A leak in any outbuilding roof, is when water permeates through the external felt covering then through the roof sheeting from the outside in and not any water, dampness or mildew on the internal roof surfaces which can be caused by condensation due to natural atmospheric humidity found in any unheated building in winter and extremely cold nights.

Everyone should be aware, that internal roof surface condensation can occur in "ANY UNHEATED" garden outbuilding if it's left locked up and seldom opened during very cold frosty days and nights. In winter months the outside roof surface of any unheated Shed, Garage or Outbuilding will become frozen. (It will be primarily the most Northerly facing surface, which is the coldest but it can sometimes be the entire roof).

Once the building starts to heat up during the day, the damp warm air will rise and when it hits the cold roof it causes condensation to develop on the internal roof surface.

This in turn will give you the impression that the roof is leaking (WHICH IT IS NOT).

If left unattended black marks will eventually form on the internal surface of the roof, which is mildew, caused by the damp roof surface not getting dried out.

During the winter "PLEASE" open the doors of your outbuildings as much as possible to ventilate them, to try and keep the internal temperature down (or fit ventilators).

Alternatively have a thermostatically controlled heater on overnight to keep the temperature high enough to prevent the outside roof surface from freezing.

THESE ARE THE ONLY 2 ways to prevent condensation forming on the internal roof surface of any timber garden building no matter who manufactured it.