Roof cowl for bathroom extract

Hi folks,
I am planning a bathroom refurb and want to put in a new bathroom extractor fan.
Due to a number of constraints (mainly due to the location of the bathroom relative to external walls, etc - the joys of an old house), the only way I can do this is to run a 100mm/4 inch pipe vertically for about 8-900mm, then turn the pipe through 30 degrees and it will then run through about 1.8-2m to an outlet fairly close to the ridge of a pitched roof. I have attached a rough sketch (not to scale!).
I intend using rigid pipework to minimise resistance and a Vent-Axia fan with a 220m3/hr extraction rate. My query is regarding the roof cowl - what type of roof cowl should I use for this? I'm conscious that a lot of the more discreet ones might not have the capacity to work with this fan.
Can anybody advise? Note I will be getting professionals to do the work, I'm just wanting to make sure it will work in my own mind and that I don't end up with a fan which isn't able to properly extract the room because the capacity of the roof cowl isn't enough.
Thanks in advance.

Why can't the duct go vertically or to the right and a nearer part of the roof?

Hi woody, thanks for the reply. Directly above (up to ridge tile) is an asbestos roof (overall roof project - about 5 years time) and "to the right" is a valley followed by an inaccessible roof over bedroom. Its a period house with a valley in the roof. In addition, I want to be able to access the whole duct in case any future maintenance is needed.
It is all I can do until I replace entire roof.

I don't think a discrete tile vent or a roof terminal will make much difference to the extract performance. The duct will be relatively short and as long as that single bend (or any other near the terminal) is not tight, then the main factor will be just the fan extract performance - for which bear in mind that a centrifugal or inline fan performs better than an axial fan.

If you are worried, you could increase the duct to 125mm, and even the terminal too - and the fan for that matter, and either of those will be adequate for a typical bathroom. Siting of the fan/outlet within the room and a suitable amount of air input into the bathroom will also affect performance generally.

A rigid duct will need to be properly and firmly fixed along its length, insulated, and could be noisy. A lightweight flexible duct may be better and wont affect extract performance as long as its cut to size and fitted properly with no excess sags or bends.

Hi Woody,
The fan I am proposing to use (Vent Axia ACM100T) can't be ran at full capacity with a number of off the shelf roof cowl products because they are too resistive to the air flow.
The vent pipework is perfectly fine, the only concern I have is how to do the roof cowl - I am thinking along the lines of a goose neck vent or the Ubbink pipe (which is a bit unsightly
But I don't know which to do or which would be better.
The only 2 things I care about are that the extractor fan works to its full capacity (wife will kill me if the bathroom has any mould/etc!) and that the roof doesn't leak etc.
Edit: Apologies didn't realise I couldn't add URL's - images attached for proposed options.

The route you show will be prone to condensation running down to the fan, so include a condensation trap and pipe it to some convenient drain.

JohnD said:

The route you show will be prone to condensation running down to the fan, so include a condensation trap and pipe it to some convenient drain.

Click to expand...

Thanks John - yes I have already planned for a condensation trap just "north" of the new fan.

Whether the fan needs to run at full capacity and whether the room needs the fan to run at full capacity are different things.
And despite any fan's capacity being correct on paper, that does not mean that the room will be free from excess humidity and localised mould.

Siting and use (of the fan and the bathroom) are more important. So it may well be that you could achieve better results with a constant trickle with boost type fan as fitted to new homes.

That second cowl looks like it belongs on a chip shop roof

Hi Woody,
Yeah it will be a bit of trial and error - the good thing is, the fan will be located in the attic so can be replaced if there is a problem.
Yeah, the second cowl isn't pretty but it won't be visible to anybody so once it works, I don't care what it looks like!

Just bear in mind the flow rate will significantly reduce along all that pipe work.