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WeekEndHack
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# Posted: 4 Sep 2013 23:44
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Hi all,
At the point of building up my roof and am second guessing my plan. I have a 22ft ridge beam comprised of 2 2x10s glued/nailed together (4ft between the splice). I plan to have 2x8 rafters attached to this on 16" centers and was originally thinking of ALSO adding collar ties a foot down from ridge as kind of extra insurance against the ridge sagging over time. My roof will be a 12/12.
This is a small build (16x20) does this like a reasonable strategy? I've already built up the beam and its ready to go for this weekend but a buddy suggested I perhaps triple my ridge as there is no supporting post midway... thoughts ??
thx!
Steve
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MtnDon
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# Posted: 5 Sep 2013 02:09
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Quoting: WeekEndHack thoughts ??
The main problem with the proposal, IF this is meant to be a true load supporting Ridge Beam, is the splice. Most glues have poor shear performance meaning the nails are carrying virtually all the load. Splices are permitted but only when directly over a support like a post.
However, even a twin or triple set of 2x12 may not be strong enough depending on the snow load. Even with a 12/12 pitch some snow capacity should be allowed for. A good engineer can calculate that using formulas developed for roof slope shedding snow. Other mortals such as myself either should build for the maximum possible snow load regardless of roof pitch or be certain our insurance (building, health and life) is paid up.
I have seen a 10/12 roof loaded with 20 inches or so of snow on metal all because it sleeted first making a rough surface for the subsequent snow to pile upon.
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I can do beam size calcs if I know the design snow load, lumber species and grade. The total beam span is 20 feet as you said there are no intermediate supports.
If we calculate using a snow load of zero, and just count the weight of all roof materials at 15 lbs PSF, a beam of three 20 foot long (unspliced) 2x12 DougFir #2 will pass the deflection and fiberstress in bending tests. But that's with no snow or wind loading added.
FYI, twin 2x10, 20 foot long... not spliced... calculate a drop/sag at midpoint of 1.36 inches (no snow load). That makes a fail.
So it boils down to whether or not this is a true structural ridge or not.
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OwenChristensen
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# Posted: 5 Sep 2013 07:29
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I know some people think a ridgeboard is doing something, but it's not. It is used as a way to nail rafters at the peak. I don't even use a ridge board, but I truss plate the opposite rafters together like a truss. Now your collar tie won't work unless it's down at the top of wall. A ridge won't sag if the walls won't spred.
Owen
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OwenChristensen
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# Posted: 5 Sep 2013 08:00
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I guess if your beam is strong and the span is 20' , and the pitch is 12/12, it's going to be OK. You really should have at least one collar tie roughly in the middle of the building. Do you have a loft. Loft joists can act as collar ties if fastened well to the walls.
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MtnDon
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# Posted: 5 Sep 2013 14:52 - Edited by: MtnDon
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Loft joists can act as collar ties if fastened well to the walls.
Incorrect terminology and/or function.
I know some folks think I'm being somewhat OCDC or even anally retentive about this, but it is my nature. If you go talk to a trained professional designer or carpenter the term collar tie and rafter tie mean completely different things. A collar tie is never located on the wall tops; that is a rafter tie.
Reference this older page
It's not just me... Here's another reference from FineHomebuilding. .... Or THIS .... Or THIS
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There is also as much confusion between the terms ridge beam and ridge board. As Owen mentions a ridge board is nothing more than a nailing aid and an aid to spacing the rafters. A ridge beam is a structural component that must be sized for the job along with the supports being adequately sized along with the fountaion & footings for those posts. A totally different animal. Hence my trying to get to know just what we are looking at here; beam or board.
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ak_flyer
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# Posted: 5 Sep 2013 15:29
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Hey Guys, new to the forum and this post prompted me to sign up and chime in. I planning the same thing, 16x20 12/12 roof and also want maximum loft same for a sleeping area.
My thoughts: why cant the beam be semi-structural so to speak. so if a normal ridge beam would support half of the live load for the roof, what if the beam was designed to support like a 1/3 or 1/4 and built it like a ridge board? would that reduce the wall spreading to an acceptable level? Or is that not going to be anything more then a weak beam?
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Just
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# Posted: 5 Sep 2013 17:11
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have a friend building a 20 x 20 cabin in the snow belt area east of lake Huron. Ont. Canada he has a building permit with stamped plans.it is a open ceiling with a 12 x 12 pitch. The plans call for 3 2x12 stranded beams glued and nailed together .my son helped raise and glue the beam in place. Each 2 x 12 weighed 150 lb. a true beam I would say .
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MtnDon
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# Posted: 5 Sep 2013 17:58 - Edited by: MtnDon
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Quoting: ak_flyer would that reduce the wall spreading to an acceptable level?
Wall top spreading is restrained by the use of rafters ties on the wall top plates in a traditional simple rafter gable roof. Rafter ties often do double duty when they are sized to support an upper floor. A ridge board is used mainly as a nailing surface and as a spacing device, which back before codes was a 1x something board. The height (width) of the ridge board is determined by the slant end cut that forms the rafter points at the ridge. The idea is to fully support that angled cut end across its entire face.
The rafter ties are needed to hold the walls vertically plumb as the weight on the roof wants to make the ridge sag and that in turn pushes out on the wall tops. One half the roof load is supported by each side wall.
Ridge Beams are often used when the designer/builder wants to eliminate the rafter ties entirely as in a cathedral ceiling design. In this case the beam supports one half the total roof load and each side wall supports one quarter. The ends of the beam must have support paths clear down to the foundation and the earth at whatever the frost depth is. Intermediate supports cut down the beam size and make it easier to raise. Those too must have a support path down to the ground.
So, No, you can't have a ridge beam that is designed to support only a partial load and expect the roof to not sag over time. Not unless rafter ties are implemented as described in the link posted above. And then you don't need the complication of a beam design.
That said there are countless DIY builders and other amateur builders who will show that their own builds that do not meet some or any of the above criteria are standing and are fine years after erection. Well, that may be in part because engineers use a large safety factor when designing and calculating. After all wood does vary within any one grade and who knows what might slip through here and there. It may also be in part due to the a bunch of pure dumb luck. Or maybe nobody has noticed how much sag there is. The fact that cabins of up to 16 feet wide have lower roof loads and forces than a building 24 feet or more wide is another factor. 12/12 pitch roofs also can shed snow more readily if metal is used and one is not unlucky with sleet storms.
A semi-structural ridge beam would be a lot like using floor joists that were a couple sizes too small. Bouncy floor is the least consequence.
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OwenChristensen
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# Posted: 5 Sep 2013 18:50
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Well said.
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WeekEndHack
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# Posted: 5 Sep 2013 20:59
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Thx Guys.
Mtn Don, here are the details:
20ft clear span, 1ft overhang each end. Doubled 2x10's, SPF, Grade 2. Roof 12/12. Snow load, ~60-70lbs (Laurentians, Quebec). each row of 2x10s is one 9ft, one 13ft with the next row going the opposite way (butt ends 4ft apart).
I have built up posts at each gable end to support this beam (on main floor, 4 -2x6 sandwiched continued up to doulble 2x6 directly under beam and 2 studs on either side to sandwich beam in place). Look at Blaine Hill build for a reference as very similar (I would just add the additional collar ties near top).
I have a 12 foot loft, with 7 ft open to below. The loft is dropped down 9" from top of wall.
Exactly like ak said, my plan all along was to do a kind of hyrbid/combination of a ridge beam support/slash tradtional rafter board. Seems it would be a lot stronger than a single rafter support as it is doubled and has full post supports down to the foundation and by adding in collar ties a couple feet down from the ridge all the away across, seems this would help fight the tempation for it to sag as much. Also, seems its a small build with a loft covering much of it??
But I'm no expert, and welcome any suggestions ... Do I add another 2x10 row to my beam while I can??
Ridge Beam
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South facing wall
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MtnDon
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# Posted: 5 Sep 2013 22:21
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Quoting: Just has a building permit with stamped plans.it is a open ceiling with a 12 x 12 pitch. The plans call for 3 2x12 stranded beams glued and nailed together .my son helped raise and glue the beam in place. Each 2 x 12 weighed 150 lb. a true beam I would say .
Not sure what is meant by a 'stranded' beam. A 20 ft length of 2x12 DougFir has a volume of 2.34 cu ft. At 33 lbs per cu ft (20% moisture) that would weigh about 78 lbs. If one of those 2x12 weighed 150 lbs I am guessing you mean they were an engineered wood product and those are substantially stronger than any natural saw lumber. Maybe those are parallel strand lumber or laminated strand lumber; both very strong and man made. The dimensions can not be transfered to sawn lumber.
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MtnDon
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# Posted: 5 Sep 2013 22:22
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Quoting: WeekEndHack I have a 12 foot loft, with 7 ft open to below. The loft is dropped down 9" from top of wall.
The 9 inch drop down the wall studs for the loft floor joists is probably okay. At least it is a whole lot better than the 2 feet and more some DIY designer/builders do. Ya might even get an engineer to approve that. Depends.
The good thing is they are close enough to the wall top plate that the moment arm (outwards horizontal force) on the short stub of wall stud is not huge. Looks like the wall studs are 2x6; good. If the loft floor joists are well nailed into the wall studs they will restrain the wall tops from spreading outwards. All good.
Quoting: WeekEndHack Exactly like ak said, my plan all along was to do a kind of hyrbid/combination of a ridge beam support/slash tradtional rafter board. Seems it would be a lot stronger than a single rafter support as it is doubled and has full post supports down to the foundation and by adding in collar ties a couple feet down from the ridge all the away across, seems this would help fight the tempation for it to sag as much.
With the loft floor joists situated as described and well nailed you do NOT need a thicker/larger ridge. As stated in previous post the ridge could simply be a 1x. As longer as the angled face cut at the rafter upper point is fully supported that 1x will suffice. Building up extra thickness of shorter, non full length 2x material simply uses more materials that could be used someplace else. The strength for your roof ridge comes from the rafter tails being restrained from horizontal outward movement by the loft floor joists and the nails used to connect the floor joists to the wall studs.
Increasing the size; thickness and depth of the ridge member does not make the ridge contribute any additional strength UNLESS it is fully large enough to support the full load. And that Can NOT be achieved with any home made splicing of the usual 2x materials no matter the amount of glue and the number of nails. In fact some adhesives are plain bad for laminating. Construction adhesive is popular. It has many uses (subfloor to joist for example). But in shear loading as in a laminated beam it exhibits creep.
Again, your roof ridge's ability to stay straight and level, to NOT develop any sag, comes from the loft floor joists ability to keep the wall tops where they are supposed to be. That prevents the rafter tails from spreading. That keeps the ridge up where it is. It does not matter if the ridge was a 2x something, a 1x something or even not there at all. You could fasten each rafter pair tip together with nails or maybe a scabbed on piece of plywood. As long as the rafter tails and wall tops do not move horizontally, all is golden.
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Quoting: WeekEndHack But I'm no expert, and welcome any suggestions ... Do I add another 2x10 row to my beam while I can??
No. No point to it at all.
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ak_flyer
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# Posted: 5 Sep 2013 22:30
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MtnDon: Thanks for clearing that up, I think I'am picking up what your laying down.
WeekEndHack: what did you do for your foundation? whats the ground like that the cabin is being built? Just curious cause from you pictures it look similar to the ground here in Alaska. And I wanted to compare Notes.
All: So using this case as an example, would beefing up the rafters in this case 2x8, to 2x10 or even 2x12 not solve the problem? Because it's issue with the rafter ties not being installed, as in a "structure design" issue. Not a issue of the rafters themselves not being able to hold the load?
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MtnDon
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# Posted: 5 Sep 2013 22:42
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Quoting: MtnDon .... nails used to connect the floor joists to the wall studs.
When the rafter ties / floor joist is connected directly to the rafter where everything is sitting on the wall top plate that connection is known as the heel joint.
The number of nails required to meet code is detailed in the IRC ...
TABLE R802.5.1(9) RAFTER/CEILING JOIST HEEL JOINT CONNECTIONS
Scroll down to the last table.
Different roof pitches are noted. Building widths may be extrapolated from the table values of 12, 20, 28 and 36 feet. Read the footnotes and note the table is relying on true 16D nails being used, not box nails which are what most air nailer nails are.
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ak_flyer
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# Posted: 5 Sep 2013 22:47
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MtnDon: In this would have screwing (4) 2x12's meet the demand for a Ridge Beam?
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MtnDon
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# Posted: 5 Sep 2013 22:48
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Quoting: ak_flyer would beefing up the rafters in this case 2x8, to 2x10 or even 2x12 not solve the problem?
Beefing up the rafter depth is a great idea for providing more space for more insulation. But deeper section rafters will only make rafters that are stronger. Larger section rafters will do nothing to prevent rafter tail and wall top spread.
A true, properly sized ridge beam and supports will prevent movement problems. Or rafter ties in the lower third of the rafter triangle. It is not magic nor voodoo; it is engineering.
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toyota_mdt_tech
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# Posted: 5 Sep 2013 22:50 - Edited by: toyota_mdt_tech
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Quoting: WeekEndHack Hi all, At the point of building up my roof and am second guessing my plan. I have a 22ft ridge beam
I have a 20 foot ridge beam and the plans called for only a 1X8. I used a 2X8 instead. I also had 2X6 rafters every 16" OC. The ridge beam isnt really load as long as there is rafters butted up directly opposite of each other on the ridge. Mine was inspected by building dept and good for 90lbs snow load per sq foot. Sounds like you are overkill now. You will need a collar tie every 4 feet, in your case, with 16" OC, every 3rd set of rafters and you DO want to do this. I assume the bottom of the rafters will tie into the ceiling joist to act as a rafter tie? My rafters went right on the side of the ceiling joist and was nailed to the top plate and to the rafter.
I will add I have an attic, no vaulted ceilings, loft etc.
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WeekEndHack
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# Posted: 5 Sep 2013 23:30
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hmm. Didn't fully understand the ridgebeam/board I think. But its kinda built now .. no problems building collar ties as I had planned this (ie. ~7ft height in loft ). May add a rafter tie in open space in bottom third for more support as well.
Seems like the focus now though it on resisting outward thrust and am wondering how best to do this w/my 4x10 beams (5 of them). Just a ton of 4-6" nails?? Each beam is sandwiched in place but given my tight stud spacing around these might be difficult to get a lot in there??
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Truecabin
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# Posted: 5 Sep 2013 23:52 - Edited by: Truecabin
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Quoting: MtnDon Construction adhesive is popular. It has many uses (subfloor to joist for example). But in shear loading as in a laminated beam it exhibits creep.
thanks don, real good to know, i wondered about that in short glue joints
but theres not a lot of shear in this situation a 4 foot splice is easy for nails and not so creepy but i'd use lots of nails in a good pattern at the splice
what adhesive doesnt have creep? I guess that would be best but weekendhack my thought is that your ok with the 4 foot splice and adhesive, I bet its the same or stronger than a 4x10
i see you clamped the boards together good so the adhesive is very thin that is good too.
next time use doug fir for structural members spf is a crapshoot by definition
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WeekEndHack
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# Posted: 5 Sep 2013 23:57
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ak, land I'm building on is mix of clay and rock. (Canadian Shield stuff)
Hauling stuff thru the bush, no roads (just quad) so piers was what I went with. 3 rows of doubled up concrete blocks filled with concrete/rebar on a crushed stone/gravel base. 11 piers total to spread weight out as much as possible.
I posted a thread somewhere on here just haven't had time to update in forever. Search on 16x20 Laurentian build or something ...
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ak_flyer
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# Posted: 6 Sep 2013 00:25
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WeekEndHack: Yea thats about what I have, but less rock. I understand your hardship with transportation, ours is Fly in or Snowmachine.
As far as tying in your loft joists, I would screw it in the best you can at an angle and try to find some metal brackets that fits close screw/nail those in and call it a day. But I'll be the first to admit I'am no expert.
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MtnDon
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# Posted: 6 Sep 2013 01:40
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Quoting: WeekEndHack Just a ton of 4-6" nails??
At times too many nails can be as bad as too few. Too many can promote wood splitting.
I would say a good way to make the connection between the beams and the wall studs would be to face nail through the studs into the beam. True 16D nails. The space is tight but that is the ideal job for a palm nailer. Something like 8 or 10 nicely spaced, not too near the 2x edges.
Or another great fastener would be Fastenmaster Timberlok or Headlok screws; also thru the studs and into the beams. These would be my choice. They have 800 to 900 lbs shear strength IIRC. Much higher than nails. Maybe 3 per beam end. ??? Maybe 4 to 5 inch length. They are structural rated. Ideally you would run them in with a drill driver. But in tight space a ratchet handle with the right adapter works; just slower and more human muscle.
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MtnDon
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# Posted: 6 Sep 2013 01:54 - Edited by: MtnDon
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Quoting: Truecabin what adhesive doesnt have creep?
Polyurethane. But it's problem is it is one of the weakest glues and will shear easily. Sudden failure. It is simply foam. For wood construction loads that are structural you will find that engineers will not advise the use of glues or adhesives. There are too many variables they have no control over. The adhesives used in engineered woods is a different matter completely.
Use adhesives for preventing squeaks as in subfloors. Great for securing wood paneling, trim, shower surrounds, etc. But if it is a part that holds up a floor, roof, etc there are better ways for DIY and general construction. Glues are great for furniture where joints are made with precidion that is unattaunable in general construction.
The picture of the ledger is a good example, even though the 2x6 gets notched more than the IRC allows. It does provide a (hopefully) solid shelf/notch for the ledger to rest on. The nails simply hold the ledger tightly in place against the stud. Then the beam rests on the ledger supporting the weight. The fasteners must then be relied on to provide shear strength against wall spread. re should be exercised in layout and avoid things like having a knot cut into where a notch is made. Also watch things like overcutting the notch; that can make a weak spot where a split may start.
Built up beams with splices should have supports under the splices, never in midstream. When splices are not directly supported the joint is only as good as the shear strength of the fasteners and the continuing integrity of the wood. Splits in wooden members remove strength.
Hope that helps.
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WeekEndHack
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# Posted: 6 Sep 2013 16:09
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This is great, thx for the help guys.
For attaching rafter tails, H1's and a few 16d nails where the birdsmouth sits on the sill??
The one other thing is my plumb cut on my rafters will be slightly larger (by ~1") then my ridge ... I was thinking of aligning the bottoms and having the tops extend by this inch over the ridge to allow for a ridge vent (ie. have my 5/8 sheathing extend just a bit beyond this to leave a ~2.5" gap). I haven't researched this much yet, just throwing it out there ..??
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toyota_mdt_tech
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# Posted: 6 Sep 2013 18:50
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Can you order 22 foot ridge boards. I ordered my 20 foot ridge board when I did mine. My ridge doesnt really have any weight on it, its all transmitted to the top plate of my walls.
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Just
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# Posted: 6 Sep 2013 19:27
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At my lumber yard you can order 2 by lumber up to 24 ft. man made lumber up to 48 ft. Canada
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WeekEndHack
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# Posted: 6 Sep 2013 19:32
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probably could ya.
Not sure why I wouldn't just use the doubled one I built though? Seems plenty strong, and along with support posts collar ties, etc...
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Just
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# Posted: 6 Sep 2013 21:59
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Quoting: WeekEndHack Seems plenty strong
I agree ,why would you special order one and wait for it ,when you can make one just as good, By the time you paid for delivery of a 22 ft. board it would cost you a lot more than you paid. IMO your plan is sound more important Don agrees !!
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MtnDon
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# Posted: 6 Sep 2013 22:22 - Edited by: MtnDon
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I thought we had already beat this to death. 
At this point with a laminated and spliced ridge board already in place, there is no need to do anymore other than get the rafters up and the roof sheathed. That is as long as those loft floor beams get tied into the wall studs before the roof is completed. Do that now before the roof has a chance to start trying to push the wall tops out.
In essence this is a regular rafter built gable roof with a fat ridge board. A continuous length of sawn lumber will not provide strength required of a true ridge beam. It would have to be of ridiculous size if only supported at the ends. And no doubt more expensive than an engineered beam.
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