Damp patches after heavy rain on roof rafters and membrane. Rafters root?

[rob_em]

Hi,

After heavy rain Im noticing in my loft areas where damp patches show between membrane and rafter. Im a bit ocd with this issue as would like to ask is it much to worry about the structure of rafters will they rot ? it only happens afer heavy rain and only in one specific area corner of the house. North corner.
I've already checked tiles above and did not find anything. Im suspecting 2 things. First is overlap and pitch which is small. But on the other side of house it's same and it's fine... The other thing I think is that the membrane is touching the bottom of the tile somehow and water is getting on it.

 

[foxhole]

buildhub.org.uk

This shows how plain tiles are doubled at gutter edge to prevent the problem you have , your tiles are also short of the gutter .

 

[catlad]

Looks under-pitched to me.

 

[^woody^]

Many of those old council system-built houses tend to have a very low or flat area at the eaves, which can have some water ponding.

That membrane should be resisting penetration regardless, so it is either creeping sideways under the lap or it's via a nail hole.
Sometimes a worn or badly scuffed membrane surface or collection of debris can cause localised internal dampness too.

Some damp that then evaporates away is not a big issue, but when water ponds and the timber stays damp for extended periods, then the risk of rot is greater

 

[rob_em]

Thank you woody

 

[datarebal]

Flat pitch , short head lap