Sistering a Rim Joist

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Old 10-07-19, 09:26 AM
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Sistering a Rim Joist

Hopefully this is the right section for this question. I want to build a small stair landing on the side of my 24' x 36' cottage. The landing will be 6' x 4'. It will be bonded to the cottage at the rim joists and sono tubes for the outer corners.

My cottage sits on 3 beams that sit on concrete pillars. The ledger board for the new stairs will be bonded to the rim joists. Those joists are 12 feet long and there are two of them going along the 24 foot side of the cottage. I want to sister those joists up and was thinking about a 12' board in the center of the joists so as to sister the two 12' joists into one 24' joists. The inside ends of the joists are sitting on a center beam.

My question is whether turning two 12' joists into one 24' joist, by sistering them, would have any detrimental effect on the structure.
 
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Old 10-07-19, 10:44 AM
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I think this may be overkill.

If your 12 foot sister is going to be outside your walls then it is an excellent place for water to collect and rot out the sister and and the joist.

I always put blocks/spacers between a deck's inside joist and the buildings joist then lag bolt that to the building.
So if using 2X8's a couple 2X8X6 will do.
Then calk the living out of the top of the blocks or if possible use a flashing.

Or you could use joist hangers to attach the deck joists to the main joist.

Make sure your door is properly flashed or seal taped at the bottom,
You did not say which direction you are running your decking but leave at least an inch of space between the building and the decking (especially important under the door.

If you have access to the joists under the cabin it would not be a bad idea to put blocks between the onside joist and the next one in just to stiffen up that outside joist.
 
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Old 10-07-19, 11:32 AM
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Because of the thickness of the siding, which had hard 2" styrofoam insulation between it and the blackjoe, there is a fairly large space I need to fill up, so that the stair's ledger board is out from the building. I don't want this styrofoam between the ledger board and the rim joist. I was going to fill some of that space up with this 2x8 wood and figured I could stiffen up the rim joist at the same time to accommodate the new stairs. The stair landing is 6 feet along the wall and 4 feet out from it.

I was planning on flashing anything that won't be covered well by siding, like the ledger board and some if not all of this extra piece of 2 x 8.

I guess what I am asking is if a sistered rim joist would cause any detrimental effect on a house. In other words, if they had 24 foot 2x8 at a reasonable price, is that what they would have used or is there some benefit of using two 12 foot 2x8 boards.
 
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Old 10-07-19, 11:37 AM
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I always put blocks/spacers between a deck's inside joist and the buildings joist then lag bolt that to the building.
------------------------------------------------------------

What kind of blocks do you use. What do they look like.
 
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Old 10-07-19, 01:15 PM
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I do not think stiffening just the outside joist is a good idea.
If you have not noticed any problems then it will just be spending money.

I would just build it out (get rid of the styrofoam) where the deck is going to be.

The blocks are the same size as the joist so if they are 2X6 that is what I use, if they are 2X8 that is what I use etc.
 
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Old 10-07-19, 01:47 PM
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OK thanks.

The ledger board for the stair landing needs to go across the break in the joists anyway, so one way or the other those rim joists are going to get joined. I think it is about 1 foot on one side and 5 feet on the other, approximately. Hopefully that is OK.
 
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