Odd discolouration on underside of floorboards?

[dbrb2]

What do people make of these floorboards?
The discolouration was on the underside only, so not visible from the room unless the boards are up, and at the ends of the boards only

My initial thought was "soaking wet" - but they seem dry as a bone even in the obviously discoloured areas, and still feel completely solid. But something has caused it....

Some kind of oil perhaps...? The issue is seen at both the internal and external wall ends of the board - they were stained some time in the past, so perhaps stain runoff with added snail trails....?
(that big white lump isn't some exotic fungus - it's expanding foam I hadn't yet trimmed off)

 

[Kearn]

We have a situation that puzzled for a while, which might be similar…. Original principal rafters in our roof and some early commons all had a darker appearance at their lower ends at the eaves. During my first survey I assumed moisture related staining or similar. The second survey there was the rare correct use for a moisture meter! Turns out these sections were slightly drier than the naked timber. We believe it to be some form of painted on moisture ‘protection’ or similar… unnecessary, but hey ho. However it’s impregnated and stained the timber.

Yours has a sheen to it which we don’t, but if it’s at the ends, I could guess someone was trying to prevent against moisture from damp walls tracking into the boards. Whether they were trying to deal with an actual problem would be of interest to me…

 

[dbrb2]

Thanks!
Hopefully you're right. As I say, I checked the wood and it felt as solid as anything. So fingers crossed!

 

[CliffordPope]

It looks to me they were painted with creosote or similar years ago. There was no point in painting the whole beam, just the vulnerable bit at the end.

 

[guitarust007]

Bitumen perhaps?

 

[dbrb2]

I think that is most likely - both because it sounds plausible, and because it's the answer I want to be correct

 

[guitarust007]

I think bitumen was used as rudimentary water proofing back in the day so I suspect as the above posters say, it was used at the ends where the beam was most likely susceptible to damp