Lean to flashing ventilation

woody1

have a client that is repairing a lean to flashing, the space under the lean to is a dormer, the underside of the rafters are the ceiling,
when it was built , breathable roofing membrane was used so the builder / client were under the impression that they didnt need through ventilation in the roof

there are air vents at eaves level, but nothing going out again

i know vent slates are the obvious answer, but their ugly, and expensive, and a lot of them would be required ..

we used to detail a vented lean to flashing in college, making an upstand against the wall with battens and plywood and then the cover flashing came over.. im just wondering has anyone come across a manufactured version of the same thing, cover flashing is on the skimpy side , so it would need to be narrow..

could core vents into the walls in the attic space between the dormer wall and the eaves, would give some air movement but not really through movement

sorry the insulation is that multi foil stuff, so the full depth of the rafter is free


open to any other ideas..

DOCARCH

This type of thing?

http://www.cavitytrays.com/ventilation/view/2/89/1/type-rav-fl-roof-abutment-ventilators

or

https://www.marleyeternit.co.uk/Roofing/Fittings-And-Accessories/Dry-Fix-and-Ventilation/Top-abutment-ventilation-system.aspx

woody1

yeah i saw both of those, they involve taking off the cover flashing and re-plastering so probably a bit too involved..

i had resigned myself to having to put in a lot of slate vents.. but i think ive figured out a better answer to the problem..

basically it involves putting in a small flat ceiling inside in the 2 rooms involved maybe 600mm in width across the length of the 2 rooms, and taking off the plasterboard and foil above this, this creates a duct that makes air available to all the rafters, from here i can put in a wall vent in the gable or a single slate vent to provide air movement ..

theres nothing major to it.. light enough ceiling members as theres no span.. maybe 2 sheets of plasterboard and a small bit of insulation..

2 of the rooms involved are a wardrobe and a hot press . so its no major loss to anyone .. no head height being lost .. the other room is a bedroom.. and whilst i dont really want to go breaking plasterboard and making a mess in that room its also the room im most concerned about in terms of air flow

will have a chat with the builder in the morning, sometimes they have simpler solutions

BryanF

How are you insulating ?
Where is the vapour barrier?

woody1

existing insulation is multi foil .. ( yes i know, but thats whats there and it cant be changed without serious work..)

how am i insulating the little flat section im proposing to build.. rigid insulation would seem to be the simplest solution, in terms of buildability..

vapour barrier between insulation and plasterboard,

its possible that when we look into this roof that there isnt a problem at all, but there should be ventilation and effectively there isnt .. what im trying to come up with is the most effective fix possible without costing an absolute fortune or having to cover the roof with vents.. as it stands there are 30 400mm bays in the roof that arent getting through ventilated.. and id really rather not put 30 vents in.. aside from cost .. 30 quid each ..plus installation.. its on the front of the house and would look shocking..

maybe if theres no sign of any problem at all we might just put in one every second or third bay .. but that seems like a bit of an " ah sure itl be grand " solution.. which is what got us here in the first place ..