Low pitch roof ventilation method?

[Chris S]

Hello, I would appreciate some suggestions with regards to providing suitable ventilation to a portion of low pitched roof. I've looked at various solutions but not found one that will work yet.

 

We're in the design phase and I'm currently creating the drawings for building control approval. 3D view and the relevant sheet of the draft drawing is attached. We have PP for the design.

 

Aim:

• To meet BC requirements in particular for ventilation. Hertfordshire BC list the following document on their website:
  https://www.hertfordshirebc.co.uk/pdf/13_-Technical_Leaflet_-__Permeable_Underlays_-_March_2011.pdf

 

Options considered:

• Air open LR vapour permeable underlay only. Herts BC accept this only if it is "used in strict accordance with the conditions of installation and use stated on the respective certificates." The certs don't state a minimum pitch, but list a minimum overlap table for pitches down to 12.5 deg only. Hence I would not be using it in accordance with the cert. Has anyone had approval to use these products at 11deg or less before? This would be my preferred method.
   
• Standard LR underlay plus ridge ventilation (equiv. 5mm continuous). The mansard edge (which is ~ half of the total ridge length) gives little room for a solution here. I looked at spacers beneath the flashing but I could not find anything suitable for the pitch (presumably because of wind driven water ingress). Would it be acceptable to just vent the center portion (i.e. the wall abutting portion)? The tiles are single pantiles (Crest BST H14) so it would be great if a vent could be installed in each valley but I can't find a product that does that. Does anyone know of such a product?
   
• Standard LR underlay plus eaves ventilation (equiv. 3mm / 7mm continuous for well-sealed or normal ceiling). This would require ridge trays/roll to get around the insulation. Since the pitch is so low, this would mean most of the pitch would be covered with the plastic trays/roll, which would appear to defeat the whole object. Another option may be to insert vent duct e.g. 100x54mm channels every so often to create a path between the eaves and the open volume - but I can't find a product actually designed to do this. Does anyone know of such products?

 

Other challenges:

• The low pitch (11 deg). This counts out many products as not suitable.
• We want to keep the roof profile as low as possible both for the neighbour's sake and to avoid the roof window being too high thus reducing the light input.

As a secondary request, I also have a gap between the bottom row of tiles and the fascia board. I can't allow the facia board any higher else there will be no slope for the eaves tray. Are there any fillers available? I could not find any. I realise it will need a bird/rodent guard.

 

Many thanks in advance, and a happy new year!

Building application drawing 31-12-18 sheet 12 a.PDF

[Mr Punter]

Why not continue the flat single ply roof out?  You could accommodate the change of pitch and keep it as a proper warm roof.  That is a very shallow pitch for tiles.

[Chris S]

 

3 hours ago, Mr Punter said:

Why not continue the flat single ply roof out?  You could accommodate the change of pitch and keep it as a proper warm roof.  That is a very shallow pitch for tiles.

 

Thank you. Do you mean make it fully flat or extend the flat so the pitched part is steeper?

 

We wanted the pitched portion for aesthetic reasons and we have PP for it. Extending the flat roof a bit further is an option but would mean reworking the rafter design (which we have already paid the SE for).

 

We'd like to keep the shape as is if possible. It seems like this is the last hurdle!

 

[Russell griffiths]

Have you found a tile to do this pitch ?

im at 10 degrees and using a raised seam metal roof. 

Im not sure I know of many pitched roof products that will go as low as that, 

from that 3d drawing I think grey metal to match the flat area would look good. 

[Chris S]

On 01/01/2019 at 10:57, Russell griffiths said:

Have you found a tile to do this pitch ?

im at 10 degrees and using a raised seam metal roof. 

Im not sure I know of many pitched roof products that will go as low as that, 

from that 3d drawing I think grey metal to match the flat area would look good. 

We've found a tile thanks. Crest BST H14 does the job nicely. PP is for tiles to match existing roof.

 

I've now realised that we can use a series of rafter trays (i.e. to fill 1 gap, not a roll) every 3rd rafter to achieve equivalent of 7mm continuous ventilation.

 

Thanks for the support.