Parapet Air Bricks

[omph]

I have a flat roof extension with a cavity wall. The guttering is currently extending into my neightbour's garden and he wants to build an extension and so we have decided to extend the height of the wall to build a parapet and move the drainage into the roof.

The section that will be parapeted has a gap spanning the entire length of the fascia (10m wall) which allows for air circulation. By building a parapet this hole will be sealed off completely and will not allow for air circulation. I am left wondering how I can resolve this. My current line of thinking is using airbricks every 1.5m on the final course of bricks, but I am not sure if this will provide enough circulation and I am also concerned that rainwater may get in.

Please advise.

 

[robinbanks]

which allows for air circulation

Circulation in what?

 

[omph]

Circulation in what?

Circulation for the flat roof. Image attached.

 

[freddiemercurystwin]

Telescopic air bricks might work but in theory you'd need one in between each joist/firring wouldn't you (stopping the rain entering aside for a moment)? What happens at the other side of the roof?

 

[omph]

Telescopic air bricks might work but in theory you'd need one in between each joist/firring wouldn't you (stopping the rain entering aside for a moment)? What happens at the other side of the roof?

The other side is sealed off. 1st side is against the property wall, 2nd side is a parapet with no airflow, the 3rd side has the fascia running across it with the 20mm opening, and the 4th side joins up with a pitched roof. There does seem to be circulation across the roof when I was doing repair work on the ceiling awhile ago.
If it were to be done "properly" how would I handle the other sides ? (for any future projects).

Also, will I need anything else for the parapet ? i,e, any form of water collection and drainage
The current parapat design will have a DPC bedded on top with the coping on top of that, but I am not sure that'll be enough considering the wall doesn't have any air bricks at the bottom due to it being a party wall.

 

[^woody^]

Airbricks on a party or parapet wall would breach the requirement to prevent fire spread.

You'd need abutment ventilation, venting above the flashing.

 

[omph]

Airbricks on a party or parapet wall would breach the requirement to prevent fire spread.

You'd need abutment ventilation, venting above the flashing.

Good point. Do I need to use a kit, or can I use lead flashing over the abutment vent ?

 

[^woody^]

You can form abutment ventilation yourself in timber and an upstand out of the roof decking board and fit proprietary plastic vents, or fit mesh and flash over.

I think there used to be a drawing in the roofing forum, else google should have something.

 

[omph]

With the proprietary vents, do they need to be a minimum height from the roof floor to avoid splashes of water getting into the vent and causing damp issues ?

 

[^woody^]

Yes, generally the flashing upstand would be 150mm, and the venting just above that.

Here's the principle

 

[omph]

Thanks @^woody^ - Do you know if i will a dpc on a single parapet wall - initially we were going with a cavity wall, but the joists have made access to the second wall impossible, so we have now opted for a single engineering brick wall with coping.

 

[robinbanks]

Fill the void with insulation instead

 

[^woody^]

Thanks @^woody^ - Do you know if i will a dpc on a single parapet wall - initially we were going with a cavity wall, but the joists have made access to the second wall impossible, so we have now opted for a single engineering brick wall with coping.

All parapets require a DPC, and you'll need to be careful with such a narrow parapet as all walls will tend to move on a DPC to some extent, and a half brick wall will have limited mass to resist this movement.

 

[omph]

All parapets require a DPC, and you'll need to be careful with such a narrow parapet as all walls will tend to move on a DPC to some extent, and a half brick wall will have limited mass to resist this movement.

Interesting point. What if the parapet is only 3 bricks high and it will be covered in EPDM on our side and felt on the neighbours side. I will be mechanically fixing some stone copings to it. which means little water ingress.
I could put the DPC under the copings but the fixings would pierce through.

 

[^woody^]

225mm high is effectively part of the roof so could potentially be covered with the roof covering as you describe. You don't need a DPC if the parapet is covered.

In this case, the issue would be how the EPDM and felt is adhered to the brick parapet, and whether any normal timber roof movement is accomodated by the covering secured to the masonry. It may well be better to form the parapet in timber.