Venting a chimney stack at roof level

[top banana]

hi guys, has anyone had a previously capped chimney stack which required venting?
I don’t want to remove bricks as they don’t line up with the flues.
I was thinking of either using a core bit to install round vents to each individual flue, or drill out perps install a weep vent.
I know the weep vent won’t have a great effect so I’m leaning towards the round vent. Just wondering what size I’d need as I don’t particularly want 4 no. 100mm vents on the side of the chimney.
Any ideas please.?

 

[catlad]

Whats wrong with a standard clay air vent?
I don't know what you mean by flies

 

[top banana]

Thanks for your reply catlad.
I’ve edited post, flies was flues!
A standard clay air vent would mean removing bricks. And the bricks don’t line up with the chambers.

 

[bobasd]

why not post pics of the flaunching and the stack?
also, why not read up on here about whats needed for redundant flues and stacks?