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Hangblague
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# Posted: 26 Feb 2020 10:12am
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I am planning for a 14' x 20' cabin with 2x6 rafters centered on 16" and pitched at 45 degrees. The peak runs with the length in the middle in a typical design, with a 2 x 8 ridge board. I do not wish to interfere with the head room in the loft which will occupy nearly half of the space. Can I get away with "tying" the rafters only with the loft floor joists and otherwise fixing the rafters together only with the ridge board itself? Would it make a difference if I ramped up to 2x8 rafters and a 2x10 ridge board?
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Brettny
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# Posted: 26 Feb 2020 10:22am
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The ridge board really does nothing for strength in the way that you need it. Use the floor joists from the loft to tie the two walls and roof together.
The real question is how tall are your loft floor joists? Is there a center load supporting wall under them and does the loft run the full 20ft?
Have you used an interior layout program? I suggest you do. 14ft wide is pretty skinny and could feel like your living in a hallway.
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Hangblague
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# Posted: 26 Feb 2020 10:57am
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Thanks Brettny. The loft joists would be 2x8s. No central supporting wall. The loft is a little less than half of the length.
Yes, the space is tight for sure. I've spent some fun time on sketch-up and with cutouts with the family seeing how this can work. It's very much a seasonal and occasional shelter. Kind of a "starter" cabin I think.
It sounds like you might approve of joists as ties!
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LittleDummerBoy
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# Posted: 26 Feb 2020 11:12am
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Put in 4 collar ties.
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Brettny
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# Posted: 26 Feb 2020 11:14am
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2x8s spaning 14ft will be pretty bouncy and noisy.
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ICC
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# Posted: 26 Feb 2020 03:04pm - Edited by: ICC
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To address the OP question; "Can I get away with "tying" the rafters only with the loft floor joists and otherwise fixing the rafters together only with the ridge board itself"
Collar ties are used to prevent winds from uplifting and separating at the roof peak. They have nothing to do with the spreading forces that rafter ties are used for.
Collar ties can be replaced with metal straps over the peak of the roof. These are to be nailed over the peak of a rafter pair with a minimum of 3 - 10D common nails per rafter, or 6 total with three on each side of the ridge peak. See IRC Table R602.3(1).
Collar ties or straps should be placed every 4 feet at a minimum. If using wood collar ties, the minimum material size is 1x4. We mostly use 2x4 because they are more common than 1x4 on a building site and often cutoffs can be used. If using straps it makes more sense to me to apply them over the roof sheathing at the rafter tops than nailing them direct to the rafters. My reason is that placing the straps on the rafters means that area cannot have the sheathing nailed to the rafter as the strap is in the way.
The rafter ties at the wall tops are there to prevent the spread of the wall tops and also used to attach ceilings to or in the case of a loft or second story floor they are then floor joists.
Floor joists are usually sized to carry a Live Load (LL) of 40 PSF when the floor is the main floor. When the floor is for a sleeping area the LL can be set to 30 PSF.
At 30 PSF a 2x8, #2 S-P-F, 16"OC, deflection L/360, can span a maximum of 13'6". So it would just squeak by a code check. Or maybe it wouldn't pass, depending on the actual on-site measurements and how "by-the-book" the inspector is. Remember the span is measured from the inside edge of one support (wall) to the other, not the overall OD span. My example could have different results with different species and grades of wood. My personal choice when the specs are being pushed like that is to go one size bigger, decrease the OC spacing or whatever it takes to get into the next upside materials group. However, on a 14 wide this loft space is so low there will be no walking around so 2x8's are probably fine unless he people are XXXL sized.
Brettny is correct in that the ridge board does nothing but make it easier to install rafter pairs. It can be a 1x instead of a 2x. Usually if the rafter is a 2x6 then the ridge board can be an 8" board. The factor that determines ridge board height is the length of the cut rafter face (the angled cut surface). The rafter board must fully contact the entire length of that cut rafter face. It is more important tp fully contact the racter end at the lower end of that rafter cut than at the top, pointy end.
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Hangblague
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# Posted: 26 Feb 2020 03:15pm
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Many thanks for this education!
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Hangblague
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# Posted: 26 Feb 2020 03:21pm
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I had gathered from elsewhere that the collar ties had a function of preventing rafter sag - a kind of gradual saddle shaping between peak and wall plate. It sounds like that is not the concern (at least not primarily).
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ICC
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# Posted: 26 Feb 2020 04:40pm - Edited by: ICC
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I re-read your post above and see I may have missed what you meant.
If the rafter was not sized right, too small, it might sag between the upper and lower ends. I suppose a tie might help with that IF it was placed in the middle of the rafter length. But collar ties are supposed to be in the top third. So I believe that was an uneducated guess at the purpose of the collar tie.
Many so-called carpenters today wouldn't know much about collar ties because more construction is dome with trusses than rafters.
Here's the complete first posting I made in response.....
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No. Collar ties are too high up in the roof triangle to resist this. The long rafter arm acts like a lever as the rafter loads push the walls apart. If rafters sag that means the rafter tie is deficient. Rafter ties are allowed to be anywhere in the lower third of the triangle formed by the rafter pair and the bottom chord (the rafter tie). If the rafter pair 'sags' then the wall tops spread outwards and the roof ridge also sags.
You will see many builders install their "rafter ties" 1, 2 or even 4 feet down a long wall stud to create a short "knee" wall above the upper floor (to increase headroom). This can also cause rafter spread and ridge sag. In some cases with a narrow building, no snow load there may not be a noticable amount of movement, but it is still not good practise.
https://www.finehomebuilding.com/2013/11/07/how-it-works-collar-and-rafter-ties
You might have to register to read the above. It is a good explanation.
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Brettny
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# Posted: 27 Feb 2020 07:57am
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ICC..So in the OPs case where the loft is not tieing the walls together he needs to either have a rafter tie in the lower 1/3rd of the rafters or put a beam/board at the same height as the loft every 4ft?
There is really not structural benefit that I'm aware of to a sharp peak Inside just below your ridge board. So adding kind of a ceiling just below it can really help tie things together. I always liked 2x6 or bigger lumber for this. You can spread the nails/screws out a bit more than a 2x4.
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ICC
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# Posted: 27 Feb 2020 03:00pm
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Quoting: Brettny ICC..So in the OPs case where the loft is not tieing the walls together he needs to either have a rafter tie in the lower 1/3rd of the rafters or put a beam/board at the same height as the loft every 4ft?
If I am understanding what you said, then, No.
There are two separate components; the collar ties and the rafter ties. Collar ties are to be in the upper third, and only need to be every 4 feet. That is a minimum, one can use more collar ties. We would do every rafter most of the time. The alternate to the collar tie is a metal strap over the ridge.
The rafter ties must occur at every rafter pair, and must either be placed at the connection where the rafter meets the wall top plate or higher, but in the lower third of the triangle formed by the rafters and a line drawn across the wall top connections. That is what code states. Every rafter pair. There is a special calculation needed if the rafter ties are lifted into that lower third. Often the rafters must be upsized to be able to handle the different forces that can be introduced by raising the rafter tie. The footnotes in the codes rafter sizing tables detail this. The footnotes are often ignored; bad idea.
When people want to have fewer rafter ties because they want more clear space with a cathedral ceiling, there is no prescriptive solution in the code. Designers/builders should turn to an engineer. Not only is it a question of having sufficient tensile strength in the fewer ties, which may be beams or steel rods, or whatever, but there is the question of whether or not the top plate of the wall has sufficient strength to resist the horizontal outward forces between the horizontal, wall to wall connections.
The code is minimum. Code is there as a guide to a few methods that have been pre-engineered and known to work within the prescribed limits of snow and wind. An engineer can always do the calculations for anything that might deiate from what code lists.
Using 2x6's as collar ties can be advantageous by allowing wider spacing of nails, thus spreading out the sheer loads on the wood and fasteners. However, using 2x4 collar ties on every rafter pair also reduces those loads because of the increased number of collar ties. A collar tie at every rafter also facilitates the installation of ceilings if the upper space is to be made habitable.
Look at older 1-1/2 story homes. Most had steeply pitched roofs with rafters, collar and rafter ties on every pair of rafters; most spaced on 24" centers. The upper floor rooms were centered down the length and only about 12 feet wide and the home width a maximum of 20 feet. The "knee" walls were inside the rafter/floor joist triangle and were 3 to four feet high. The center of the ceiling was flat and 4 feet wide at the most. The rest of the ceiling was sloped and attached to the pitched rafters. Some of those homes had a dormer or two, other only had windows on the gable ends.
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Hangblague
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# Posted: 28 Feb 2020 09:22am
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Again, thank you for the careful clarity and detail of this information. Now I am wondering about how to compromise between adequate rafter ties in the non-lofted half of my planned cabin and the unpalatable prospect of having a naked 2 x 8 spanning my open "cathedralized" space every 16" at the top plate level.
It seems that one option would be to spread my rafter spacing to 24", at least on that half. Another thing I wonder is if I used beefier rafter ties (the lower ones), say 4x8's instead, if I could get away with tying every other rafter set - maybe installing a tie at every 4' that way.
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Hangblague
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# Posted: 28 Feb 2020 09:34am
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As I re-read, I guess you already answered this, ICC ("not without an engineer"). That will be tough to swallow. Glad you got me thinking this through.
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razmichael
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# Posted: 28 Feb 2020 11:15am
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Is it practical (design and cost) to switch to using a Ridge beam instead? A built up LVL or similar product should span the 20' in the right configuration (manufacturer dependent). Some added design complexity as the end supports need to be have a support path to the foundation but this can be direct or through properly sized headers if over a door or window. Hangers are available to really make the process of putting up the rafters easy. A beam is structural so carries half the load thus the rafters "span" on the walls is only 7'. This process opens up the loft area providing way more space.
When I built my cabin I sized the LVLs for more width but less depth (to increase headroom). I also planned on hand lifting them in place and then bolting them together - ended up being delivered with a crane so this made things much easier. With 10' high walls and a dropped floor I have almost all use of the space in the loft (Gambrel rafters) - I can stand up in about 13' of the 16' width. I'll note that, although I span the 24', 8' is an overhang over the covered porch so the beam is supported at 0,16,24.
Certainly this added to the cost but was also easy to put up myself and makes a huge difference in loft space and overall look and feel (Cathedral ceiling).
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Brettny
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# Posted: 28 Feb 2020 01:57pm
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Hang they don't need to be bare 2x8 in your lofted ceiling. There are other options. Cables or logs come to mind.
I'm going to use logs on my 20ft wide cabin
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Hangblague
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# Posted: 28 Feb 2020 05:18pm
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More great input. Thank you. The ridge beam is certainly the obvious solution - but it also means I depart from keeping a tiny modest project from being tiny and modest in some ways. I don't have the confident know-how to depart from the plan I have that fundamentally. But I'll keep mulling that over.
Log ties will look neat, Brettny. Cables also an interesting idea.
Here's another possible idea for a solution I now wonder about. (The topic has become one about rafter ties rather than collar ties).
I keep 2 x 6 rafters (conserving a little precious head space) under the loft with proper ties for each set. I increase my rafter spacing to 24" (while leaving 16" spacing for wall studs and floor joists). On the other half of the cabin (the non-lofted part), I compromise on the standard rule by beefing up the rafters up to 2 x 8's on the non-lofted part while adding rafter ties only to alternating rafter sets (4' apart).
I propose getting away with this on the basis of A: extra beefy (8") rafters on a high, 12:12 pitch over a short span (7'); B: It compromising only about 10 - 11 feet of cabin length; C: No inspections where I build. I'd try to get a structural engineer's informal blessing maybe.
Somebody stop me.
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ICC
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# Posted: 28 Feb 2020 06:20pm - Edited by: ICC
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The idea of using a larger member as the rafter tie, spaced at 4 foot intervals has merit. The fasteners used for the connections must also be stronger.
You can do some calculating with some online tools.
The following website has some good calculators for sizing timbers, rafters, and joists, etc. http://timbertoolbox.com/ I know the guy who owns it. He is not an engineer but engineers we both know have said the results from his pages are accurate, when the input information is correct.
FYI, design loads (floors, roofs) have 2 components; dead load, DL = weight of the structures materials and the live load, LL = the weight of the contents, people, furniture and for a roof wind, snow, rain, etc.
If you use DL for a roof to be 20 PSF you will generally be safe. For a living space floor 40 PSF is the normal, and for sleeping space 30 PSF.
So use the sq ft of the roof;DL + LL including snow. If no snow or little snow the minimum LL used should be 20 PSF. Then divide by the number of rafters including gables and you approximate the load on each rafter. Then you can estimate the horizontal thrust of each rafter tie in the loft area.
For the open ceiling area the thrust would be approx doubled, as the number of ties are about half.
A 4xsomething would maybe look better across the cathedral ceiling space than a 2xsomething. Or log timbers as suggested by brettny.
If you are going to be looking at calculators two other very useful ones are the span calc by the AWC and the connections calc, also by the AWC
https://www.awc.org/codes-standards/calculators-software/spancalc
https://www.awc.org/codes-standards/calculators-software/connectioncalc
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Hangblague
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# Posted: 29 Feb 2020 05:51am
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Great. Thank you!
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Brettny
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# Posted: 29 Feb 2020 07:04am - Edited by: Brettny
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Also a normal 2x material used in the open area could be replaced at a later point or dressed up/down at a later point with trim.
Also you could use a structural ridge beam only over the open area. This would require a post that goes from the ridge beam down onto the concrete foundation and could possibly need a wall attached to it to help with racking the length of the building.
You could also do a gambrel roof, gaining you the most head room in the loft. Another way would be knee walls for the loft. Or build just a common roof and add dormers if you feel that you need the head room later. There are a bunch of ways to get head room in the loft. Skimping on roof rafters so gain 2in of head room is not something I would do.
There are rafters calculators out there that can help with how much head room you have. When you find one you like get out a tape measure and a fire pieces of string to lay it out and see if it's what you expect. Idk if you mentioned it but what is your roof pitch?
We are doing a 20ft wide with 12/12 pitch, I'm going to use 2x10x16' boards every 16in. This should give me just over 8' head room at the peak and have a 12in overhang at the eve. I used the rafters calculator that blocklayer offers. You have to remember to subtract the floor joist height from the interior head room. I did measure that there's roughly a 6 or 7ft area that s got more than 6ft of head room in the loft...I'm 6ft tall.
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snobdds
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# Posted: 1 Mar 2020 09:53pm
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The best thing to do for a loft is build high walls, 12 footers. Then you can put the loft floor in at 8 feet with tgi's supported by 2x6 jack studs on the outside walls. Then put rafters on top of the walls, giving a 3 foot knee wall on outside walls. I have no outside wall wasted space. My collar ties are at 8 feet on my 10/12 pitch roof. My loft is spacious and it doesn't feel cramped. Again, the key is high walls.
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Hangblague
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# Posted: 2 Mar 2020 12:14am
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Nice looking design!
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Brettny
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# Posted: 2 Mar 2020 12:32pm
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That is nice snobdds.
Some thing to keep in mind is that a town/county may not tax the loft as livable space. How much head room or if they allow vertical walls in it I'm not sure.
Also if you have ever slept in a loft with a wood stove it can be hot as all he'll up there. I plan on enclosing mine.
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ICC
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# Posted: 2 Mar 2020 02:02pm - Edited by: ICC
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Quoting: Brettny Some thing to keep in mind is that a town/county may not tax the loft as livable space. How much head room or if they allow vertical walls in it I'm not sure.
Building code has headroom limits on habitable spaces. IRC chapter R.305.1
Seven feet in the minimum with exceptions that cover sloped ceilings and bathrooms.
Exception #1. For rooms with sloped ceilings, at least 50 percent of the required floor area of the room shall have a ceiling height of at least 7 feet (2134 mm) and no portion of the required floor area may have a ceiling height of less than 5 feet (1524 mm).
THose short side walls would not be permitted under the std IRC rules regarding rafters and rafter ties. As pictured, the floor joists are being asked to do the job of the rafter tie. But they are located lower than the rafter triangle, so cannot be called rafter ties.
The chances are that on a narrow building with a steeply pitched roof the outward horizontal forces created by the rafters, will not be sufficient to cause a problem. Time will tell. However, that type of construction would never be drawn and stamped by a licensed engineer, nor would it be passed by a building permit inspector who was following the rules, following the codebook.
Brettny is spot on about lofts being hotter than downstairs. They are always warmer, hotter than the lower level, no matter the time of year.
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ICC
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# Posted: 2 Mar 2020 02:03pm - Edited by: ICC
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For reference the bathroom exceptions in the IRC are:
2. Bathrooms shall have a minimum ceiling height of 6 feet 8 inches (2032
mm) at the center of the front clearance area for water closets, bidets, or sinks.
The ceiling height above fixtures shall be such that the fixture is capable
of being used for its intended purpose. A shower or tub equipped with a
showerhead shall have a minimum ceiling height of 6 feet 8 inches (2032
mm) above a minimum area 30 inches (762 mm) by 30 inches (762 mm) at
the showerhead.
There are also basements exceptions. It's all there in R305
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snobdds
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# Posted: 2 Mar 2020 02:54pm
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Rafter ties don't necessarily need to be in the building. When our engineer designed the building, the roof was made to be tied into the main building and the overhangs. The sistered roof rafters are anchored in by two birds mouths on the framing. Then the rafter ties were triangulated on the outside. This free'ed up space on the inside.
Pushing the limits of cabin building.
My cabin is at 10,000 feet. I appreciate having a warm loft even in the summer. There might be one day a year where it gets warm enough to open the windows up there.
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ICC
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# Posted: 2 Mar 2020 03:24pm - Edited by: ICC
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. Yeah, 10000 feet can be cool to cold most anytime of the year. Most of the viewers are nowhere near that high. But even at 8000 I never liked the lofted areas much for most all the year. That may be just me, but I need cooler spaces to sleep and UP always is warmer than DOWN to me. . Nice appearance though.
FWIW, Rather than simply post some pictures that show the use of knee walls, I think it would have been good to note that an engineer was involved and did more than just make 12 foot tall walls. Many people use pictures of what was done as proof that the concept is okay. When there is unstated info like "an engineer calculated this" others may copy what they think they see and can end up with future problems that could have been avoided.
A "papered" engineer has the knowledge and tools to design "things" that are outside of the codebook box. They can be worth their weight in gold, so to speak. How much did his expertise and rubber stamp cost?
My non-degreed experience does does have me wondering just how the outward horizontal forces generated by the roof system are safely absorbed, or resisted. But if he /she had a degree and a license I'd have to say, great!
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snobdds
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# Posted: 2 Mar 2020 03:44pm
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No one should be building off of pictures of someone else's cabin. These are to show that one does not need to be an in the box thinker and there are other ways to make a place comfortable and at the same time spacious but not be confined to old ways of framing.
It's good to stretch the mind and come up with ways to build without the cookie cutter approach.
My engineer was my roommate in college. Brilliant engineer but can't figure out how to use a hammer. I remodeled his bathroom and he designed my cabin and workshop.
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ambroseluther
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# Posted: 10 Jul 2020 12:34pm
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First post prompted by running across this thread while searching for solutions for my own small cottage build.
snobdds, how does your awesome looking loft satisfy the rafter tie requirements since the rafters do not appear to be tied directly to the loft floor, but rather to the top plate? There does not appear to be a structural ridge.
I'm still fully trying to understand the ins and outs of this issue: a small cottage, with a partial loft and a cathedral ceiling, without a ridge beam. I see a hundred built examples online, but there does not seem to be a clear prescriptive path in the code that my monkey brain can pick out.
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ICC
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# Posted: 10 Jul 2020 02:46pm - Edited by: ICC
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Quoting: ambroseluther ...but there does not seem to be a clear prescriptive path in the code...
That is because there is no prescriptive path in the IRC that would permit the technique of building what are sometimes called "kneewalls" and placing the rfaters on top of them. The ceiling joists / floor joists for the upper level are required, by code, to be connected to the wall studs/top plate at the upper end of the studs at the same level as the rafters.
The saving grace on these small, relatively narrow cabins is that the roof loads are probably not high enough in most cases to cause the rafters to generate enough horizontal outward force to bend that kneewall length and cause a structural issue. However, that is only fair to say if the wall studs extend from the bottom plate to the upper plate. If 2x6 wall studs are used that would be even better. Building separate short kneewalls would create other issues.
IF a cabin was being built under a building permit and if it was being inspected by an inspector who was following code, such a wall, loft floor, rafter assembly would not pass.
An engineer could be hired to check and state the design was okay and that would then meet the code and then be passed by even the most "by-the-book" inspector. Inspectors do have leeway to make exceptions if they want to. That is up to the individual inspector. Some may also be somewhat lax; whether or not that is good is another topic.
The engineers I have worked with over the years would not approve such designs.
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ambroseluther
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# Posted: 10 Jul 2020 10:24pm
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I somehow missed the part where ICC pointed out the apparent code conflict and snobdds mentioned that an engineer had solved the rafter tie issue on the outside.
Total failure of a first post.
It does seem strange to me that there is not some prescribed way to tie a rafter to a knee wall, or at least to set a rafter on top of a loft floor system and faithfully complete the rafter tie requirement.
My plan (which I’m obviously going to have to change) was a 16’ x 26’ cottage with a cathedral ceiling with raised rafter ties every other rafter in the front and a loft floor acting as rafter ties in the back third. I was hoping to at least set the rafters on top of the loft floor system to steal an extra foot of head space. But it seems like that is hard to do without an engineer.
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