I recently went through a very similar repair. There are a lot of
things to consider that your post did not mention. Since you said the
joist was rotted out from water, I am led to wonder if the joist is a
roof support truss on a flat poured tar roof.
Is the joist that you are replacing identical to modern 2x8 lumber, or
are you referring to the sawmill sizes that were common up until
somewhere around the 30's or 40's? If it's an old sawmill joist,
chances are it's more like a 2.5 by 8.5 in today's lumber sizes. It
may also be made of a much higher quality of wood than the pine you
may be getting at your local home improvement store.
In any case, you need to consider how much weight is to be on this
joist. If we're talking about a 1920's poured tar roof (which is what
I was dealing with on my truss replacement), you're talking about a
potentially tremendous amount of weight. Think about how many layers
of tar are on that roof, and then recall how heavy one of those
5-gallon buckets of cold-app roofing tar are. Then add the weight of
the sheathing, any repairs, and the consderation that the original
roof was likely 3-5 layers of tar and tar paper, then top that off
with a large snowfall, and you've got the potential for disaster.
Yes, in many circumstances a quantity of screws or bolts will work.
But this is not the method that I would choose. Instead, I would try
to replace or supplement the entire length of the joist. If you can
install your new joist such that it rests on the load-bearing elements
that the existing joist rests upon, you have a much more reliable and
In the house that I am rehabbing, I actually built a jacking assembly
out of floor jacks and a wooden frame of 2x8s and 2x4s to create a new
load bearing assembly to lift the roof back to its proper location,
then up a little higher. I completely removed the old truss, and
seated a new 16 foot 2x8 between the opening in the brick wall and the
center girder. The new truss was shimmed up to compensate for its
slight difference in size from the original truss. Then the roof was
lowered back down onto the new truss.
This technique worked out very well for me. I also don't have to
worry about a pieced-together truss breaking in the future, nor do I
have the concern of pieces of old rotted truss breaking loose, or the
potential that a small split developing in the wood could have very
It's also worth noting that the garage with this house was repaired a
long time ago by the previous owner using the joist bolting technique.
This joist, however, was a main girder to support all the trusses at
their center point. Apparently after some snow, the entire roof
collapsed in because it was enough weight to split one of the bolted
On the other hand, if we're talking about a regular floor joist over
an unfinished basement, that changes a lot. It'll be accessible to be
watched or repaired in the future, and it's not going to be holding a
massive load. Here bolting in a new joist would be fine. However, I
would still get the new joist segment as close as you can to whatever
elements are supporting the existing joist, that way you have more
room for spreading out the bolts over the length of the joist run, and
it minimizes the potential of the joist assembly flexing under a large
load or sagging over time.
This is just a bunch of information I have put together from my
experience with some similar situations. I am by no means an expert,
just another homeowner hoping to provide some intellectual stimulation
through the communication of my ideas and experiences.
-R. Neil Covington