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Grizzlyman
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# Posted: 23 Jan 2021 17:16
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I am building a cabin that I designed - 22’ wide x 28. and the only real problem I am potentially worried about is the lack of rafter ties. Code is not required in MN where I am building.
My lofts have kneewalls that are part of a continuous wall from the floor where 12†only will protrude above the floor of the loft- effectively creating a knee wall. and the joists for the loft will span the entire width of the cabin- thereby tying the walls together. Yes- the joists are outside of the rafter triangle, but I THINK they are close enough (12†only) where they will still do the job of resisting outward force of the rafters. Pic of the design below. Loft 1 is is 10’, and loft 2 is 8’. The lofts may or may not be joined by a small gangway. Therefore there is a 10’ section with no ties at all. If there is a gangway- a tie will be made through the gangway. if not, then a “TRUE†rafter tie will span the distance in the lower 1/3 of the rafter.
My main question is this. In your opinion. Do I need to actually tie the rafters together? Or is a wall that is tied by joists 12†below the top plate sufficient? I think it is.sufficient but it does have me thinking about it.
Thanks in advance!
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toyota_mdt_tech
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# Posted: 23 Jan 2021 18:51 - Edited by: toyota_mdt_tech
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Boy. I think the formation of the triangle as a result of the rafter tie really stiffens up the roof, ie ridge in addition to supporting the walls from bowing.
What about collar ties?
I'm waiting for ICC to chime in.
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Grizzlyman
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# Posted: 23 Jan 2021 19:13
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I will use straps instead of collar ties. Collar ties I don’t believe do anything to resist outward force at the walls. Needs to be in the bottom 1/3
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spencerin
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# Posted: 23 Jan 2021 20:21
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Joists 12" below the top plate are sufficient.
Collar ties DO help prevent spreading walls, but their primary purpose is actually to keep rafters from spreading apart at the top.
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Fanman
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# Posted: 23 Jan 2021 20:34
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If your loft joists can extend through the exterior walls to meet the rafters, you don't need collar ties, except where the loft is open. Or if the exterior wall studs could extend up to the rafters, rather than the knee wall being framed separately, that'd work too. Obviously the header beam into either scheme requires some thought...
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ICC
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# Posted: 23 Jan 2021 23:08 - Edited by: ICC
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Here we go again. Collar ties do nothing to restrain the outward forces on the rafter tails. A collar tie is properly installed in the upper 1/3 of the roof triangle. The collar tie can be a 1x4, or larger. It is there to help keep the rafter peak together. As such one can use a metal strap over the peak and nailed to the upper surface of the rafters to do the same thing.
Rafter ties are often the ceiling joists. A rafter tie may be raised up into the lower 1/3 of the rafter triangle. If the rafter tie is raised there may have to be adjustments to a larger size rafter to maintain rafter strength. There is also a minimum size and number of nails listed in the IRC for the rafter tie to rafter connection.
Looking at the illustration I see a problem on the right end where it appears the loft is above an open porch. Those 12" "kneewalls" are going to push outwards at the top where the rafter tails are connected. That is seriously deficient. Put some snow on the front end of that roof and those stubby studs will be turned outwards at the top end.
IF you run the wall studs continuously from the bottom plate up past the inset ledger and loft joists to the upper plate where the rafters connect that is better but still deficient from an engineering perspective. You will never get a PE who wants to keep their license to give that a blessing with their stamp.
Try this. Get some small sticks like popsicle sticks or similar. Pin a triangle together like a proper gable roof. Overlap the three pieces to make a triangle, 2 rafters and 1 rafter tie. Push a single pin into each overlap. Don't pin the assembly down to the table top, leave the pieces free to move if they can. Because you have formed a triangle the unit will remain rigid. Now cut two short pieces and reassemble the 5 pieces the way that the front section of the roof is drawn. Pin each connection with a single pin. Again do not pin the assembly down to the support board. Stand it up. Is it stable? No. More pins in each connection will help, but that places stress on the pins and the lumber.
Questions... what size are the wall studs? How are the ledgers attached to the wall studs? What is the designed slope of the roof? Steeper would provide more loft headroom and reduce outward force on the sidewalls.
If the illustration is an accurate portrayal of the proposed build the window headers are insufficient. Those long walls are load bearing and require properly sized headers as well as jack studs to support the header, or the approved Simpson header brackets.
Quoting: Grizzlyman My main question is this. In your opinion. Do I need to actually tie the rafters together? Or is a wall that is tied by joists 12†below the top plate sufficient? I think it is.sufficient but it does have me thinking about it.
You are quite correct to question whether or not that assembly is a strong design. It has an inherent weakness.
For that ceiling area over the front porch, which direction are the joists running. I am not sure what the design is there. It appears to me that the ceiling over the porch has the joists running parallel to the ridge or ninety degrees to the rafters. Is that what is drawn or am I confused by the drawings shading?
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Grizzlyman
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# Posted: 23 Jan 2021 23:56
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Thank you icc for your comments.
No the design isnt 100% accurate.Headers are doubled 2x8s with jack studs.
Not using collar ties-someone else mentioned that. I know they do not stabilize the outward force- I will be strapping the rafters over the ridge board.
You are correct in that studs run up from the base in the big loft. No actual small frame knee wall. Standard 2x4 studs. 10 ft long. 2x10 joists supported by the load bearing wall that makes the bedroom. Joists Will be likely fastened to the studs themselves with lags and also sit on a ledger that spans that width of the loft.
The porch loft has been redesigned since this pic- my bad for posting outdated design. The porch will have a 10ft 2x4 running to the top plate of the “knee wall†just like in the other loft, but will feature a 10ft 4x4 post in the middle and at the corner- In other words in the drawing put a 2x4 between the 4x4s and make them 10ft tall instead of 8 specifically for the purpose of providing a structure for the knewalll instead of framing a short 12†wall. Joists are indeed running perpendicular to the rafters. There are 4 rafters over the porch- 2 of which are “tied†to load bearing walls. My hope was that the deflection on the “bad raftersâ€would be limited enough by a combination of the 2 “good “rafters, the 10ft studs, and the 3/4 sheeting on the loft tying all the perpendicular joists together - though this might not be the best plan!!
Icc I’d appreciate your feedback- sound like you know your stuff!!
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ICC
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# Posted: 24 Jan 2021 00:21
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Quoting: Grizzlyman Joists Will be likely fastened to the studs themselves with lags and also sit on a ledger that spans that width of the loft.
An array of nails are better than lag screws. A nail array spreads out the stress points whereas a lag screws concentrate load at fewer points. If forces cause a split at the lag there is no strength left.
How will the ledger be attached to the wall and what size will the ledger be? A stud should not be notched more than 25%; a 2x4 stud should then not be notched any deeper than 3/4". You can use a S4S 1x4 as a ledger board and notch it into the 2x4 wall stud. The joist ends would then be face nailed to the stud as well as be sitting on the 1x4. Use 4-16D box nails or 3-16D common nails per connection.
Cut the notches on the floor before assembling the wall with the studs clamped together in a bundle. We would use a router with temporarily nailed in place guide boards. Quick and clean. Just measure twice.
Porch roofs and their floors bring design and construction problems.
What is the snow load for your part of MN? What is the pitch of the roof? You may be asking too much of the "bad" rafters or rather asking too much of the connections those make to the other structural members. For a 22 ft wide the loft clearance seems to indicate a low pitch. Low pitch increases the outward horizontal vector. Without seeing more detailed drawings and knowing more about snow and wind loads this is all just seat-of-the-pants guesstimating.
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Grizzlyman
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# Posted: 24 Jan 2021 00:45
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ICC
42psf roof from my understand. See photo. It’s above the line.
Pitch is 5/12.
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ICC
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# Posted: 24 Jan 2021 09:37 - Edited by: ICC
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I forgot to ask the rafter spacing, so I'll run both 16" OC and 24" OC through the rafter thrust calculator. Using a rafter span of 22 feet and a 5/12 pitch with a 42 PSF snow load:
16"OC = 739 lbs. outwards force at each rafter tail.
24" OC = 1108 lbs.
Using the above the line 60 PSF value, which could be the value used if you were applying to the state for a building permit and sizing the rafters to their criteria:
16" OC = 1056 lbs.
24" OC = 1584 lbs.
Yesterday when I mentioned notching I meant to add a comment and forgot. That notch for a ledger reduces the strength of the stud right at the point where one needs maximum strength. The rafter horizontal outward force is applied above the notch. Instead of the stud is 3-1/2" it is only 2-3/4" with a 1x4 ledger. Any miscutting with a saw can eat into that. Routing is more precise.
FYI, a 10/12 pitch, 16" OC reduces the rafter thrust to 528 lbs. instead of 1056 lbs.
************ OOPs I made an early morning error. I entered only the snow load and forgot to account for the weight of the building materials. The numbers should be redone with an extra 15 PSF added.
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Grizzlyman
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# Posted: 24 Jan 2021 11:10 - Edited by: Grizzlyman
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ICC
So you think at 1584 -24†on center this is a fail and I should redesign?
Also. A more accurate photo.
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ICC
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# Posted: 24 Jan 2021 14:21
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Quoting: Grizzlyman So you think at 1584 -24†on center this is a fail and I should redesign?
Yes. I am not an engineer, but I have worked with my brothers' company and talked with some of the engineers they work with. Any time I have asked the question about "kneewalls" they responded with the advice that it is poor design, and not to do it. So I never have, but you can take that however you want to.
The best way to eliminate the outward horizontal forces is to design and build with a load carrying ridge beam instaed of a ridge board. That however, will necessitate support posts in the gable ends, plus footers to the rock. The length at 28 feet would mean a very large structural beam, or more mid span support posts and footings. But that totally eliminates horizontal forces other than the wind against the walls and roof. That is no real issue unless a tornado drops in.
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Grizzlyman
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# Posted: 24 Jan 2021 19:01 - Edited by: Grizzlyman
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I have this second design that I’ve been going back forth with. Basically- just cutting out the double loft and ending structure at the 20’ mark I lose that space of the double loft but gain a lot in structural integrity- and I think looks a little better too.
Both would have had a seperate external deck anyways just wasn’t in the first plan. And the deck is by no means designed yet- just a basic shape for now.
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ICC
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# Posted: 24 Jan 2021 22:32
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That is better without the extra loft IMO. Kneewalls are still not ideal though, but I am finished with comments on them.
I'll just mention that in my experience lofts are always too warm for me to sleep in unless the ground floor is very cool. Often the loft spaces are also lacking in windows or other ventilation. However, lofts do seem to be built into a great many cabins so not everyone shares my aversion to them.
I am reminded that I had a further question, a further potential concern. It appears the built-up beams run from under the enclosed cabin to extend out under the porch floor. That would mean that when it rains the beams may be thoroughly wetted. Are the beams PT? They will get wet and more than likely some water may be wicked under or blown under the cabin end wall. It is possible a floor joist or two may get wet enough to eventually cause problems. It is not clear because the way the porch floor boards are drawn it appears the joists for that run across the width, but the beams remain a question.
If the joists that are near that end of the cabin end are PT that would be better. However, if one takes the long term view, that can eventually end up with some rot under the floor or in the floor. It is better practice to design to keep the joists and beams dry at all times. Better to design the porch as an add-on to the end of the cabin. Porches and decks are easier to replace and rebuild than the cabin floor assembly.
I should not complain too much as a good portion of the business my brothers built was rebuilding foundations, floors, walls, windows.... that had been built with faults years before.
Q? Do you get those Nosseum flies or gnats up there? I've been fishing in areas north of northern MN and further north and grew to hate those things.
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Grizzlyman
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# Posted: 24 Jan 2021 23:16
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Yes it’s all pressure treated.
No- just lots of biting black flies and even bigger mosquitos- screened in porch is a necessity.
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Brettny
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# Posted: 25 Jan 2021 06:27
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I can second that lofts are to hot to sleep in. You can vent them but then your also cooling the lower floors by quite a bit in the winter.
Why not just run the floor joists in the second loft like the first one? Have you thought about moving the floor joists up ontop of the wall top plate and changing the roof style?
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Grizzlyman
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# Posted: 25 Jan 2021 08:55 - Edited by: Grizzlyman
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Quoting: Brettny I can second that lofts are to hot to sleep in. You can vent them but then your also cooling the lower floors by quite a bit in the winter. Why not just run the floor joists in the second loft like the first one? Have you thought about moving the floor joists up ontop of the wall top plate and changing the roof style?
Kids will be sleeping in loft. Loft will have window.
It’s 22 ft wide porch- need to support joists somehow. Joists are supported by a center wall on the big loft- not sure how that would be accomplished without a beam on porch and that would kill the headspace- which at that point might as well just raise the floor of the loft to be the actual rafter ties.
Good question about pitch. Went back and forth about the pitch/headspace in loft.but I think the kneewall design is more optimal for space as Raising the floor and increaseing pitch to 6/12 decreases space- I don’t really want to raise the pitch anymore as I have to build this and safety becomes a concern for me off grid with a higher pitch.
See pic for area under triangle- cross section of loft to scale... and more importantly useable area.
***EDIT. Put better picture in with more triangles and labeled ****
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Brettny
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# Posted: 25 Jan 2021 12:55
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You could do a gambrel roof.
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Grizzlyman
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# Posted: 25 Jan 2021 15:54
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Quoting: Brettny You could do a gambrel roof.
At first thought It sounds like that’s a good idea but in practice to have the roof be a continuous gambrel it would require me building an entirely different cabin- it would be the size of a regular house! For 22’ wide the roof would end up being something like 10’ plus above the loft I believe - which is 20’ at the cathedral And I’m not certain how it would be framed over the open catherdral portion- would probably need to be 2 full stories ...wouldn’t want to use support posts in the open portion. Surely the complexity, time, and costs to do this are ratcheted up more than a few notches for a simple cabin.
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ICC
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# Posted: 25 Jan 2021 17:52
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A gambrel has all the same issues that need to be addressed that a gable roof on kneewalls has, only with four long sticks instead of 2 long and 2 short. Same problem just a different geometry.
Sorry, but I do not have a better solution other than what has been suggested or factory-made roof trusses which are not possible given the location.
Old stick-framed homes we often 20 to 24 feet wide with an upstairs. They had load bearing more or less central walls running parallel to the ridge that had a support beam and foundation supports. They built the upper floor by using 10/12 or 12/12 pitch roofs. The rafter tails connected to the wall top plates and the rafter ties were laid across from wall to wall and doubled as floor joists for the upper level. So the roof assembly was a solid rigid triangle. Upstairs they framed short walls inset maybe 4 feet from the long house sidewalls. They might have been 4 feet high. Then the ceiling sloped up attached to the rafter underside faces. In the center of the ceiling there would be a long narrow horizontal ceiling about 4 feet wide.
__________________
I understand not wanting to work on a steep roof. I have never liked roof work myself. For working on a roof I like them to be no more than 6/12 at the most, 4/12 or 5/12 is nicer to be on.
***************
So it would seem that if you are comfortable building a gable roof on knee walls that is what you will do even if (a) we know that is not the best structural design and (b) we know that IF you had to build to IRC code that would be a problem issue.
If a proper stronger roof assembly takes priority then it would seem (a) the loft space will be more restrictive or (b) forget the loft and then the main floor plan would require adjusting, increasing, in order to make more sleeping space available.
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snobdds
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# Posted: 25 Jan 2021 18:22 - Edited by: snobdds
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I know that ICC does not like knee walls, but as long as they're designed right. They work great for getting more space.
Rafter ties need to be there, but if you remember they prevent the walls from spreading only, there are multiple way of going about this. My ties are on the outside of the structure on the covered porch under the main roof.
Here is how my engineer designed the rafter ties...might be an idea or two in there for you.
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Grizzlyman
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# Posted: 25 Jan 2021 18:26
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Quoting: ICC So it would seem that if you are comfortable building a gable roof on knee walls that is what you will do even
I do think this is what I will do- but clarify not on separately framed kneewalls- rather the kneewall as a continuation of the wall itself and tied with joists 12†below the plates. I am comfortable with this. I will be forgoing the 2nd loft.
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snobdds
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# Posted: 25 Jan 2021 18:42
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Quoting: Grizzlyman I do think this is what I will do- but clarify not on separately framed kneewalls- rather the kneewall as a continuation of the wall itself and tied with joists 12†below the plates. I am comfortable with this. I will be forgoing the 2nd loft.
Why can't you just ballon frame the cabin?
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Grizzlyman
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# Posted: 25 Jan 2021 18:44
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Quoting: snobdds Why can't you just ballon frame the cabin?
That’s exactly what I’m going to do
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snobdds
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# Posted: 25 Jan 2021 18:54 - Edited by: snobdds
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Quoting: Brettny I can second that lofts are to hot to sleep in. You can vent them but then your also cooling the lower floors by quite a bit in the winter. Why not just run the floor joists in the second loft like the first one? Have you thought about moving the floor joists up ontop of the wall top plate and changing the roof style?
I love lofts in Wyoming. At 9,500 feet in elevation, it's nice to have the loft heat up in the day and when the temps fall to the 50's at night, the loft stays around 60 at night.
Will a loft work in Arizona in the summer, No. But they sure work good in places where the temps drop fast.
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snobdds
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# Posted: 25 Jan 2021 18:56
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Quoting: Grizzlyman That’s exactly what I’m going to do
OK, mis read your post.
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Brettny
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# Posted: 25 Jan 2021 19:28
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A gambrel wouldnt need a knee wall and yes the ceiling would be half the width. Minus the floor joist height and rafter thickness.
Running the rafters the floor joists the other way in the front loft over the porch would tie the walls together. Then you put a gambrel over it and you would have plenty of head room. Your taking out the second loft from the design? Either way a gambrel roof adds quite a bit a head room and usualy for not much more money. Your dealing with alot shorter boards but would need to make trusses on site. We built 19' wide gambrel trusses and after we figured out how to raise them they went pretty quick. The height almost dosnt matter because you can move then on the top plate from the bottom. Theres no need to get to the peak until you sheeth them.
You could even add a gambrel with a lower kick out (for the lack of a better term) if you wanted to keep the drip line away from the foundation/walls.
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gcrank1
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# Posted: 25 Jan 2021 19:41
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1)Set aside your knee wall idea for now
2)I had a loft for a bedroom area. Had the usable headroom issue, roof needed to be steep to accommodate people so too steep, imo not so great and I wouldnt do it again other than for some storage so roof height wouldnt be much of an issue. They are HOT, mine was typically 10*f hotter on a still, muggy summer night; ie, not useable. I wouldnt want kids falling out of my loft in a night stupor anyway.
How about this:
Conventional wall height
Loft joists on top of top plate like you were doing a second story
On that loft deck sits the 1st part of the gambel raftering angling upward. Where it transitions to the roof proper a support stud wall goes to the loft floor triangulating it together. Enclosing this would provide some storage and only narrow the loft area a bit.
That provides an inwardly braced support wall for a narrower and lower pitch 'conventional roof' on top.
Now you would have a real stand up loft with some side storage and the roof top would be a pitch you could work on without sliding off (mine ended up almost too steep to walk on with sticky shoes and asphalt shingles. Then there were the pine needles...
The gambrel sides are managed with a ladder quite easily.
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