I just read the thread with the subject heading of, "DWV pipe through studs"
and it reminded me of a question I have.
I am having work done on a house and it includes a washer/dryer closet that
will be next to an existing bath which is also being redone. The dryer vent
pipe needs to go about 8 feet across the ceiling above the tub and toilet to
reach the exterior wall and vented to the outside. I was thinking of just
running it along the corner of the ceiling and wall above the bath and
toilet (the ceiling is high enough to do that) and having it boxed in by
building a soffit(?) around it.
One contractor said that since the ceiling will be open anyway, it would
make more sense to run it through the 2x8 ceiling joists which run
perpendicular to the way the vent pipe will run. That would leave it hidden
in the ceiling instead of having a soffit showing. My concern is that
drilling 4 inch holes in the joists would create a structural problem since
there is a room and floor above that. He said it would be okay because a
sister joist could be added to each existing joist where the holes would be.
Does that seem okay as a way to go? It is more work and would cost more up
front, but I am not worried about that part. But, would the sister idea be
okay, and do the sister joists need to go the whole length of the joists, or
could they just be a short section that is sistered on to make up for the
holes being cut into the joists?
Without engineering, the maximum hole size for the middle of a joist
is depth/3. So for a 7.25" joist, that's under 2.5". I strongly
advise against putting 4" holes in a 2x8. Can you hit a different
outside wall by running parallel to the joists?
A rectangular opening would not weaken the joist nearly as much. A
2.5 inch high by 6 in wide duct would work as is is about the same
square inches opening. The are rectangular ducts available for
dryers, I would use whatever is the standard duct size. I would still
sister the joist for as much of the span as can easily be reached.
As to the relative merits of cutting holes in joists versus a 15-foot
longer horizontal dryer run, the longer horizontal dryer run wins
hands down, in my opinion. You can check the total dryer run against
the manufacturer's recommendations (an example of this is at, e.g.,
<http://www.applianceaid.com/vent-length.html ), to determine if youare OK. If not, you may be able to just upsize the duct to 5", not
sure about that.
Fifteen feet is okay.
I had one that went up 8' to the attic, then another 7' to the roof - and
that was straight UP. I replaced it with an up 8' and horizontal 22' to the
soffit. Works swell.
If, in your case, your dryer doesn't have enough oomph to push air 15', you
can get in-line vent fans to help things along.
If the contractor mentioned is the one doing the work I would consider
not letting him touch anything else in your house. Do not cut your
joists and weaken the structure for a dryer vent.
Talk to an inspector (you do have permits for the work being done) and
get his advise. That is their job.
You do not want to run flex duct for the dryer exhaust. Rigid metal duct is
the best way to go and will be very difficult to get through bored holes.
The holes would probably need to be larger for rigid metal duct to go in.
How are they going to sister around 4" holes? It sounds as though you will
have double joists with 4" holes in them. Removing more than half a joist
width doesn't sound healthy, but the ultimate structural affect is in
relation to how close it is to a supporting wall. I vote for the soffit.
Call your inspections dept. They've always been willing to answer Q's for
Is a permit required for the work being done? Was one obtained? Would be
better to find out if it's not OK now rather than when it's all done.
Hole is made and no inspection required/done. Upon sale of home, buyers
home inspector says this is no good. Inspector would say for buyer to
request an PE's (Professional Engineer) doc stamped with seal to say it's
OK. $350 for PE. And if PE says NG then also cost to rectify.
Hole is made. Fails building inspection. PO'd contractor removes and
sisters to cover hole. Upon sale of home, buyers home inspector says
joist has been altered. Inspector would say for buyer to request a PE's
(Professional Engineer) doc stamped with seal to say it's OK.
Just my pessimistic projection of shit happens.
I like this idea best but does it require an ~ 8 ft riser in the laundry
room to get up to the ceiling in the first place, before it makes the turn
for the bathroom wall ?
Trying to 'Push' hot wet air UP 7-8 ft may result in condensation in the
vertical pipe which could run back down causing problems. Otherwise, I do
like the ceiling/soffit idea
In this case, the rise will be less because it will be a "laundry closet"
with a full-size stackable washer/dryer set -- front loading washer on the
bottom and dryer on top of that. So, the rise will be about 5 feet, then a
horizontal run of either 8 feet (if I do the soffit) or 15 feet (if I run it
in the other direction through the open channel created by the parallel
If you have standard 2x8 joists, the largest hole you could safely drill is
about 2.4 inches (1/3 of 7.25"). Anything larger would be against code and
significantly weaken the structure. A 4" hole would be out of the question.
You mentioned a room "above" the washer/dryer closet, but what is "below"
the closet? If at all possible, I would opt for running down to a basement
or crawlspace, then out to the side wall. I know many dryers vent up to the
roof, but I personally don't like the idea of trying to blow lint uphill
and having it fall back down when the dryer shuts off.
Have you investigated other routes? You could run 4" vent pipe parallel to
the joists without any problems. It may be easier to take another route,
even if it's slightly longer.
Remember the flex pipe and elbow coming out of the dryer and up/down to the
vent line count as elbows in the total run of the dryer duct. I recommend
keeping the duct under 12' long if possible.
Well, duh, I forgot about that option. What is below the laundry closet is
a kitchen which now has an open ceiling and that is being completely redone.
And, although the kitchen design plan isn't finished yet, it may turn out
that there is going to be a soffit above the top cabinets along the same
wall that I was going to use if I went through the bathroom one floor up.
So, it may be that the dryer vent can just go down and into the new kitchen
soffit, and then 8 feet out through the back wall.
If the kitchen ceiling is already open, that sounds like a much better
route. Clothes dryers vent near the bottom anyway, so going down and out
would eliminate the 8 feet of duct running up to the closet ceiling.
Hope it works out for you.
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