That wire screen over your rooftop dryer vent may well be keeping out critters, but it's also creating an extreme fire hazard. It's not supposed to be there.
What is the regulation regarding dryer vent covers?
Though exceedingly common, for reasons we'll explore later, it is against International Regulation Code, and generally an unsafe practice, to place or keep any kind of screen over a dryer exhaust vent. The specific code, M1502.3, regarding dryer duct termination, is worth quoting in full:
Exhaust ducts shall terminate on the outside of the building. Exhaust duct terminations shall be in accordance with the dryer manufacturer's installation instructions. If the manufacturer's instructions do not specify a termination location, the exhaust duct shall terminate not less than 3 feet (914 mm) in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.
Why is it dangerous to keep a screen over a dryer vent exhaust?
Though screening is commonplace over chimneys, furnace flues, bathroom exhausts, etc., for the main purpose of keeping animals out, it is ill-advised over a dryer vent exhaust, because in addition to exhausting moisture and heat, dryer vents also exhaust small bits of lint. Moisture exhausted acts as a glue that adheres the lint to the screen, and, over time, the holes in the screen become completely choked with lint, cutting off airflow and creating further blockages down the vent line. This buildup of lint, which is highly combustible, creates an extreme fire hazard. The restricted air flow also impairs the efficient function of your dryer and can cause the thermal fuse (a device specifically designed to trigger in the case of a high-heat event) to blow out, leaving you without a dryer until it can be replaced.
If I remove the screen from my dryer vent, what keeps animals from entering?
Your dryer vent, behind the screen, should have a louvered flap that swings open when the airflow from the dryer opens it and then falls back into the closed position when the airflow ceases. If the vent does not have this feature, it should be replaced. You'll want to inspect the cover every so often to make sure it's intact and none of the louvers are missing, which might present a welcome invitation to a furry critter seeking warmth.
Why the heck do dryer vent exhausts even have screens on them?
It seems the default is that rooftop vents, whether bathroom or dryer, come with screens over them and are installed as is. Why are they not removed in the case of dryer vents? I posed this question to several roofing companies in the Twin Cities, all of whom were forthcoming and generous. One replied that they do not remove them so as not to expose the homeowner to the possibility of animal entry. Another explained that they are kept in place for reasons of liability, presumably referring back to the animal-entry scenario, as well as the fact that the screen is installed by the manufacturer. A third explained that indeed they do remove the screen when the vent cover they're installing is designated for a dryer, and they mark them for this purpose, so as not to neglect that final step once the job is complete.
What's the consensus?
Remove it. Regardless of how the screen got there, it should not remain. If you cannot remove it yourself, your dryer vent cleaning company will (or should) do so during the course of the cleaning. Unfortunately, the screen is often not detected until the dryer's performance begins to suffer and the homeowner calls a dryer vent cleaning or repair company seeking explanation.
Coincidentally, one of the roofers I spoke to said that he had just been on his own roof the night before to clean out his dryer vent. He noticed the screen and removed the pesky thing himself.
Wanna dig deeper? Download our free tipsheet: 5 Questions to Ask Before Hiring a Dryer Vent Cleaning Company.
Many thanks to our technician Ben S for lending his expertise to this article, as well as our friendly neighborhood roofing companies.