4" aluminum dryer vent through joists

[alta47]

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?

[Wayne Whitney]

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?

Cheers, Wayne

[alta47]

I could, but that would mean about a 15-foot horizontal run to the outside wall. I am not sure that such a long horizontal run would be a good idea, but maybe it's an option.

[hrhofmann]

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.

[Wayne Whitney]

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., ), to determine if you are OK. If not, you may be able to just upsize the duct to 5", not sure about that.

Cheers, Wayne

[HeyBub]

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.

[Gordon Shumway]

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.

G.S.

[John Grabowski]

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.

[Red Green]

"alta47" wrote in news:mYGdnWsJpKAbp8zVnZ2dnUVZ [e-mail]:

Call your inspections dept. They've always been willing to answer Q's for me.

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.

Scenario 1:

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.

Scenario 2:

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.

[Rudy]

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

[John Grabowski]

I had a problem like that since my duct passes through the attic. I insulated it with duct wrap and the problem went away.

[alta47]

Thanks. That chart helps and it looks doable for me to run the duct 15-feet if I end up choosing that option.

[alta47]

All good points. I'm liking the cut-the-holes-and-sister-the-joists idea less and less.

[alta47]

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 joists).

[RicodJour]

At no point should you "like" the large holes in joists with bandaids approach.

R

[HerHusband]

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.

Anthony

[S. Barker]

Usually up to 25' horizontal and straight are ok. check the dryer specs. and use smooth pipe and no screws.

s

[alta47]

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.

Amazing.

Thanks.

[HerHusband]

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.

Take care,

Anthony

[RicodJour]

Don't be tempted to tie in the stove hood to the dryer vent. Very bad idea.

R