There should be no openings underground for rats to get into. If you mean can you fit mesh over a opening in a manhole, the answer is no. If you have an open ended pipe underground there is something wrong with your sytem. The only place a soil pipe sistem should ventilate to open air is 900mm above the highest window.
You accuse the forumof having no experts to solve your rat problem and state this should be an easy problem for a builder. It is not and as correctly stated above there are at least 100 ways a determined rat can get into a building if it wants to including, in our own experience, chewing through a live electric cable to get access to the void it filled. The only answer but one you choose to ignore, is a pest control officer who will eliminate the present rats and any future infestations. There is absolutely no such thing as a rat proof house and they must have chosen your house for a reason so give the pest control officer a chance to find out why.
Thanks for your reply, i think my question wasnt clear though as you havent answered it, so i try again
{The question is not about manhole covers (i said underground pipes), the easy question to answer is not the rat problem but the specific question eg see next paragrapgh where i try to clarify, pest controllers know nothing about building i dont think as have tried several}
Question again in more detail:
If it helps to clarify, the open-ended pipe is under a manhole cover like a normal drain 1/2 metre down - there are 2 pipes merging
into 1 there, and sorry if it wasnt clear, my question is can i fit any kind of flap or wire mesh over these 2 merging pipes?
(the rats are going are in the drains, the person who built the extension moved the drains and apperas to have forgotten to block the old ones which are under the extension and must have opened up a hole into caity walls.)
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