Thread: Dryer Vent Disharge On The Roof

This is the second time I have found the dryer vent discharging onto the roof surface. I know that there is a maximum run of 25' plus account for the elbows, add 4' per elbow. They both have had measurements of 24'. I recommended that they monitor this for cleaning. I also told them to check their dryer manufacturer manual if a booster fan would be needed.
Our high humidity levels would seam to keep lint all over the roof.
Does any one else see this and what do you usually recommend?

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Brad,

I am sure you meant subtract 5' for each 90 degree turn. I do not believe dryer "boost pumps" are allowed. I do know where I work they are not allowed.

Bob,

When figuring length, you ADD for the elbows not take away and booster pumps are allowed by many AHJ's.

rick

I see that type of vent on all Centex homes in my area. They are crap and will clog after a period of time.

I don't recommend anything if it is connected properly and I can see everything. Problem is that I can't see everything 75% of the time so I note that in my report and go on with life.

Originally Posted by Rick Hurst

Bob,

When figuring length, you ADD for the elbows not take away and booster pumps are allowed by many AHJ's.

rick

One man's adding is another man's subtracting. You don't mean the vent can be longer due to added elbows, I know.

Originally Posted by John Arnold

One man's adding is another man's subtracting. You don't mean the vent can be longer due to added elbows, I know.

No, he means you measure the duct length, then *add* for each elbow to get the effective length.

*That* effective length is what is used for the duct length.

Originally Posted by Scott Patterson

They are crap and will clog after a period of time.

Yeah, the damper swings up, the lint catches on the turned down front lip of the goose neck hood, then slowly builds up, eventually blocking the damper from opening much, or occasionally, when the damper is open the lint builds up enough to not allow the damper to close that one last time.

Recommended action: Clean the lint out of it.

Here is the the ITC 2006 code.

M1502.6 Duct length.

The maximum length of a clothes dryer

exhaust duct shall not exceed 25 feet (7620 mm) from the dryer
location to the wall or roof termination. The maximum length
of the duct shall be reduced 2.5 feet (762 mm) for each
45-degree (0.8 rad) bend and 5 feet (1524 mm) for each
90-degree (1.6 rad) bend. The maximum length of the exhaust

duct does not include the transition duct.

So how am I incorrect?

Jerry is the correct word equivalent or effective?

Exception 2. Where large-radius 45-degree (0.8 rad) and
90-degree (1.6 rad) bends are installed, determination
of the equivalent length of clothes dryer exhaust duct
for each bend by engineering calculation in accordance
with the ASHRAE Fundamentals Handbook

shall be permitted.

I started seeing a lot of those in townhouses around my area a few years ago. It's funny now that they've been in use for a few years (and of course nobody cleans them) you see white lint crap streaking down all of the roofs.

I agree with the others that this is an overall poor idea and installation... but is allowed.

Originally Posted by Bob Spermo

Jerry is the correct word equivalent or effective?

Probably both mean one and the same thing, however, being as the exception used the word "equivalent" I would go with that.

Remember, though, that code only applies when the manufacturer of the dryer does not allow for longer runs, and some allow for up to 90 feet.

I wish the code required the "equivalent" length of the dryer duct be stated in a label permanently attached to the clothes dryer connection.
Because, one a homeowner moves out with a dryer rated for 90 feet and the next home owner comes in with a dryer rated for 50 feet, and the "equivalent" duct length is 60 feet, the new dryer makes that installation no longer code compliant.

Thanks Jerry. I did not know that a manufacturer could build a dryer that could use a longer vent. Nice to now know that!

I can't tell in your picture, but these things come with a screen that pops in for use as an exhaust fan vent termination.
The screen is not allowed to be there on dryer vents.
M1501.1 General
... Screens shall not be installed at the duct termination. ...

I regularly see the screens left in place and even have builders claim the AHJ requires it to be there. This is one time I quote the code in the report.

The ones I've seen with screens do not have the required dampers. The ones I've seen with dampers do not have screens.

That does not mean all are made that way, only that is the difference I've always seen.

So then, I just read this thread and her is what I have come away with:

1. Equivalent is a synonym of effective. That's a novel idea, and sure to come in handy someday.

2. Like inspectors, some clothes dryers blow more hot air than others. I knew this one already, but it is always nice to be reminded of the important things in life.

3. Hood + damper = no screen, and hood - damper = screen. Interesting math formulas have always intrigued me.

How will I use these little gems of wisdom?

1. I shall run right down the local Mercedes Benz dealer and order a new 500S AMG. When my bank account looks a little slim for this $130K purchase, I will simply inform them that my funds are effective, therefore equivalent to what is needed. I will then have to finally convert to Telesteps.

2. I intend to start a new thread informing the lurkers on this forum to purchase and install personal boosters in order to equal the effective smoke-blowing capacity of the forum's resident elders.

3. This will come in handy for totally confusing AHJs, builders, contractors, et al. Any things containing the words hood or screen will be open game. The possibilities are endless.

..or everything you wanted to know about dryer venting and then some! All dryer vents are prone to lint build up. Remember to clean your lint trap after each dryer load!

If you can get a Mercedes 500S AMG its a real bargain. Get two!

Okay ... can't resist adding to this one


I've heard (not yet personally seen) that the dryer vent pipe distance has been extended in the 2009 IRC that is now available.

No turn deduction distance is now 35'.

EC J can likely confirm. I've not yet received my copy of the new IRC.

Now ... as for the Mercedes .....

Nope, don't have mine yet either.

Originally Posted by Jerry Peck

Nope, don't have mine yet either.

JP: And that is because you prefer Limey Felines to German Craftsmanship.

Late to the thread (hopefully Brad still subscribes) but in SC they (the State) have an exception to the 2006 IRC (number 30 is the exception from July last year) that allows 35' of run.

New construction applicable; otherwise dryer manufacturer rules apply, and still the normal adds of 5' and 2.5' for the bends.

I don't like the roof exhausts either. I put comments in the report about cleaning regularly and the hazards of climbing on the roof to do so. I pretty much always tell clients to consider changing at some point. I've had a few clients call over the years asking if I knew a contractor who would come out and change it. This call usually occurs after the 1st attempt by the husband to clean the lint away.
Another problem in our area at least is squirrels using the vents as entry points into the attic. They don't seem to have a problem chewing away the plastic.