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Old 11-14-2008, 09:05 AM   #1
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Strucutral help


I have a guy who wants to make his walk in attic into a family room and at the same time vault the ceilings of the rooms below at the knee wall. The style of the house is gable type with a bearing wall running down the middle. There is no problem with the structure down to the basement floor and appears will built and well supported.

The space has 2x6 joists @ 16" OC spanning 11 1/2' which is too long to be a weight bearing floor. These are old growth joists with really tight grain. There is a 2x4 knee wall about 5 feet from the center of the space holding up an 8x12 roof with 2x4 rafters. Also well built and solid from what I can see.

What I want to do it to sister in alternating 2x8's on every other 2x6. The first 2x8 will go from the left outside wall to the right knee wall. The next 2x8 will go from the right outside wall to the left knee wall and so on.

Once that is done I will take out the non sistered 2x6's from the knee wall to the outside wall and then cap the remaining sistered 2x6/2x8 to clean it up.

Anyone see any red flags in this idea?

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Last edited by Marvin Gardens; 11-14-2008 at 09:08 AM.
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Old 11-15-2008, 10:20 PM   #2
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Strucutral help


Is there a permit and structural design for the modifications?

Terms like "it appears like" and is "well built" only apply to what is now there and amy not have any relation to the addition.

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Old 11-15-2008, 11:29 PM   #3
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He has a permit and the inspector says he doeesn't see a problem with it but has really never seen anyone do this before. No money for an architect.

I figured I would run it by here and see if there is anything that sticks out.

So far I have not found anyplace that has information on sistered joists. More than likely it is considered a manufactured joist and therefore has to be tested.
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Old 11-16-2008, 09:21 AM   #4
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Quote:
Originally Posted by Marvin Gardens View Post
I have a guy who wants to make his walk in attic into a family room and at the same time vault the ceilings of the rooms below at the knee wall. The style of the house is gable type with a bearing wall running down the middle. There is no problem with the structure down to the basement floor and appears will built and well supported.

The space has 2x6 joists @ 16" OC spanning 11 1/2' which is too long to be a weight bearing floor. These are old growth joists with really tight grain. There is a 2x4 knee wall about 5 feet from the center of the space holding up an 8x12 roof with 2x4 rafters. Also well built and solid from what I can see.

What I want to do it to sister in alternating 2x8's on every other 2x6. The first 2x8 will go from the left outside wall to the right knee wall. The next 2x8 will go from the right outside wall to the left knee wall and so on.

Once that is done I will take out the non sistered 2x6's from the knee wall to the outside wall and then cap the remaining sistered 2x6/2x8 to clean it up.

Anyone see any red flags in this idea?
Maybe I'm missing something, but I don't follow what will support the 2X8's at the knee wall.
The initial plan is to remove the non-sistered 2X6 joists. Would they, then be replaced with 2X8's?
Also, does the knee wall have top and bottom plates? Or are the knee wall studs nailed to the 2X6 joists?
If there is a bottom plate, how would the new 2X8 joist be fitted?
These are some questions that come to my mind, for what its worth!
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Old 11-16-2008, 09:31 AM   #5
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Maybe I'm missing something, but I don't follow what will support the 2X8's at the knee wall.
The initial plan is to remove the non-sistered 2X6 joists. Would they, then be replaced with 2X8's?
Also, does the knee wall have top and bottom plates? Or are the knee wall studs nailed to the 2X6 joists?
If there is a bottom plate, how would the new 2X8 joist be fitted?
These are some questions that come to my mind, for what its worth!
The non sistered joists would be cut off at the knee wall. The knee wall 2x4's are resting in top of the 2x6's. There would be a double 2x8 header going from each sistered 2x6 and nailed into the end of the cut off 2x6 so it wouldn't just be hanging out there.

It's a complicated weight distribution setup but I can't see why it wouldn't work.

My only concern is the weight bearing ability of the sistered 2x6's at 32" OC. But the weight is going to be on the bearing wall and not a lot on the joists since most of the room will be in the center.
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Old 11-16-2008, 05:03 PM   #6
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Quote:
Originally Posted by Marvin Gardens View Post
The non sistered joists would be cut off at the knee wall. The knee wall 2x4's are resting in top of the 2x6's. There would be a double 2x8 header going from each sistered 2x6 and nailed into the end of the cut off 2x6 so it wouldn't just be hanging out there.

It's a complicated weight distribution setup but I can't see why it wouldn't work.

My only concern is the weight bearing ability of the sistered 2x6's at 32" OC. But the weight is going to be on the bearing wall and not a lot on the joists since most of the room will be in the center.
Working back from what the finished job will be, I think that you will require 2X8 joists running from the plate of the outside walls and installed on 16" centers.
Rather than disturb the 2X6 joists, could you install the new 2X8 joists between the 2X6's.
Then, the new floors and walls would be supported from the outside bearing walls.
Its hard to visualize what you are planning without a sketch or at least some pics!
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Old 11-16-2008, 05:21 PM   #7
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Quote:
Originally Posted by Wildie View Post
Working back from what the finished job will be, I think that you will require 2X8 joists running from the plate of the outside walls and installed on 16" centers.
Rather than disturb the 2X6 joists, could you install the new 2X8 joists between the 2X6's.
Then, the new floors and walls would be supported from the outside bearing walls.
Its hard to visualize what you are planning without a sketch or at least some pics!
We considered that and open beams at 16 OC would make the vault too crowded.

I will try to make up a diagram and post it here.
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Old 11-16-2008, 07:13 PM   #8
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We considered that and open beams at 16 OC would make the vault too crowded.

I will try to make up a diagram and post it here.
I think that I'm starting to get the picture!
From the knee walls to the outside bearing walls will be an open vault.
Sound like a great place to gather dust!
Keep in mind that the attic 2X6 joists are really collar ties that prevent the walls from being forced outwards from the downward pressure exerted by the roof rafter load.
I think that you will find that the 2X8 joists will have to span from bearing wall to bearing wall.
Another option may be to double up the 2X8's on 32" centers, then remove the 2X6's completely!
At least this would cut down on the clutter in the vault.
The attic floor would have to be stiffened somehow due to the 32 centers.
The knee wall studs would have to be supported by a bottom plate,also!
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Old 11-16-2008, 10:47 PM   #9
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Here is a diagram. The floor would have a 14 1/2 OC of sistered 2x6/2x8.

The weight would be on the center bearing wall that is solid and has good support all the way to the basement floor.
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Strucutral help-attic_modification.jpg  
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Last edited by Marvin Gardens; 11-16-2008 at 11:14 PM.
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Old 11-16-2008, 11:20 PM   #10
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Quote:
Originally Posted by Marvin Gardens View Post
Here is a diagram. The floor would have a 14 1/2 OC of sistered 2x6/2x8.

The weight would be on the center bearing wall that is solid and has good support all the way to the basement floor.
Looking at the sketch i now understand!

I would suggest that the left vaulted area would be mirrored on the right side.
Every 32" the 2X8's would run from the left bearing wall to the right bearing wall.
This would ensure that the roof rafters cannot force the outside walls outwards. Secondly the short 2X8's between these, would be supported by a header, which is in turn supported by the main 2X8's.
its the loss of the collar tie that concerns me!

Last edited by Wildie; 11-16-2008 at 11:28 PM.
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Old 11-17-2008, 12:51 AM   #11
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Quote:
Originally Posted by Wildie View Post
Looking at the sketch i now understand!

I would suggest that the left vaulted area would be mirrored on the right side.
Every 32" the 2X8's would run from the left bearing wall to the right bearing wall.
This would ensure that the roof rafters cannot force the outside walls outwards. Secondly the short 2X8's between these, would be supported by a header, which is in turn supported by the main 2X8's.
its the loss of the collar tie that concerns me!
That makes sense. I could do a double header between the sistered joists and run the from outer wall to outer wall. Then for the joists that are cut off I could sister in shorter 2x8's.

I will look at that and run it by the inspector this week.
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Old 11-17-2008, 11:40 AM   #12
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That makes sense. I could do a double header between the sistered joists and run the from outer wall to outer wall. Then for the joists that are cut off I could sister in shorter 2x8's.

I will look at that and run it by the inspector this week.
I will be interested to hear the comment from a pro.
I once built my own home from scratch and have renovated two others.
I don't consider myself as to be considered to be a pro by a long shot. But, I have managed absorb a few of the principles.
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Old 11-19-2008, 09:34 PM   #13
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I will be interested to hear the comment from a pro.
I once built my own home from scratch and have renovated two others.
I don't consider myself as to be considered to be a pro by a long shot. But, I have managed absorb a few of the principles.
The inspector took the plans in and we should hear back from him by the end of the week or next week.

But I am headed out the door for 2 weeks at the vacation home....yeeeee haaaaaa.

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