If it is an oldish house (which I guess because it has been reroofed) then it is probably ventilated from the eaves. Have a look at the insulation and see if it has been laid so it flops into the eaves. If so, cut it back with a big pair of scissors (like wallpapering scissors) or an old breadknife. You only need the insulation to cover the plaster ceiling and flop onto the edge of the wall. Wear a dustmask.
If the eaves are closed with a wooden soffit, you can drill holes in it, but you will need quite a few. You might find it easier to take a brick or two out of each end, or hire a core drill and make a 105mm hole at each end of the loft, fitting a grille to keep birds out. You need holes at each end/each side so that you get a through current of air.
On the underside of the felt it will be condensation.
If you are sure you have a good through=flow of ventilation (check that it has not been blocked by flopping insulation) then you need to look at where the humidity is coming from.
Some possibilities:
- Hot water in the loft tanks due to pumping-over or other plumbing fault
- No cover on water tanks in loft
- Steam rising from bathroom due to not using an extractor fan during and after baths and showers
- Excessive humidity in house due to leaving wet washing about the house or
draping it over radiators
- Bathroom ceiling cracked or gappy allowing steam to rise into loft (especially around holes for light fittings, pipe and switches)
- Incorrectly fitted bathroom ventilator or extractor venting into loft
- Badly fitting loft-hatch allowing excessive amounts of warm moist air to rise through gaps.
- Use of "condenser" tumble drier. Most of these allow a lot of water vapour to escape
- Use of vented tumble drier with no hose going outside; or faulty hose
- Use of paraffin heaters or gas fires that do not have a flue
Water vapour is lighter than air, so it rises through the house even more than warm air does. This is why clouds are above us.
