Most houses do not have adequate ventilation, this can lead to condensation build up in the loft and for timbers to rot damaging the structural integrity of your roof. Having sufficient ventilation in your loft will allow air to circulate, and stop the formation of condensation on the underside of the felt which can lead to premature aging of the roof. Ventilation can be achieved from replacing old non breathable felt with a breathable version. This allows moisture to escape your loft space whilst still being completely water tight from external ingress. If your homes felt is still in good shape then vented soffits and ridge can be installed to achieve the same effect. Fresh cool air enters through the soffits at the bottom of the roof and humid warm are exits through the ridge at the top of the roof.