Gas pipe in roof void above a cavity wall

Can some fellow 'competent person' or CORGI member please tell me the reasoning behind the regulations that do not permit installation of a gas pipe in a cavity wall?

Is it in case someone drives a nail or drill through the pipe and then gas would leak into the cavity?

The reason I ask this question is that I have found a convenient run to my new gas boiler between the eaves in the roof space ABOVE the cavity of a wall.

Note that this is not IN the wall, itself but the void is immediately above the cavity. There is adequate ventilation and in fact insulation material will be fitted between the pipe and the cavity.

Is it ok to install the pipework in this space or should the space be sealed from the cavity in some way?

The alternative route is to bring the pipe into the next rafter gap but this would mean an extra 90 bend in a 22mm pipe.

What is wrong with an extra bend in 22mm pipe

This will only add 0.5m equivalent length to the pipe run

If that is a problem, I would suggest that the 22mm is too small already.

Tsk Tsk, .3m for bends .5 for elbows.

Thanks for that Ray I assumed he meant an elbow

'Salright Dave, I had to look in the book to make sure before I posted anyway.

Thanks for your help guys but I am really interested in the reasoning behind the regulations. The route above the cavity is neater because the pipe from the boiler can run straight up the wall next to the boiler and enter the roof void between the rafters. The pressure drop is not an issue as the pipe run from the meter is to be in 28mm up to the last 2 meters before the 24kW combi boiler. You are right though, I meant an elbow, not a bend. I guess in practice there must be lots of situations where pipe runs are close to the top of cavity walls in single storey extentions etc.