The steam then condenses on the colder roof.
Normal for inside of roof to look wet.
Roof leaks can occur when the flashing around a vent pipe or chimney separates from the roof and creates a gap.
If you have ever noticed a lot of icicles hanging from the edge of your roof chances are your house is leaking significant and expensive amounts of air and heat into the attic.
Patching material may not adhere properly if the roof area is damp.
High ceiling beams and cathedral ceilings with water spots can be confused with an active leak when in fact it is a result of condensation.
If you ever find damp patches on walls after heavy rains or see wet spots on your ceilings after rain storms leaks are likely the culprit.
Cracked masonry and failing flashing around windows and doors are other potential problem areas.
Water can sneak through roof penetrations like skylights vents and chimneys causing wet walls.
Either of these can result in a slow but steady stream of steam from the open vent pipe above the header tank.
Faults in the roof structure including damaged or missing tiles poor flashing and broken or cracked gutters and drainpipes can be the cause of dampness in the house.
Stains or even wet areas on the under side of roof decking and on rafters can appear to be a roof leak but in fact moisture may be entering the attic not from above outside and through the roof but by rising through a building suffering from leaks a prior fire extinguishment or most common a wet basement or crawl space.
Newer roofs typically have an ice and water barrier installed along the eaves where the dams typically form to prevent the water from leaking inside.
Best way to check see if it dries out during dry periods and get wet again onunderside of the sheathing in rains.
If you have a standard central heating system with header tank etc.
There are ice dam cables you can install on.