Counter Battens?

[Allex]

I understand these follow the line of the rafters and are nailed on top of the underfelt to allow any rain run-off getting under the tiles. Are they really necessary if I allow a bit of slack in the underfelt under the tile battens? Thanks.

PS I've got to fit a Velux window and am also wondering if counter battens are necessary for one of these.

[Crofter]

You need counter battens if you have sarking, but a tented membrane draped across the rafters doesn't.

[cwr]

Velux windows are designed to work with or without counter battens.