Question for you guys, have any of you seen many problems with bats using this still of roof vent? I have a job now with some of these vents and I have find some dropping by and under them. there are some with loss screens on the under side but on the inside a bat would have to maneuver and shot straight up to get out

Most of those screens I've seen on the bottom of those are wimpy and usually a bit warped where a bat could crawl from the ceiling to emerge at night. Seems like many makers just spot weld the screen in a couple of spots, or maybe solder is the best word. If I have them on a house I make sure they are excluded with something made to last on the inside (yet still allowing all the airflow - just to be clear).

The only issue with screening them off from the inside is birds love to nest under them. Usually the factory screen rips and the nesting material fills the attic. By reinforcing the screen you make it more convenient for the bird since less material will fall through.

I would suggest screening from the outside and you eliminate any future problems.

Unless of course you live in an area where birds are not know to nest under roof vents.

Good point Nate!

In my experience, most bats that enter from these vents do so by crawling under the flange, not from climbing the side and entering through the can area. Jason Grimm has seen bats doing that in his service area, but in mine I haven't. I'd love to see video of bats actually using the can opening to enter and exit the structure. Justin has a good point that if the screen is still in place but loose, the bats could be using it to crawl on or just to hang on if they get too hot. Either scenario can have them leaving their calling cards underneath.

If you are going to screen them either do over the top as previously suggested with 1/2 x 1/2 inch (back issues of WCT Magazine contain plans on how to make them), form the screen into a cone with the lip bent over, put a good line of sealant in a circle on the lip and then push it up into the can until it reaches the top then finish it by putting some sheet metal screws in to hold it in place until the sealant dries or replace the vent with the a newer style one that does not have the mesh but rather slits.

In my service area those vents are about the only thing on a house that critters don't bother - even the plastic ones. In all the years I've inspected those, top and bottom, I've only had one raccoon rip the top off, and one gray squirrel and one flyer chew thru the louvers on the side to get into an attic. We have several different styles installed (most have the louvered slits on the exterior), but overall I'd say three breaches in all these years makes them a fairly safe product that we rarely need to 'proof.
On the other hand, attic fans (aka rooftop power ventilators) are a common entry point which we always retrofit with a heavy duty stainless screening (for metal topped vents) or HY-C's Roof Vent Guard 3030 (for plastic topped vents). The last 6 RVG3030s HY-C sent had smaller diamond screening than earlier models, which will do a better job of keeping even small critters like mice and bats from squeezing through. Also of note is that one of the attic fan companies (unsure which one yet) is using a real nice heavy duty diamond screen on their fan units now. I've seen three or four installed and the screen is pretty much raccoon proof.

Just had two more occurrences of bats roosting on top of the mesh inside the can vents, in the past two weeks. These in particular are stamped expanded steel on the inside and the bats can not get into the attic, but they will roost on top of the grate on the inside. While working on a roof and walking by a vent, 2 bats flew out and every single one had guano up inside and around the units. Next week I will be starting a exclusion that has bats roosting in them and while I was in the attic you can see them stacked in there. I will take a picture when we start the project.

If the can vents have thin mosquitoe screen on the inside we automatically screen the outside as the inside screen will not hold up to regular roosting activity and they will eventually use it for a entry point, as I learned the hard way on a old 1874 school house that was converted into a residential home. All other areas of the structure was completely sealed but yet they were using the roof vent as main entry.

It would be neat to install camera's to watch their activity around these vents, but we are way too busy to monkey around with setting up the electronics. Maybe someday if we ever catch a slow time. I can also show footage of bats entering at the foundation level, that would be great footage to watch and better yet a great learning tool.