Refelting Garden Shed Roof - Advice please

[Magnet]

I've bought some 36Kg top-layer felt for my 5m x 3m shed/workshop (lowish angle apex running lengthways).
The current felt is OK, just worn at each end, I guess where the birds use it as a take-off area! There are no leaks yet but I want to do the job before that happens. The current felt is not rippled particularly, and has been very well nailed every couple of inches along each length, may even be glued too (not sure)
I'm tempted to put the new felt on top of the old. I would not then be worried about the weather forecast and not have the risk of damage to the (glued shavings)l roof ripping all those nails out.
Just a bit concerned about laying the new felt on the gritted old felt. Will this prevent it laying flat enough, or will it be Ok to do this? Any comments appreciated. Thanks

 

[noseall]

as long as the substrate is flat then the new felt will sit flat.

the only thing is, you will need to offset the new felt lap from the old felt lap, as doubling up the seam will be bulky and bad practice.

this may mean cutting the first layer of felt down its length giving you a split piece with which to start each side.

 

[Magnet]

Thanks for your response, Noseall. By substrate I think you refer to the existing felt which is "nobbly" (due to the grit in the felt) so not flat as with a sheet of board, but I think will be OK. I will use battens as well.

A very good point about staggering the overlap which I shall take into account. Having one overlap on top of another would probably weaken the board with such a concentration of nails in close proximity to each other.