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Old 07-08-2008, 03:49 PM   #1
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Cantilever floor joists


We have a cottage with a 3 season porch that has a shed roof. The cottage is 24x40 including the porch which is 24x8. I had the cottage raised, removed crumbling block foundation, had 7' concrete walls poured on footings and installed a laminated beam on concrete piers as main support.

The original cottage was 24x32 and the porch was added later. Floor joists in the 32x24 run parallel to the 24' side (16" oc). The floor joists in the porch are parallel to the 32' side. These are 2x8 (16" oc) We would like to renovate the 3 season porch to make it wider and a little more headroom (with different roof line in order to have proper windows). In general discussion, I'd like to cantilever the porch 2' to make it 10x24. Is there any concern with cantilever design? Are there suggestions or gotchas to be heeded?

We are located in Ontario Canada.

I have edited post to include picture of the porch.
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Cantilever floor joists-porch1.jpg  


Last edited by orange; 07-08-2008 at 04:10 PM. Reason: spelling
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Old 07-08-2008, 04:01 PM   #2
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Where I'm from 24" is the longest cantilever you can have without involving an engineer. You would of course need at least three times that length of floor joists on the interior side.

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Old 07-08-2008, 04:07 PM   #3
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Where I'm from 24" is the longest cantilever you can have without involving an engineer. You would of course need at least three times that length of floor joists on the interior side.
Thanks for your response. I think I was attaching a photo when your email arrived.

Is there any rule of thumb or specification that deals with cantilevers? I can't seem to find particulars on Google.
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Old 07-08-2008, 04:26 PM   #4
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Cantilever floor joists


Basically you can cantiliver the depth of the joists with no problems at all and no specific design consideration. Here, the longest you can have is two feet, and you'd have to have at least 6' of the cantilevered member on the interior side.

Check your local codes/regulations. Your city inspector should be able to tell you.
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Old 07-08-2008, 04:32 PM   #5
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Thekctermite is correct as usual
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Old 07-08-2008, 06:16 PM   #6
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Thekctermite is correct as usual
Not always, but thanks. I've been known to spew incorrect information from time to time!
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Old 07-08-2008, 06:38 PM   #7
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Originally Posted by thekctermite View Post
Not always, but thanks. I've been known to spew incorrect information from time to time!
Does your 2' cantilever allowance indicate anything about supporting a roof?

Would it make any difference if I placed a post at the position where the current wall and roof meet over the foundation say at both ends and 1 in the middle? That is at 0, 12 and 24' positions and had a beam on top of those posts to support the roof; and continued the new roof out those 2 feet?
I could even put ceiling joists and/or some sort of truss set up for the new roof.
The cantilever would end up supporting the exterior wall primarily.
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Old 07-08-2008, 08:30 PM   #8
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Not always, but thanks. I've been known to spew incorrect information from time to time!
WE ARE ALL HUMAN !
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Old 07-08-2008, 08:45 PM   #9
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Does your 2' cantilever allowance indicate anything about supporting a roof?

Would it make any difference if I placed a post at the position where the current wall and roof meet over the foundation say at both ends and 1 in the middle? That is at 0, 12 and 24' positions and had a beam on top of those posts to support the roof; and continued the new roof out those 2 feet?
I could even put ceiling joists and/or some sort of truss set up for the new roof.
The cantilever would end up supporting the exterior wall primarily.
2' cantilevers can typically support some of the roof, as well as the exterior wall. Your idea about supporting the roof at the line of the foundation wall incorporating a header is a good idea, and I have seen it done, although it is not always necessary. The amount of cantilever possible is directly related to the roof load sitting on the wall. Lots of roof means less cantilever. A roof with good load distribution (purlins, hip/valley mid-span supports, etc) means less load on the exterior wall, which allows longer cantilevers of up to 24".

Longer cantilevers are possible using built up floor joists, but will definately require the support of an engineer...That is something I'd definately recommend either way.
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Old 07-08-2008, 08:48 PM   #10
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From what I am reading, And what you want to do with the posts At 0,12,24 you will be having a post 2ft in from the new wall and in the middle of the room. Why not go the extra 2feet and run the new floor beams along side the existing and back to the house Leger board. And yes it will support a roof load.
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Old 07-09-2008, 07:18 AM   #11
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From what I am reading, And what you want to do with the posts At 0,12,24 you will be having a post 2ft in from the new wall and in the middle of the room. Why not go the extra 2feet and run the new floor beams along side the existing and back to the house Leger board. And yes it will support a roof load.
The post 2' in, in the centre of the room, could be used as part of an interior wall. Part of our thinking is to divide the "new" porch into 2 areas - a bed/sitting room and a traditional porch area. Seems 10'x12' would be adequate for each.
I'm not sure I follow your point regarding "Why not go the extra 2feet and run the new floor beams along side the existing and back to the house Leger board."

My plan would be to sister 2x8's to the current floor joists and cantilever these on the foundation wall. 2' overhang outside and 4' on the inside of the foundation. I'm not sure how the ledger board ties in.
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Old 07-09-2008, 07:15 PM   #12
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We would like to renovate the 3 season porch to make it wider and a little more headroom (with different roof line in order to have proper windows). In general discussionI'd like to cantilever the porch 2' to make it 10x24Is there any concern with cantilever designAre there suggestions or gotchas to be heeded
The porch floor joists are 8' I would use 2x8x10' sister them up along side the existing floor joist and nail them into the ledger board. (The 2x8 that is fastened to the house wall that the existing floor joists are fastened to). If your going out 2' you will have to run the joist back 6'.
if your going to change the roof line then what I would do is put a double joist under each jack stud location. My reason being is that A single joist is going to carry much more of a load then if they were doubled and if you ran them back to the house wall you will have less of a chance of lifting the floor in these locations.
I would not go with putting a girder over the existing foundation, you will be eating up your head room height that you wanted to gain. just remember that you will be losing some pitch to the roof.
HOPE THIS HELPS BOB
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Old 07-09-2008, 10:06 PM   #13
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We would like to renovate the 3 season porch to make it wider and a little more headroom (with different roof line in order to have proper windows). In general discussionI'd like to cantilever the porch 2' to make it 10x24Is there any concern with cantilever designAre there suggestions or gotchas to be heeded
The porch floor joists are 8' I would use 2x8x10' sister them up along side the existing floor joist and nail them into the ledger board. (The 2x8 that is fastened to the house wall that the existing floor joists are fastened to). If your going out 2' you will have to run the joist back 6'.
if your going to change the roof line then what I would do is put a double joist under each jack stud location. My reason being is that A single joist is going to carry much more of a load then if they were doubled and if you ran them back to the house wall you will have less of a chance of lifting the floor in these locations.
I would not go with putting a girder over the existing foundation, you will be eating up your head room height that you wanted to gain. just remember that you will be losing some pitch to the roof.
HOPE THIS HELPS BOB
The idea of the "girder over the existing wall" was to get good support for the roof.Since I plan to change the roof line I could move a ledger higher than the current shed roof. Bring the new roof line to seat on top of the girder.I could even have some truss structure to attach to the new ledger and to the girder that would carry the roof over the cantilevered section.
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Old 07-10-2008, 10:05 AM   #14
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Cantilever floor joists


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Does your 2' cantilever allowance indicate anything about supporting a roof?

Would it make any difference if I placed a post at the position where the current wall and roof meet over the foundation say at both ends and 1 in the middle? That is at 0, 12 and 24' positions and had a beam on top of those posts to support the roof; and continued the new roof out those 2 feet?
I could even put ceiling joists and/or some sort of truss set up for the new roof.
The cantilever would end up supporting the exterior wall primarily.
Run your new joists back to the house and be done with it, that's all you need to do. The roof can sit on the wall, that's all you need.

Why don't you just frame a gable roof to match the existing house roof with the rafters running on the side walls with the same pitch?
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Old 07-10-2008, 11:57 AM   #15
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mu question is: wouldn't it be much safer, yet not that much more difficult, to have the extension sit on maybe three pillars that can be 4x4 or bigger PT lumber set in concrete, rather than cantilevered ?

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