We have cockroaches. Probably not. I think they are water bugs or palmetto bugs, rather than cockroaches per se, but from a home owners point of view, I'm not sure the difference matters much. They mostly live outside, in the trees or under rocks, but occasionally come inside. We've tried home remedies, like boric acid power, and roach motels, and maybe those work, but they certainly don't get everything.
The house is pretty well sealed. Good windows, weatherstripping on all the doors, screens on all the windows. We have had situations where they apparently try to slip thru a door at the hinge side while it is open; occasionally I find one smashed between the door and the frame -- not quick enough to get out of the way when the door closed. But they tend to show up either in the utility room/kitchen area or the bathrooms. In the utility room/kitchen, they could be coming in from the garage thru the kitchen door.
But for the bathrooms, they either have to come thru a door and then make their way to the bathrooms, or they are coming directly into the bathrooms. Texas A&M says these are "one of the most common cockroaches in sewer systems", so it seems they could be coming up thru the pipes.
And there are two directions for them to come. Up from the sewer, or down thru the vent pipes. We have a set of pipes on the roof that are part of the plumbing. These pipes vent sewer gas and allow air to enter to equalize air pressure.
But at least on our house, the vents are uncovered on the top, sheathed in lead (the stack flashing) and then painted.
So it might be possible for a bug to enter the pipe on the roof, go down until it meets a branching pipe which would take it to a p-trap. The p-trap is filled with water, but a swimming bug could go thru that and then be in the house. I don't know if that actually happens, but they are called "water bugs".
But all the vent needs to be able to do is to allow air to vent, so it seems reasonable that we could put an insect screen over the top of the vent and prevent this from happening. I would imagine a wire cap sort of like:
And there is also the problem of keeping the screen cap securely on the vent pipe.
Let us consider the scope of the problem. We have 4 vents in the front of the house for the Master Bathroom: the sinks, the toilet, the bathtub and the shower.
These vary in size from 2.25 to 3 inches and are not totally round. The lead flashing sometimes sticks out more in one direction than another.
The back bathroom has 3 vents: the sink, the toilet, and the bathtub. For some reason, the vent for the sink is larger: 3.75 inches while the other two are only 2.25.
The central bathroom has two sinks, and 4 vent pipes: the two sinks, the toilet and the tub.
These all seem to be 2.25 inches, but the variation of the lead flashing takes it to 2.5 or 2.75 inches.
There is one vent pipe for the kitchen sink, 2.25 to 2.5 inches.
and one last vent pipe for the washing machine and the garage sink, at 2.25 inches.
That makes 13 vent pipes total. One is 3.75 inches while all the others seem to be 2.25 to 3.00 inches.
Our approach is to put a patch of insect screen over the vent, and hold it in place with a worm gear clamp:
I will need clamps that adjust to a range of 2.25 to 3 inches for most of them and one that goes to 4 inches for that one big one.
A trip to Home Depot gets me most of what I need. A roll of silver gray fiberglass screen (36x84) from which I can cut 7 inch squares of screen. I have one clamp here already, and bought one 4 inch clamp for the largest vent. And a bag of ten stainless steel 1-3/4 inch to 2-3/4 inch clamps to hold the screen in place. And just in case the clamps are the wrong size, one 1-3/4 inch to 2-3/4 inch clamp separately, so that if it does not fit, I can return the bag of ten without having opened it. (But they work, so no problem there.) $34.24.
I put the screen patch over the vent pipe, slide the clamp on, and tighten it.
We had 13 clamps. Two of the vents are too big for the clamps we have; we needed to get slightly larger ones.: 2.31 to 3.25 inches, another $5.61.
One possible problem is that we got the fiberglass screen. That should be sufficient. However, we noticed that a couple of the tops of the lead flashings have been chewed on, presumably by squirrels. This happened once before and we had to have the flashing replaced. Obviously if squirrels start chewing on the fiberglass screens, they will not last long.