Sistering floor joists...connection question

[jimtheengineer10]

Attached is a drawing for your reference. I am working on some house plans and the current plan is to run pressure treated 2x12's from one wall to another but they will cantilever out 4' for a deck. Instead of installing pressure treated 2x12's on the interior portion of the house can pressure treated 2x12's be used for the deck and be sistered onto the interior 2x12's? If so how far should the deck floor joists extend back into the house and what kind of connection should be used? Thanks.

 

[dik]

You'd have great difficulty in transferring the moment... would likely be a lot less costly to use the PT joists throughout...

 

[dhengr]

jimtheengineer10:
Those joist/wall penetrations are real dogs to flash, caulk and make water tight, so make the top plates and studs PT too. That just has not proven to be a very good detail, all kinds of water problems. If you do it, cap flash the first 18" of each joist away from the wall so that water can’t run back into the wall/bldg. on the top and sides of each jst. The wall end of this cap flashing goes behind and up under the regular wall flashing and counter flashing which keeps the wall water away from these jst. penetrations. That seems like a smart and practical structural solution, but it stinks from the standpoint of water migration into the wall and the thermal break. You can actually have those jsts. sweating a foot back into the bldg. on a cold day.

 

[msquared48]

I have done that before.

With 2X10 floor joists and 2X8 PT deck joists, I extended the 2X8's back into the house twice the deck width with a 1/4" per foot slope for drainage. Depending on the width of the deck, you might have to rip the top of the 2X8's inside the interior joist space to get the slope.

Mike McCann
MMC Engineering

 

[jimtheengineer10]

msqaured48, how do you fasten the 2x8's to the 2x10's?

 

[XR250]

@DLK
No problem transferring the moment. Assuming a 4' cant. and 16" O.C. and 50 psf on the joists, and assuming a 4 ft. backspan (8' PT joists - I would use more, but this is for arguments sake) Asmuming simply nailing at the end of the 4 ft. lap on the interior, the shear in the nails is:

(50 x 4' x 4'/2 x 1.33' O.C.)/4' = 133 lbs (summing moments about the wall top plate)

No problem for a few 10d nails. I would use more, but you get the point that the moment transfer is easy.

 

[4thorns]

I recall using a program that would not allow a situation where the cantilever was more than 1/3 back span (not sure what it was but I think it was for LVLs. Is this just product specific?

 

[TXStructural]

Be certain the joists are fully braced over the top of the wall. Avoid fastening the PT joists to bracing or interior floor decking. I would consider bracing with detail that lets you slide PT joist into placed and bolting some distance back from the exposed end.

Plan for water to enter the wall, one way or another, if the climate is not dry all of the time. PT wood will dry and can carry moisture in to the untreated wood and fasteners. You might consider a rubberized asphalt flashing/barrier between the joists and untreated wood wherever they touch. Stick the barrier to the untreated wood, with the non-adhesive side on the PT. Attach the joists using stainless or hot dip galvanized bolts. The membrane and the use of bolts should make it easier to repair if/when it needs replacement.

 

[moltenmetal]

dhengr has it right. I'd NEVER do this, because water penetration into the house is going to generate a very, very costly repair for someone in 10 to 20 years at most- unless this is located in a desert.

 

[jimtheengineer10]

4thorns, it would be 2x12's and pressure treated 2x12's.

What is we just installed pressure treated 2x12's and have them cantilever out the 4'. Are there still going to be water issues doing it this way?

 

[msquared48]

The 2X12's for the deck would have to be sloped away from the wall, or there will definitely be water penetration issues at the wall. It would have been much better to use 2X8's cantilevered four feet and sloped at 1/4"/ft.

What material are you putting over the 2X12's, and how much of a vertical separation to the interior floor do you have at the slider or door?

Mike McCann
MMC Engineering

 

[jimtheengineer10]

Thanks for all the help. After talking to the owner we have decided to stop the floor joists at the exterior wall and run the deck floor joists perpendicular.