|
| Author |
Message |
SandwichBear
Member
|
# Posted: 23 Apr 2013 15:15
Reply
I'm building my first small house using mostly an old carpentry textbook and Sketchup. In an attempt to get some more loft headroom, I added a 3' knee wall around the loft.
Should I be concerned about the roof being heavy enough to bow out the knee wall framing at the center of the walls?
The cabin is 8'x12' built with 2x4 walls and 2x6 rafters and ridge beam.
(This is my first post here... sorry if this has been covered 100x)
|
|
MtnDon
Member
|
# Posted: 23 Apr 2013 16:05 - Edited by: MtnDon
Reply
Nice use of sketchup
Quoting: SandwichBear Should I be concerned about the roof being heavy enough to bow out the knee wall framing at the center of the walls?
Yes, especially because of the "hinge" area where the short studs attach to the horizontal plates. Roof materials weight plus snow, rain, wind can all add up to cause a slow spreading along with the resultant ridge sagging. Some folks believe running studs full length in one piece is best, bottom to top. It would be better that is for certain but still not as strong as it should be. The way it is drawn the roof, upper walls and loft floor make a 5 sided figure. Not very strong.
It would be better to raise the upper walls even more and then use rafter ties properly placed at the lower third height to achieve the desired headroom. Note that if rafter ties are higher than the lower third mark they become ineffective at restraining horizontal forces. And higher up collar ties do nothing at all for the side walls.
Or use a ridge beam not a ridge board, along with column supports at each end down to the foundation footers. As drawn that is a ridge board as there are no end supports.
Then the problem also becomes one of height and the building narrowness. How will this be anchored against strong winds?
2x4's are strong enough for the walls but what about space for insulation? Any planned?
Maybe your details are not completed in the sketchup image but the windows in the sidewalls do not appear to have proper headers or trimmer/jack studs. That is important on sidewalls, but not needed on the gable end walls. Tables for that can be found in the IRC.
|
|
MtnDon
Member
|
# Posted: 23 Apr 2013 16:12
Reply
I'll go out on a limb here and say that the narrowness of the building is good. That may be the saving grace if you built this as drawn. That the the roof steepness which reduces the horizontal thrust as compared to a shallower pitch roof. But it still would not be a design for optimal building life.
|
|
Hick
Member
|
# Posted: 23 Apr 2013 16:34
Reply
I would use a 2x8 ridge beam.
|
|
MtnDon
Member
|
# Posted: 23 Apr 2013 16:45 - Edited by: MtnDon
Reply
Quoting: Hick I would use a 2x8 ridge beam.
Good catch I was re-reading and saw that and was about to edit in the following.
2x8 would be better but not best...
If the rafters are 2x6 the ridge board should be a 2x6 plus a 2x4 stacked vertically. The 45 degree cut end of the rafter needs full support across the cut end and a 2x6 + 2x4 will provide that. (All a ridge board does is give something to nails to rafter peaks to and to stabilize the spacing.) If the tips were supported between a 2x6 that leaves the bottom edge of the plumb cut unsupported and it is possible for splits to develop into the rafters. Not a good thing.
|
|
SandwichBear
Member
|
# Posted: 23 Apr 2013 16:59
Reply
Some of this is already an existing structure. I have everything except the rafters. I already put 7/8 OSB on the lower 8' walls as well. (The windows do have headers and trimmers... the angle of the image just doesn't show it.)
So it sounds like the best plan for a long-lasting structure would be to remove those upper walls. If I were to do that, I would be left with the lower walls with the loft framing sitting on top of the double top plate. It would look like this:
I know this is unconventional. How would you recommend I proceed from here, preferably with the least amount of back-tracking?
I'm sooooooo open for suggestions here. I've been going at this all alone, and as you might imagine, I'm a little tense, worried and overwhelmed.
|
|
MtnDon
Member
|
# Posted: 23 Apr 2013 17:21
Reply
Or use the upper walls as you illustrated in the first image, BUT be sure to install rafter ties across the rafters at a maximum of 1/3 the height of the rafter triangle. This will reduce the headroom, but you'll still have more headroom than with a conventional loft using no sidewalls at all. The 2x6 rafters are still strong enough under most snow loads even with the span de-rating that results with raised rafter.
If you are not sure of what I mean by "install rafter ties across the rafters at a maximum of 1/3 the height of the rafter" look at the drawing at the end of this link... Sorry I don't have a drawing of exactly what I mean, but basically it means moving the rafter tie from the lower position as shown in my drawing, to the first dashed line up from the tie, where it is marked "upper limit for effective rafter tie".
|
|
SandwichBear
Member
|
# Posted: 23 Apr 2013 18:44
Reply
Thank you for your help - I'm absolutely understanding this so far. I've made a few adjustments to my Sketchup model to compare options.
1) Leaving the upper walls, but installing rafter ties at a height of 1/3 the vertical distance (which is 15" up from the top plates) gives a head room of about 47 inches.
2) Removing the upper walls and installing 2x6 ceiling joists on top of the plates would yield about 40" of headroom at the peak.
3) What if I were to attach 2x6 joists below the top plates, screwed into the existing lower wall studs? If the top of the joists were 3" below the plates, for example, this would give about 51" of headroom at the loft peak, and still leave 7'1" headroom below.
I like option #3, but I'm not sure how best to support the joists - if screws are not enough, could I support the joists vertically with a 2x4 below, sistered to the neighboring wall stud?
Please bear with me - I'm hoping that I'm not making this any more convoluted than it need be. And your help is helpful. Thanks.
I can also provide more screen shots if these words are not clear enough.
|
|
|
suburbancowboy
Member
|
# Posted: 23 Apr 2013 19:06
Reply
I did a 12X16 bunkhouse similar to yours with 10 foot walls. So every 16 inches on the 16 foot sides I attached 2 feet down on the wall a 12 foot 2X8 from one side to the other. This solves the spreading problem. It also gave me 7 feet floor to ceiling upstairs.
Another option would be to use scissor trusses. This is what I used for my big cabin. It has 12 foot walls and is a 1 1/2 story. on the upper floor it is 9 feet floor to ceiling.
|
|
SandwichBear
Member
|
# Posted: 23 Apr 2013 19:27
Reply
suburbancowboy,
If I could start from scratch - or at least the part where I built the walls - I would certainly opt for different design. At this stage, I am trying to save most of what I have since it is built already - and I don't have much money (I'm down to my last few thousand of meager savings, so a total do-over is out).
But, point taken - if I run an 8 foot 2x6 from one side to the other, I can solve the spreading problem. Should I just screw the 2x6s into the neighboring studs, or should I also support them from below with a 2x4 running down to the sole plate? Is there a better way?
Thank you for the input.
Chris
|
|
MtnDon
Member
|
# Posted: 23 Apr 2013 19:46 - Edited by: MtnDon
Reply
Quoting: suburbancowboy So every 16 inches on the 16 foot sides I attached 2 feet down on the wall a 12 foot 2X8 from one side to the other. This solves the spreading problem.
Ask any licensed structural engineer, he will strongly disagree with that assumption. It is bad design. It stresses the wall studs in a manner they are not designed to handle.
Quoting: SandwichBear But, point taken - if I run an 8 foot 2x6 from one side to the other, I can solve the spreading problem.
If the 2x6 is below the top of the wall where the rafters sit, NO. see above note regarding what an engineer will say. I've asked three different engineers and that is what they say.
"Just because something has been done by someone and has not failed, doesn't mean it is good design."
|
|
SandwichBear
Member
|
# Posted: 23 Apr 2013 19:58
Reply
MtnDon,
I agree with your signature quote, and I would like to do this the right way. That leaves me with option #2 from above (using 2x6 ceiling joists on top of the double plates, next to the rafters, for a conventional loft.) Or, rafter ties.
|
|
MtnDon
Member
|
# Posted: 23 Apr 2013 20:37
Reply
Yep, #2 is simple and solid...
Raised rafter ties is more complex, more parts needed, but also comes with approvals if done as described.
So it seems it is...
...a smaller loft vs a loft with more height,
and...
...a less expensive loft vs a more expensive loft.
And it's your wallet. I appreciate the dilemma, the choices to be made...
Best of luck. I do think it is always best to choose a solid design choice over hoping your luck is better than the design.
Another related thought: If the loft choice is #2 (no extended height side walls) that pretty much means the loft loads are going to be minimal. If that was storage only or a loft limited to children sleeping you could use 2x4 for the ceiling joist / loft floor joists. #2 Douglas Fir 2x4 on 16" centers can span 8-11 for loads in the 20 psf LL + 10 psf DL range. Something to think about. Even if placed on 12" centers that would save a little cash for you. ???
|
|
leonk
Member
|
# Posted: 23 Apr 2013 21:16
Reply
gambrel roof? They're quite popular in other parts of the world and provide more headroom.
I know OP doesn't want to redo the walls, but I am curios - would balloon framing (long continuos studs) solve the 'hinge' problem?
|
|
Borrego
Member
|
# Posted: 23 Apr 2013 21:27
Reply
Quoting: leonk I know OP doesn't want to redo the walls, but I am curios - would balloon framing (long continuos studs) solve the 'hinge' problem?
Yes, I was thinking he could 'sister' some full height studs after removing (or cutting) the top plates....
|
|
Just
Member
|
# Posted: 23 Apr 2013 21:35
Reply
If you rotated the roof 90* and used 3 2x6 for a ridge beem 8 ft. long you could have standing room of 6 ft. in the loft.that makes it usable by adults. "just" saying
|
|
MtnDon
Member
|
# Posted: 23 Apr 2013 22:15
Reply
Quoting: leonk I know OP doesn't want to redo the walls, but I am curios - would balloon framing (long continuos studs) solve the 'hinge' problem?
Continuous length sidewall studs are better but still imperfect. An engineer will not apply his stamp. The stubby upper ends of the studs still have the same horizontal thrust from the rafters. Most walls done like that have notches in the inside face of the studs for a ledger. That notch weakens the stud and provides a perfect place for splits to occur. The highest so called kneewall any engineer I have spoken with on this has been "6 to maybe 8 inches". When sidewall knees become measured in feet they all agree it's not good or acceptable design.
There are ways to achieve more headroom in a loft / upper floor. I've mentioned ridge beams and upper floor sidewalls with raised rafter ties. suburbancowboy mentioned scissors trusses. A truss company engineer can design those to have very light horizontal loading; they are usually not zero. There are also trusses called attic trusses; those work best on buildings over 20 feet wide. Don't forget that balloon framed walls were discontinued because they require the extra work and materials for blocking if the fire hazard is to be avoided.
There are many details to be planned with lofts and cathedral ceilings. Sufficient insulation in the roof. Lofts are always warmer than the main floor in any season.
|
|
MtnDon
Member
|
# Posted: 24 Apr 2013 10:06
Reply
Quoting: SandwichBear 3) What if I were to attach 2x6 joists below the top plates, screwed into the existing lower wall studs? If the top of the joists were 3" below the plates, for example, this would give about 51" of headroom at the loft peak, and still leave 7'1" headroom below.
??? How about adding a 2x2 to the interior face of the wall studs and placing a 2x4 ledger horizontally on top of them. Use structural rated screws or just nail them. The weight is supported on the 2x2 and then the loft floor joists sit on top of the ledger 2x4. The loft joists also get face nailed to the wall studs. That with the raised ties will be strong.
|
|
Truecabin
Member
|
# Posted: 24 Apr 2013 12:01 - Edited by: Truecabin
Reply
Referring to the original sketchup, and keeping the knee-walls. You can do this. It's a tiny building. You've been in it and you know!
1) Take the loft floor joists off the wall.
2) Sheet the inner and outer studs with plywood, covering the joined wall studs with continuous plywood sheet with 8' orientation vertical. Your extended wall is now continuous.
3) Then set "loft" joists on a ledger or use hangers on the studs (thru the plywood) with longer nails. These loft joists don't hold much weight they are not floor joists.
Other notes: As mentioned add ridge beam. Collar ties up higher will add some strength too from spreading. This is a tiny building and does not seem necessary though. Regarding headroom: Remember its a loft not a floor and does not experience standard floor loads because you are always spreading the load on hands& knees. You may be able to span this 8 feet with 2x4s if its DF, maybe put them 12" OC but I'd use a 2X6 or 4X6 at the open end.
|
|
SandwichBear
Member
|
# Posted: 24 Apr 2013 14:49
Reply
There are multiple good suggestions here. I think I have a plan. Since this is the first house I've ever built, I'm going to back up a few steps and keep it simple. Simplicity was my original goal.
Today, I'm going to remove the upper walls and the loft decking. I'm going to run new loft/ceiling joists across the top plates, and erect the rafters in the normal fashion. All by the book.
I don't love setbacks, but I knew I'd have them. I'm doing this to learn. As with everything, the learning curve is easier when you don't overwhelm yourself. This will not be my last small structure, so I can re-use the materials for other fun projects. I'll have time to get fancy when I'm more experienced.
Thank you all who chimed in to help me make an informed decision.
Chris
|
|
Hick
Member
|
# Posted: 24 Apr 2013 17:37
Reply
One more thought on how to get more room in the loft. Use 2x10x12 and let them over hang the walls by 2' on each side. build your rafters on that. Sort of like what this attached picture shows.
|
|
SandwichBear
Member
|
# Posted: 24 Apr 2013 17:55
Reply
Well, I just spent the last few hours removing those upper walls and the loft framing. I'm feeling great about putting on a completely normal roof and not spending more money or adding more complication. I'm feeling excited to make progress toward completing my first tiny house - completely by the book with no untoward pizazz.
There are a couple new housing developments planned near where I am living. I intend to frequent the dumpsters for supplies to make more houses - even smaller - just for cheap fun. This little 8x12 should serve me well for now. I'm Sketchup-ing a 6 1/2'x8' to be built from found scrap and cutoffs. It will be the "Study Shack," placed in a quiet spot to read books and drink coffee. (No more tall houses for awhile).
|
|
OwenChristensen
Member
|
# Posted: 26 Apr 2013 07:49 - Edited by: OwenChristensen
Reply
It can be done. I built mine with a two foot knee wall. I understand the concern about side pressures. The reason it works is it's only 12' wide, the roof sheathing is 5/8 plywood with staggered joints, and a 12/12 pitch roof doesn't hold much snow.
About the plywood; There's about a ten foot surface of that plywood. It's very hard to bend it on it's edge. If it won't bend then the walls can not spread. This cabin was built in 1990 and has absolutely no sag in the ridge or spread in the walls. I understand not wanting to build this way though. I can't figure out what holds up a spiral staircase either.
|
|
OwenChristensen
Member
|
# Posted: 26 Apr 2013 07:58
Reply
I guess you can't see it from the front, as the gable you're looking at doesn't have the knee wall. From the end the eye brow porch hides the height.
|
|
toyota_mdt_tech
Member
|
# Posted: 26 Apr 2013 14:05
Reply
Dont overlook dormers on each side of the loft either for extra headroom. I think Dons idea of a load bearing ridge, then have it supported to the foundation via the gable walls was also a great idea.
|
|
SandwichBear
Member
|
# Posted: 26 Apr 2013 14:22
Reply
Well, I already took the upper walls down - so my first house is going to be built strong. Possibly even overbuilt. But, I want it to have the potential to last a very long time. I also might want to move it to a new location some day, so 'extra' strength is a good thing.
I'll be sure to incorporate some more complicated carpentry on future projects - once I get more comfortable with the basics.
Nice looking cabin. Also, good point about the dormers... that's something I'll probably build into the next one.
|
|
OwenChristensen
Member
|
# Posted: 26 Apr 2013 15:43 - Edited by: OwenChristensen
Reply
That's good. Here's one that's been standing for a hundred years.
I've also done a few with twelve foot studs and a notched in ledger board to hold second floor joists down a couple feet from the top.
Just 2x4 rafters, full 2x4
|  | | |
|
|
SandwichBear
Member
|
# Posted: 26 Apr 2013 15:52
Reply
I like the notched-in ledger idea. In my head it still seems like I'd want rafter ties too. In my head, though, everything is probably very over-built. That's the whole point of this thread, I suppose - nothing was feeling right about the loft framing.
|
|
OwenChristensen
Member
|
# Posted: 26 Apr 2013 16:15
Reply
I don't like the shallow roof pitch, and no headers what so ever.
I made a living building as light as possible with no come backs. When building for yourself, It's not much trouble to over build, although if you build to lite, you alone suffer. If I had any come backs , it would financially ruin me. It's a fine line.
Owen
|
|
|
