Sistering Cantilever Balcony Joists

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Old 08-24-13, 07:27 AM
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Sistering Cantilever Balcony Joists

I'm in the process of repairing a Balcony. It is 25' wide, 4' deep, 2 X 8 16" On Centre joists.

Problem is each joist has varying degrees of dry rot most likely requiring removal of the front 4" of the joist and approx 1" off the top of each. Leaving each joist at 3' 6" extending from the home. I'm looking at sistering a new 3' 6" piece of Pressure Treated 2 X 8 to each joist utizling Premium Grade Construction Adhesive, 1/2" Carriage Bolts (9" on centre (2" from top and bottom of board alternating every 9")) then adding GRK 2 1/2" construction screws above or below each carriage bolt.

The front of the joists will have a new facia of 2 X 8 PT attached via Simpson Strong Tie Joist Hangers

Bead of Silicon sealant will be place along top joint between existing Joist and new sister plus a membrane on top of each to minimize or eliminate any water ingress down the road. The new decking will be supported by the new sistered 2 X 8 at a minimum, via the carriage bolts & screws to the exisiting now modified to approx 2 x 7 joists.

Is that overkill? or am I in the ball park? Perfect world would be to pull the existing joists completely or sister the full length but that is cost prohibitive at the moment.
 
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Old 08-24-13, 08:26 AM
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Post some pictures.
Sure sounds like a very bad idea to me.
Those joist should have been part of the whole flooring system when the house was framed.
They should have been run at least 8' as a minimum all the way into the main house.
 
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Old 08-24-13, 12:27 PM
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joecaption1: The original joists were installed into the original framing of the house. From what I've gathered from the original owner the joist extended 8' into the house with 4' out giving the normal 2/3 1/3. The 4' cantilevered out has dry rot on the leading edge (approx 4" where the original railings tied in) and most of the top of the joists have some dry rot on the top of the joists where the original decking was touching and nailed (up to a maximum of 1" deep on some joists).

Once I've removed the dry rot the joists would be now extending from the house approx 3' 6" and be approx 7" thick vice the original 8".

As mentioned intent would be to sister a new 3' 6" piece of PT 2 x 8 attached via premium construction adhesive, carriage bolts (5 per board at 9" OC) and GRK 2 1/2" wood screws.

Load of decking would rest on the new PT 2 x 8 transfered to the old joist via the carriage bolts and screws.
 

Last edited by flingwing; 08-24-13 at 12:30 PM. Reason: spelling
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Old 08-24-13, 12:49 PM
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minor correction. original joist would extend to 3' 8" vice 3' 6" and sistered PT 2 x 8 would be 3' 8' as well.
 
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Old 08-26-13, 11:12 AM
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Wood rot, like rust, usually goes much deeper than expected. Sistering outside the foundation won't do anything to improve load capacity of your compromised joists.
How high off the ground is this balcony?
 
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Old 08-26-13, 05:06 PM
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Balcony is approx. 9' above the deck. Attached is photo after removal of old decking & railings. Ends have been cut back 6" so they are now cantilevered out 3' 6". As you mentioned Dry Rot was deeper than initially surveyed.

Though there is dry rot on top of some of the joists (to be removed (dry rot not the joist)) not all will have to be trimmed as deep as expected. So when all said and done portions of the new decking etc will be solely carried by the sistered 2X8. Other portions will be carried by both existing Joist and Sistered Board. Upside with the sister is, I'll have new PT to drill decking into. All now double wide joists will be covered by Vycor Joist Protection memeber which seals around screws and nails and will give good long term protection to the soon to be rot free old joists as well as new structure.

I'd post a picture but have no clue how to get it down to 50kb
 
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Old 08-27-13, 12:59 PM
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I think all your solutions to keep the joists dry; will keep them from draining/drying when they get wet. Bolt (2 each end) the new joists after waterproofing both old/new; add silicone caulk in the bolt holes, add four (or so) flat stacked galv. washers between joists for 1/4" drainage. Leave tops open/no adhesive anywhere. Keep bolts 2" from top/bottom joist. Back- one page; The Design of Renovations - Donald Friedman, Nathaniel Oppenheimer - Google Books

Gary
 
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Old 09-08-13, 07:05 PM
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Gary: Didn't read your post till today. I racked my brain on the best solution for keeping the surface between the two joists dry and I though about the possibility of spacers between the two to allow drainage and air drying. The reason I didn't go that root is I felt that I would then get too much of a leverage movement/arm when the load transfers from the new 2 X 8 into the old 2 X 8 as on some of the joists it will be only the new 2 X 8 carrying the load of the decking board and anybody standing on top of it. That said with the Vycor Joist protector over the top of the joists I'll eliminate water getting in from on top.

I did go min 2" away from the edge of the joists with the carriage bolts. as well used the silcon sealant on the bolt holes as well will overspray each bolt head and washer/nut end with Techniseal Instant Leak Sealer.

Basically I've ended up with 20 16" OC 3' 3 1/2" Joists All Sistered (there's between 8' to 10' of the original joists that run into the main structure of the House.) Will be adding the face board (2 X 8 PT) later this week utilizing Simpson Strong Tie Double Joist Hangers w/ Simpson SD Screws. Decking will be 2 X 6 Western Red Cedar and Railing posts will be 4 X 4 Western Red Cedar attached by Simpson Strong Tie Railing Post connectors. Haven't figured out the railing caps or infill between railing posts yet.

Thanks

Fling
 
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Old 09-09-13, 12:43 AM
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Your repair plan sounds great, with one significant exception--you're beefing up what's left of the compromised cantilevers, while doing little (or nothing) to actually strengthen them.

Do you have any idea how far the rotten condition extends into the interior of the joists? Notice that I didn't say "dry rot," as your joist deterioration was caused by organisms aided by the presence of excessive water and moisture, not "dry" conditions. No amount of sistering or joist hangers can heal the damaged joists where they emerge from the house proper, the tops of which are critical locations in terms of bending moment stresses developed. Depending on the extent of damage, a much better repair would include sistering every other joist at least 4' into the interior of the house.
 
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