 2 would sit on top of each of my side walls to allow for the 6.5' extension out the front.
The rafters would be 2x6x12' that would be gusseted together with 3/4" plywood and the plywood gussets would sit on top of the ridge beam. The rafters would also be tied to the sidewall beams with gusset plates as well. There will be a 6.5'x16' Loft in the back of the cabin which will help tie the side walls together, but I am still worried about the ridge beam sagging or the walls bowing out.
The pitch is going to be about 9/12 with a tin roof so snow load won't be much of a factor.
I also live in a remote area with access only to conventional lumber, all of which must either be hauled by snowmobile (16' is about as long as I want to pull) or flown by float plane (at great expense).
Any thoughts?
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Bldginsp
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# Posted: 14 Jan 2013 15:45
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Do the rafters have collar ties?
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SubArcticGuy
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# Posted: 14 Jan 2013 17:01 - Edited by: SubArcticGuy
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I wasn't planning on collar ties. Although it would work well to help avoid any bowing and should also help transfer some load from the ridge beam to the side beams. It is just a matter of giving up headroom in the loft.
I guess if I just used open collar ties (as opposed to running the ceiling across them I could leave the ties out in the loft space (since the walls will be tied by the loft joists) and only put them in the remaining space.
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MtnDon
Member
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# Posted: 14 Jan 2013 17:20
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Let's see if I understand this correctly.
Enclosed building is to be 16 feet long. You want a vaulted or cathedral ceiling, part above a loft part over the unlofted area. You do not want any interior midspan posts at all. Is that correct?
I ran that information first for a RIDGE BEAM. I used DougFir grade #2 as you did not mention local species availability. DF is one of the stronger ones but not available everywhere. You didn't specify a climate zone or snow load in PSF so I guessed a bit. Std DL figures for materials is generally calculated at 10 PSF, but steeper pitches use more lumber per sq ft of horizontal area so I used 15 PSF. There will be some snow load; generally the designer/planner should work with 20 PSF as the LL. That will also account for some wind load. So the roof load might be 35 PSF. Using that 16x16=256 x 35 = 8960 lbs roof load. Half (4480) will be supported by the ridge beam.
Using 4480 pounds evenly distributed along the beam along with the values for DF I came up with some results for building the beam from standard dimensional lumber; the DF #2
The doubled 2x8 as a Ridge BEAM: Fails badly for fiberstree in bending. That is it will sag at that load.
Doubled 2x12: also a Fail in Fb.
Triple 2x12: A Pass for Fb
FYI, triple 2x10 also Fail in Fb.
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Using the same loads for the porch roof ridge BEAM doubled 2x8's DF #2 : Pass Or use a real 4x8 for its more pleasing aesthetics.
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Back to the main cabin Ridge BEAM...
If there was a post at the edge of the loft floor, 9.5 feet in one direction and 6.5 feet in the other... The double 2x8 would do the ridge over the loft.
Then doubled 2x10 equal a Pass.
Keep in mind those are for what I would call very light roof loading (the bare minimum, IMO. If it sleets and freezes then snows a bunch the snow can quickly build up to greater numbers. What is the actual ground snow for the area in PSF?
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The side walls under the enclosed interior space do not need beams, proper wall framing will suffice. The Ridge Beam supports one half the roof load. Each end must have a properly sized column. Each of the columns must also have a path to the ground and foundation with a properly sized footing at frost depth. I have not bothered to calculate the columns as I was just guessing at the roof design load.
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The ridge carries half the load each sidewall carries one fourth the roof load. That can be safely carried by standard wall framing. Because this roof has a structural Ridge Beam (not a ridge board) there is no horizontal outward force on the wall tops. There is no danger from that. Because the walls are only 8 foot high you can also get away with no rafter ties, no lateral bracing on the walls in the 9.5 foot open ceiling area (rafter ties are the component that traverses from one side to the other on top of the walls.). Collar ties, the members located up near the ridge should be use. They can be every 4 feet and snug up against the bottom of the Ridge Beam, although a case might be made for the plywood tie plates fulfilling the needs of a collar tie. That's open to interpretation.
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The roof structure over the porch should have a beam down each side to support the eve end of the rafters. For this using the same
doubled 2x8 (or 4x8) as used for the porch Ridge Beam. The porch ridge would also require proper column supports.
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All column supports may be split into two columns with a suitable beam to work around windows and doors.
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MtnDon
Member
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# Posted: 14 Jan 2013 17:24 - Edited by: MtnDon
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There is much confusion with the terms "collar ties" and "rafter ties" and what they do. They are different. Collar ties do NOT help with wall spreading; only rafter ties do. BUT with a structural ridge BEAM rafter ties are not required as there is NO horizontal component to the rafter thrust. That only happens with a Ridge BOARD.
Collar Tie = upper third
Rafter Tie = lower third
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MJW
Member
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# Posted: 14 Jan 2013 17:25
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MtnDon,
Can you tell me how you make the calculations?
I will be building a 20 x 32 with pretty much the same set up (12 ft of the 32 will be a loft - 20 feet open cathedral) and am curious what you use to calculate what the ridge beam should be.
You are such a great help to so many of us. I really appreciate it.
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SubArcticGuy
Member
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# Posted: 14 Jan 2013 18:01
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Thanks for the feedback. You are correct in the 16' length with no support posts. That would be 6.5' of loft at the back (in which the Joists would act as rafter ties) and 9.5' of vaulted ceilings in the front with collar ties.
The ridge beam would actually be 24' total since it would stick out 18" off the back to provide an eve and 6.5' out the front to provide a cantilevered roof over the porch. Spruce is the only lumber available here, I am tempted to go ahead and cut 3 white spruce trees to use as a ridge and side beams (to cantilever the roof over the front deck), I just figured using 2x material would be easier to keep square and close up. I also don't want to build with green logs as it may cause troubles down the road.
Snow loads are not huge up here...we get a couple of feet of light powder with little or no freeze thaw cycles (it gets cold...damn cold, nice, then melts in spring). It should all slide off before much weight accumulates (with the exception of around the chimney where I will have snow stops to protect it).
I was hoping to use the side beams strictly for the porch section to cantilever the roof so that I don't have to use columns on the front side of the roof. Once I put on the deck I suspect I will go ahead and put log columns in place. I guess if I commit to that now I can forgo the cantilever beams and just put a short beam from the front wall to the log column at the front of the deck.
Although now I am thinking logs would look really nice....hmmm.
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MtnDon
Member
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# Posted: 14 Jan 2013 18:06
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I use a javascript fueled calculator made by a friend. We hope to have a home for it in the future where anyone can use it; along with a beam tutorial and other stuff. It is good but because we're not engineers it is not, will not be, accepted by most permit departments. But it is informative.
It uses simple inputs, some which have to be looked up online or in a book for some wood design values.
In the meantime I'll run a few numbers if you will advise on the ground snow load for the location.
USA now load info here for many locations. Any place on the map marked CS needs local info. Scroll down about 4/5 the page for the snow load map...
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MJW
Member
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# Posted: 14 Jan 2013 18:20
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Looks like we are in the area designated a 15.
I am assuming the wood being used will be yellow pine.
Thanks!
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MtnDon
Member
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# Posted: 14 Jan 2013 18:37 - Edited by: MtnDon
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Quoting: SubArcticGuy The ridge beam would actually be 24' total since it would stick out....
Beam length is calculated between supports for each section to keep it simple. A cantilever requires different calculations; 18 inch should be no problem. The 6 1/2 foot cantilever would require an engineer to get an answer that would satisfy any inspectors. It could be done but I would advise against it. With a steep pitch roof that is a lot of upper area for the wind to act on. It would be a beam with a large cross section. I've seen some 1850's pioneer barns done like that for a hay loft that protruded about 8 feet. IIRC the beams were about 10 x 16, but tapered towards the end to maybe 10 x 10
If you cut trees it should be dried first. I have had some personal bad experience with timbers used too green. Lots of twisting.
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MtnDon
Member
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# Posted: 14 Jan 2013 20:59
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Quoting: MJW Looks like we are in the area designated a 15. I am assuming the wood being used will be yellow pine.
20x20 roof x 15 psf DL + 15 psf LL = 30 PSF total = 12000 lbs total roof load
One half load is on ridge, 12000 / 2 = 6000 lbs
20 feet between supports
Southern Pine, grade #2 (not dense)
The calculator Fails everything up to a quad layer of 2x12, using #2 SP. Those would have to be 20 feet long, no splices at all. If better grades were available the beam could be a three layer 2x12; such as #1 dense SP. Or select structural SP would work with doubled 2x12. That's all non guaranteed but should work.
A glulam engineered beam might be the better answer. You could compare the pricing between the lumber grades and a factory glulam. Any lumber dealer that sells them should be able to take the span dimensions and roof loads info and come up with what would work and supply the engineering papers that would be required if there were permits and inspections involved.
IF that 6000 lb beam load was equally distributed over two columns, one at each end, then a SP #2 4x4 would be sufficient if it was braced at least every 8 feet. It should be large enough to fully bear the beam though so with a three layer beam the column should likely be a 6x6. It does not need to be a continuous one piece column. One length can bear on another or on a beam, as long as the path from beam to footing follows straight down.
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bobbotron
Member
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# Posted: 14 Jan 2013 21:12 - Edited by: bobbotron
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That will be one heavy chunk of a beam!
We built our subfloor out of 2x12"x16' PT, pretty much the heaviest board I can handle by myself before things get ridiculous. Sure you're not dealing with the PT weight but, a 6x12x20', phew!!
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MJW
Member
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# Posted: 15 Jan 2013 10:06
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Yowsers!
That is one big beam...
Thanks MtnDon...
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CabinBuilder
Admin
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# Posted: 15 Jan 2013 10:20 - Edited by: CabinBuilder
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Quoting: MtnDon I use a javascript fueled calculator made by a friend. We hope to have a home for it in the future where anyone can use it; along with a beam tutorial and other stuff.
MtnDon,
Let me know if this is something you could share, to be placed on this site.
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razmichael
Member
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# Posted: 15 Jan 2013 11:59
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Quoting: MtnDon A glulam engineered beam might be the better answer.
You may find a LVL or similar beam is not as expensive as you think. I have a 16'x16' cabin with an 8' extension out the front over a deck and containing a loft. I went with a structural beam in part because I wanted the open ceiling and was using Gambrel rafters. Can you utilize a post down to a foundation at the front of the deck to act as a third support? Working with my lumber yard I went with 3 LVL beams 91/2" deep each (which really was overkill). Could have done it with 2 deeper beams easily but wanted to preserve the headroom. Originally I was planning on getting the beams in 16 and 8' pieces to ease the installation but the lumber yard did not see any problems getting a boom truck in for delivery so I went with the 3 full length 24' beams and put them up in one piece.
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MtnDon
Member
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# Posted: 15 Jan 2013 12:39 - Edited by: MtnDon
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Quoting: CabinBuilder Let me know if this is something you could share, to be placed on this site.
Thanks for the offer of a place to host our calcs. We do have a home site for them; it just is not quite ready for 'prime time'. Because we may need or want to tweak something from time to time our intent is to host them ourselves, but have an open site for anyone to use. When we release them I'll make a link if that is okay.
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MtnDon
Member
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# Posted: 27 Mar 2013 21:32
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Cabinbuilder, said calculators are now available on another cabin and home website with forum; over two dozen assorted rafter, beam, column and other construction related calculators. If you are okay with a link there, respond affirmatively to this and I'll post the link.
Regards
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CabinBuilder
Admin
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# Posted: 28 Mar 2013 11:53
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Go ahead MtnDon, I will have a look.
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MtnDon
Member
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# Posted: 28 Mar 2013 13:13
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The site main page is located at theOwnerBuiltCabin
Calculators link is part way down the page.
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