Feasible to retrofit an "over-fascia" vent?

[lineweight]

I'm going to replace a rotten timber eaves board & ropey guttering with new. Will probably replace with a PVC eaves board, and I'd like to improve the ventilation at the same time.

 

The eaves board is tight against the brickwork - no soffit to speak of, so soffit vents aren't an option.

 

Can anyone tell me if it works to retrofit an over-fascia vent, eg this sort of thing

 

https://www.roofingmegastore.co.uk/manthorpe-25mm-over-fascia-vent-37-x-46-x-1000mm-black-pack-of-20.html

 

- or is that not really possible without removing the bottom row of slates? I am thinking that I would attached it to the top of the eaves board before installing the eaves board - then push it up under the slates and fix.

 

[Mr Punter]

I don't see why you can't.  Is there a clear path for the airflow?

[Oz07]

No problem with doing as you say. Be a bit fiddly and you need your fascia to be thick enough to get a fixing ontop. Also clear path as Mr punter says. My place has treated timber sub fascia with over fascia vents on these. The capping fascia slightly blocks the airflow but it's a full hipped roof with ridge vents so like a cyclone in the loft. 

[ProDave]

Yes, but I would use this type of vent https://www.roofingmegastore.co.uk/corovent-1m-low-profile-overfascia-vent-with-10mm-air-gap.html

[lineweight]

33 minutes ago, Mr Punter said:

I don't see why you can't.  Is there a clear path for the airflow?

Yes there is. The only thing slightly nonstandard is that it's a mansard roof so the roof pitch is pretty steep where it meets the eaves.

[lineweight]

4 minutes ago, ProDave said:

Yes, but I would use this type of vent https://www.roofingmegastore.co.uk/corovent-1m-low-profile-overfascia-vent-with-10mm-air-gap.html

Any particular reason you'd use that rather than the one in my OP?

[ProDave]

7 minutes ago, lineweight said:

Any particular reason you'd use that rather than the one in my OP?

Yes.  In my particular case on top of the facia board was a strip of (I think it was called) Easy Verge or similar, a plastic strip to divert any run off from the felt down into the gutter rather than just drip down the front of the fascia.  Using the one you linked with the vents pointing downwards would have just about blocked them in my plastic strip.

 

But if it works in your case no reason not to use them.

 

Here is an example of one of my eaves showing the straight through vent works but the bottom outlet one would not.

 

[lineweight]

Hm, in that scenario I guess I would have thought the plastic strip would want to sit over the ventilator, rather than being under it, but maybe I am misunderstanding.

[ProDave]

15 minutes ago, lineweight said:

Hm, in that scenario I guess I would have thought the plastic strip would want to sit over the ventilator, rather than being under it, but maybe I am misunderstanding.

It's a warm roof, so insulation above rafters, then breathable felt, then counter battens and battens.  All the vents are ventilating is the gap between the tiles and the membrane.  If the plastic went above the vent then the membrane would also have to go above it and the vent would be venting nothing.

 

 

[lineweight]

3 hours ago, ProDave said:

It's a warm roof, so insulation above rafters, then breathable felt, then counter battens and battens.  All the vents are ventilating is the gap between the tiles and the membrane.  If the plastic went above the vent then the membrane would also have to go above it and the vent would be venting nothing.

 

 

Ah, I see. In the photo it looks like the membrane is sitting on top of the vent strip (ie the vent leads to space under the membrane) but I think I se now it's actually not and it's just a trick of the camera angle.