Subscribe or Renew Membership Subscribe Renew

Sistering a joist

Tonys1228's picture

I need to sister an existing 17ft long 2x10 floor joist.

Reason is , after sistering the one joist I need to attach a 6ft long beam at 90 degrees... And will be hanging (3) 2x10s off it.


Problem is.....the original joist are spaced only 12" apart and each end of the 17 joist has 6"sitting on the sill plate.  How can I get a joist to fit in that small of a space?  

Will shortening the additional sister joist b y 6"... (leaving 3" on each sill plate) reduce the strength too much?

I'm not even sure I can squeeze it in by shortening it 6"

Btw....I'm creating an opening in the floor joist for a spiral stair case.


no need for bearing if nailed (post #215241, reply #1 of 7)

no need for bearing if nailed properly -

lookup lateral shear

Thank you for your (post #215241, reply #2 of 7)

Thank you for your response.... So you are saying the added joist ends don't need to rest on the will plate as long as its fastened correctly to the existing joist?  I am spanning 17' 8".

After doubling i will be connecting a header about 6' from one end .  header will be about 6' long and have 3 joist hanging off it.

Tony (post #215241, reply #4 of 7)

You should be able to add joists that do sit on the wall a couple inches on each end.  Bevel the top of the joists you are adding so they will tip up easily. Stuff one end in and over to one side, pull back to center after you get the other end up.

why go wrong when you can go right?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Got her done (post #215241, reply #7 of 7)

Thanks to everyone for your help. 

I was able to get 2.5 inches of bearing on both ends.

All I did was clipped about one half inch of the top corner of one end.

Not bad since I was alone and had no help lifting..

Spann Distance from sill (post #215241, reply #3 of 7)

Spann Distance from sill plate to sill plate edges is 16'8"

Joist length is 17'8"

Tony (post #215241, reply #6 of 7)

heres another reason to make the joist long enough to sit on both walls.  You can do this solo.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


The main thing to understand (post #215241, reply #5 of 7)

The main thing to understand is that, if the original joist is sound and evenly loaded, the most stress in on the middle third.  If you anchor the sister to the outer third on each end (and, of course, also tie it to the middle) then the original joist is greatly strenghtened.

If you can manage to have the sister rest on the supports on each end so much the better, but that's not necessary in most cases.

Do use something substantial to do the joining.  Through-bolts are the best choice.  But don't put the fasteners too near the ends, to avoid splitting.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville