Lead Pumping

Lead pumping was a phenomenon well known in Victorian times but little appreciated today. Many Victorian homes had lead box and/or valley gutters. The houses were not as warm as homes would be today but none the less were significantly warmer than outside, the air inside them could carry more moisture in the form of water vapour than the cooler air could outside could, the air was by no means humid but when it came into contact with cool surfaces condensation readily occurred, typically on windows.

There is an unfortunate area where condensation could also take place and this was on the underside of the lead sheets that formed horizontal box or valley gutters, these could get extremely cold and would gather condensation to such an extent that the wooden lining boards underneath this leadwork often became saturated and then started to drip onto the ceilings below making it appear that the roof was leaking. Actually neither the roof nor the valley was leaking but the cold lead was pumping moisture from the house and condensing it rather like the collector inside a dehumidifier does, the difference being that a dehumidifier is designed to capture the condensation and safely store it whereas the valley gutter lining is designed to keep water out from above but has no way of dealing from water that gets under it. The pumping is similar in nature to a siphon and is happening far more than we realise, cold surfaces literally suck moisture from the air In the house and this happens without any air movement and it is not prevented by the presence of the ceiling which is “vapour open”.

Almost every valley gutter that I have ever been involved in repairing showed signs of rot in its boards, more surprising was that this evidence was also present even when the linings were not leaking. This showed that the underside of the lining was getting damp by some other mechanism, presumably from condensation. In general this is not too much of a problem for real wooden boards that are well ventilated but as insulation levels increase, not only will there be less ventilation to carry away the moisture but the valleys will be colder and more often cold than previously as less heat is escaping from the building to help keep them warm.

I think that we will see an increase in the number and severity of problems being caused by “lead pumping” and this will not be restricted to lead linings but also to the underside of other materials used to line or cover roofs and valley gutters.

Tony Cowling — December 2018

Posted By: fostertomIt describes what happens whenever there's interstitial condensation happening - water vapour 'disappears' from the air as it condenses to liquid; this leaves a partial vapour vacuum, which draws more of that particular vapour even without any bulk air movement; and so on ad infinitum - a one-way pump.

Indeed, it's how heat pipes work. I suppose the flock or felt underlayer will do a similar job to the wick in a heat pipe - namely transport the liquid water away.

Perhaps it has slight insulative value, which kills the extreme radiant-cold chill of a metal surface facing a clear night sky. Perhaps it disperses the condensation so it doesn't form drops and runs. Perhaps it presents a larger wet surface for re-evaporation.

ie between the lead and the boards!!!

I have heard it called this before but a good long time ago

Very happy to rename it if you know of a name for it

thx