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-   -   Building Knee Walls in the Attic (http://www.diychatroom.com/f19/building-knee-walls-attic-127029/)

Jimmybrp 12-19-2011 09:45 AM

Building Knee Walls in the Attic
 
I have a bungalow cottage with a nice high 7/12 pitch gable roof and 2 X 6 rafters, leaving a nice space up in the attic for finishing. There is also a load bearing wall roughly down the middle of the cottage, along the 36' length of the 34' X 36' footprint, and perpendicular to the roof rafters. This middle wall also has another wall on top (up in the attic), extending right up to to about 1' from the ridge of the roof, and this wall would of course stay because it helps carry the roof.

The problem is that there are braces in the attic running from about 1/3 of the way up the rafters down to the load bearing wall (at an angle of course), so they really bugger up the potential usable space up there.

In order to open up the attic space in the middle, I want to replace these angled roof rafter supports with a knee wall on each side, the studs for which would drop straight down to the top of the attic floor, which is in turn supported by 2 x 10 joists that run parallel to the roof rafters. (So, the knee wall would run perpendicular to the attic floor joists, about 4' from the outside wall.) The roof rafters are spaced at 2' o.c., so the attic floor joists will likely be spaced at 1' o.c. because 2' o.c. is too wide and 16" o.c. would leave some of the knee wall studs not sitting directly on a joist. The plan for the future, by the way, is to make sure that the floor joists for the attic (which of course are also the ceiling joists for the main floor below) are beefy enough and spaced closely enough so that they can serve as floor joists for a full second story if I decide to raise the roof in the future.) That part I can deal with, but the issue in the meantime is with the knee walls.

The question is, will these attic floor joists take the load of the knee wall, so I am looking for guidance on the specs for this sort of thing. (I do have the option of adding more joists if I need to, in order to take the increased load from the knee wall.)

I am North of Toronto where we do get snow, and the 2 X 10 joists span 12' over the two back bedrooms, 13'8" over the third back bedroom, and 16' over the LR/DR. All of these can be made to have 1" o.c. spacing if needed.

So all you DYIers, does anyone have any sage advice they would care to pass along? It would certainly be much appreciated!

Jimmy

BTW, I am also planning on adding collar ties as well, just as extra support.

joecaption 12-19-2011 09:51 AM

Post a picture of what you have for better info.
Sounds like you have some form of trusses which may not be able to be removed.

Marvel 12-19-2011 02:33 PM

Pics would help but it sounds to me like the roof has been stick framed. I would suggest that you "draft" a full cross section of the existing condition and the proposed and have it reviewed by an P.Eng. or certified designer in your area. Anyone else see the need to align the proposed knee wall studs with the supporting joists below? Sounds like it could be accomplished in some fashion but requires professional input. Is there a basement?
BTW Building Permit !!!!!

Jimmybrp 12-19-2011 06:18 PM

1 Attachment(s)
Quote:

Originally Posted by Marvel (Post 797621)
Pics would help but it sounds to me like the roof has been stick framed. I would suggest that you "draft" a full cross section of the existing condition and the proposed and have it reviewed by an P.Eng. or certified designer in your area. Anyone else see the need to align the proposed knee wall studs with the supporting joists below? Sounds like it could be accomplished in some fashion but requires professional input. Is there a basement?
BTW Building Permit !!!!!


Yes, the roof was stick framed, sometime before 1938. Here is a hand-drawn diagram. I suppose I could go with 16" o.c. floor joists and simply use a double bottom plate on the knee wall, but it is only 7 more joists per side to go with 12" o.c., so not a lot of extra expense.

Also, building permit will come later, but I want the details worked out first.

Wildie 12-19-2011 09:09 PM

I built my house back in the 80s. Originally it was a summer cottage, but I retired here in 1998.
Its a 1 1/2 story building with a 11/12 gabled roof. It is 24 feet wide.
The ceiling/floor joists are 2X8s. The first floor was a standard box frame then it has 24" knee walls on the rim and 6 foot knee walls resting on the floor joists, in the same manner as you are planning. Then the 2X6 rafters have ties at the seven foot level. These rafter ties form the 2nd level ceiling joists.
Your plan to install floor joists between the existing ceiling joists should be adequate. Mine are only 2X8 and I had 4 feet of snow on there last December, and all was good!
I installed diagonal bracing on the outside of the of the knee walls.
Of course, codes have changed since I built mine, so you will require the services of an engineer to certify your plans. To get a building permit, signed engineers drawings will have to be submitted.

Jimmybrp 12-19-2011 09:57 PM

1 Attachment(s)
Quote:

Originally Posted by Wildie (Post 797992)
I built my house back in the 80s. Originally it was a summer cottage, but I retired here in 1998.
Its a 1 1/2 story building with a 11/12 gabled roof. It is 24 feet wide.
The ceiling/floor joists are 2X8s. The first floor was a standard box frame then it has 24" knee walls on the rim and 6 foot knee walls resting on the floor joists, in the same manner as you are planning. Then the 2X6 rafters have ties at the seven foot level. These rafter ties form the 2nd level ceiling joists.
Your plan to install floor joists between the existing ceiling joists should be adequate. Mine are only 2X8 and I had 4 feet of snow on there last December, and all was good!
I installed diagonal bracing on the outside of the of the knee walls.
Of course, codes have changed since I built mine, so you will require the services of an engineer to certify your plans. To get a building permit, signed engineers drawings will have to be submitted.

Thanks, Wildie, and hello from a fellow Ontarian! Looks like you got away with no load bearing wall down the middle of the attic. If I could eliminate mine, the space up there would not only be some 35' long, but also a lot wider without the middle support wall. I'll check this out further.:yes:

P.S. Are you on a Lake? We are on Kawagama Lake, near Dorset, just South of Algonquin Park.

P.P.S. In fact in the back half of the cottage I removed the old (2 X4!) ceiling joists in order to replace them with 2 x 10 joists. (They just used the old 2 x 4 joists to nail strapping and crappy old tentest ceiling tiles to, but all of that is now gone and I will be using something nice like T & G pine for the ceilings. Here is a photo of the old bedroom ceilings:

James

Wildie 12-19-2011 10:19 PM

Quote:

Originally Posted by Jimmybrp (Post 798045)
Thanks, Wildie, and hello from a fellow Ontarian! Looks like you got away with no load bearing wall down the middle of the attic. If I could eliminate mine, the space up there would not only be some 35' long, but also a lot wider without the middle support wall. I'll check this out further.:yes:

P.S. Are you on a Lake? We are on Kawagama Lake, near Dorset, just South of Algonquin Park.

James

Yep! I'm near the Pinery Prov. Park.
I would think that you could remove the central wall, but this is where an engineer would be needed.

Marvel 12-20-2011 08:53 AM

Getting back to some basics I'm wondering how you intend to incorporate a stair to the second level. If its proposed to be parallel to the bearing wall than there will be a lot of joists to head off. Have you considered going the other direction and incorporating dormers front and back so that the developed space runs front to back vs the longitudinal direction?

Jimmybrp 12-22-2011 09:23 AM

Quote:

Originally Posted by Marvel (Post 798318)
Getting back to some basics I'm wondering how you intend to incorporate a stair to the second level. If its proposed to be parallel to the bearing wall than there will be a lot of joists to head off. Have you considered going the other direction and incorporating dormers front and back so that the developed space runs front to back vs the longitudinal direction?

Good point, Marvel, and actually as you can see from the diagram there are two interior load bearing walls in addition to the outside walls, with a 5' space between them for the main floor hall. That is where the attic stairs will be (I already bought them at Lowes - they are the kind that pull down from the ceiling), and there will be no problem boxing in the opening, since the joists span only 5' in that area, and there will be no attic supports at all over the hallway.

AGWhitehouse 12-22-2011 09:35 AM

Looks like it would be worth your while to consult a structural engineer. Those 2x6 diagonals are currently transfering roof loads directly to your bearing walls. you proposed sketch diverts that load to the mid span of your ceiling joists. Couple in the proposed live and dead loads associated with an occupiable space and you may be causing strain upon those floor joists.

Jimmybrp 12-22-2011 10:34 AM

Quote:

Originally Posted by AGWhitehouse (Post 800263)
Looks like it would be worth your while to consult a structural engineer. Those 2x6 diagonals are currently transfering roof loads directly to your bearing walls. you proposed sketch diverts that load to the mid span of your ceiling joists. Couple in the proposed live and dead loads associated with an occupiable space and you may be causing strain upon those floor joists.

Yes, that is certainly the issue, which is why I want to also use collar ties, and I may even upgrade to 2 X 12 joists @ 12" o.c..

The Ontario Building Code requires that "Dwarf Walls) (knee walls) supporting rafters midspan be treated as loadbearing walls, and that means they can be no farther than 600 mm (about 2 ft.) from another bearing wall beneath it, "unless the joist size is designed to support such walls"

I have an architect coming up soon, so will see.

James

AGWhitehouse 12-22-2011 10:51 AM

Quote:

Originally Posted by Jimmybrp (Post 800315)
The Ontario Building Code requires that "Dwarf Walls) (knee walls) supporting rafters midspan be treated as loadbearing walls, and that means they can be no farther than 600 mm (about 2 ft.) from another bearing wall beneath it, "unless the joist size is designed to support such walls"

Yep...it can get a little tricky determining the load factors unless you know what you're doing with it all. Good start with the Architect...

Collar ties don't do too much to distribute loads. They are mostly to hold the spread forces. The rafters will still deflect in a vertical direction with collar ties and the knee walls will take the brunt of it, hence your codes reasoning for considering them to be load bearing.

Best of luck!

Jimmybrp 12-23-2011 02:22 PM

Quote:

Originally Posted by AGWhitehouse (Post 800327)
Yep...it can get a little tricky determining the load factors unless you know what you're doing with it all. Good start with the Architect...

Collar ties don't do too much to distribute loads. They are mostly to hold the spread forces. The rafters will still deflect in a vertical direction with collar ties and the knee walls will take the brunt of it, hence your codes reasoning for considering them to be load bearing.

Best of luck!

Yes, and there is a somewhat unclear reference in the OBC to what is required underneath the floor joists, depending upon whether the knee walls support another floor above (which they do not).


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