Hipped roof ventilation

Hi all

We have a 1950s Hipped (Traditional cut) roof bungalow configured in an L shape. So we have no gables anywhere. Currently we are upgrading the loft insulation of a loft conversion that was carried out in the 1980s - We are doing other bits including relocating loft access stairs (Spiral to a proper staircase) and hence Building control are involved.

BCO has asked for a 50mm ventilation gap between the underside of roofing felt and the insulation as we are insulating at rafter level. My understanding is the aim is to have airflow from the eaves vents up through each rafter 'bay' then down the other side of the roof or out of a ventilated ridge.

With this aim I am struggling to see how I can maintain this crossflow air path with the hipped roof construction. We have Hip rafters at each corner, with Jack rafters leading from the eaves up to rest on these Hip rafters. So these effectively are 'dead ends' and airflow would never reach the ridge. Then when it comes to a rafter that does reach from the eaves to the ridge this airflow is blocked by the timber trimmers of Velux windows. So only a handful of the rafter bays will have a true airflow path.

What's the most common way to overcome this issue? I'm not sure drilling holes all along the ends of the rafters does much good to structural strength!

The existing roof membrane is a breathable type, as the bungalow had a new replacement slate roof only 10 years ago. So I'm not overly keen to be stripping any of It off if I can help it.

Any Ideas?

Many thanks in advance