Oak and lead

[Keithj]

Seeing DJH's post on the leaded lights thread reminded me...

The green oak in our repairs and garden room is gently leaching out tannin, which is drying on the lead flashing underneath. Somewnere, I came across a warning that this will chew its way through the lead and cause all manner of problems. I've therefore been cleaning off the tannin (using a damp cloth with no other chemicals) on a regular - six monthly or so - basis. Then I wipe over the cleaned and shiny area with patination oil, of which I have a litre can. The lead is a consistent dull grey.

Am I doing the right thing, or should I leave the tannin for longer/not as long? I can't detect any obvious pitting or thinning of the lead, but it's early days.

What (if anything) should I do about the tannin that's run into the cracks where the lead is pushed into the oak?

 

[DJH]

A common fault Keith. Wherever lead sheet comes into near contact with oak whether it's over oak boarding or lead joists below softwood boarding, a separating or isolating layer should be inserted between the lead and the substrate to protect the lead from corrosion. This layer can be a bituminous or 'damp stop' type building paper or two coats of a hard setting bituminous paint applied on the underside of the lead.

Any attack on the lead from the oak is witnessed by a dark brown stain leaching out onto the surface of the lead. I'd say that what you are doing Keith, in the circumstances, is the best thing so keep it up. If you could manage to slip some building paper in between the lead and the oak so much the better.

It's a mistake to think that patination oil protects lead from attack by other substances. It's designed to be applied to lead sheet or flashings at the end of the day, or before rain showers can cause unsightly lead carbonate to form on the surface, which is what happens to newly fixed lead after rain falls on it and before it's had chance to form its own patina. The patination oil is supposed to degrade over time allowing the lead to achieve its own patina at the same time as its adjoining sheets so they all appear the same colour.

An application of patination oil should always be applied to the underside of the leading edge of the lead so that any water drawn up under the edge (whether by suction or because of wind holding up the water running off the lead allowing it to be drawn up by capillary attraction) does not turn to lead carbonate. If this is not done then white stains will eventually appear down over the surface of the adjoining roof as lead carbonate is washed down over it. It is particularly noticeable on slate roofs and can be difficult to remove.

Doug

I've added quite a bit to this post in a subsequent edit beause I forgot it the first time...

 

[Keithj]

Many thanks, Doug.

When it stops raining, I'll go and have a closer look!

 

[DJH]

I've just rummaged through my pile of photos of jobs done and things made to find an example of under sheet lead corrosion and found these pictures of the North Aisle of what was my local church in Somerset. These clearly illustrate the power of oak 'fume' on lead.

The job I had was to remove the failed and leaking lead sheet covering, carry out repairs to the substrate along with associated works to the merlons. The roof construction consisted of oak joists/beams with a stained board ceiling under and softwood boarding over to support the lead sheet. It was obvious from the start that the lead sheet had failed because of the splits that were evident across the sheets and that water was leaking through into the church below.

Here's what we found when the lead and boarding was removed....







You can see the little piles of white lead carbonate on top of the oak joists/beams corresponding to the gaps in the boarding above where the 'fume' from the oak has corroded the lead...

I should have saved it I suppose and made up some lead paint. :wink:

Doug

 

[DJH]

I should have added to the last post that the situation was exacerbated by a few other things one being that the roof, being on the North side of a hitch clerestory suffered from a shadow line from all the way along its length which meant in hot sunshine ( remember?) a shower of rain instantly reduced the temperature of the hot consequently subjecting it to all sorts of stresses from expansion and contraction when half of the sheet was cold in the shadow and the other half turned from hot too cold in an instant.

All this happened over a 'cold' roof' which wasn't too bad until a benefactor paid for a glass screen to be erected across the entrance to the church with knock on effect of the ventilation inside being reduce to virtually nothing and the church attaining much higher temperatures because the heating worked more efficiently. Add to that a popular vicar and some lusty singing producing much moisture to settle under a cold roof and you can see the result...

Doug

 

[Keithj]

Wow!

We're in the process of fixing all our church roofs (and more) but there's very little lead up there, and what there is is "belt and braces" over slate/timber joins where there is also Tyvek underneath to keep the wet out.

I will be paying a lot more attention to the lead round my house!

 

[88v8]

....the situation was exacerbated by a few other things one being that the roof .... suffered from a shadow line

I never thought of that in the context of lead.

Had a problem with the garage roof in our previous house, where the back of the front parapet, and the side of the side parapet, were in the sun. The double-asphalt roof came away from the parapet at the upstand junction, and the front parapet cracked, due to thermal movement, while the back parapet and the upstand onto the house - both in shadow - were fine. Repairs repeatedly cracked.
The solution was to train Hydrangea Petiolaris over the front parapet, and ivy along the side parapet, they kept the sun off and repairs were no longer necessary. The roof was 29 years old, and leak-free, when we sold. The new owner was muttering abut 'removing that ivy' so I expect he's enjoying his new garage roof by now.

Growing ivy over the church roof has much to commend it, providing someone gets up and clears the gutters now & then

Ibor

 

[Keithj]

Growing ivy over the church roof has much to commend it, providing someone gets up and clears the gutters now & then

It's when it sneaks under the tiles that the problems really start! We've removed it all, and keep it that way.

 

[DJH]

Following on from those horror pictures of under sheet corrosion on the North Aisle about 6 years later I won the contract to re-roof the South Aisle of the same church. This roof, like the other one each bay was covered in one continuous sheet of lead from the top abutment against the clerestory down to the parapet gutter. It was decided that both roofs would be divided into two bays with a drip (step) dividing them. This had the effect of cancelling out the 'shadow line' problem and solved the problem.

The drip (step) was formed by the use of firring pieces on the highest bay placed on top of the existing joists which ran fro 2" to nothing. This helped maintain the original flashing height against the clerestory and window cills. A pragmatic view was taken about the slightly different angle of fall between the lower and higher bays as the benefits far outweighed any visual objections and the roof can't be seen from the ground.

This is the West end of the Aisle nearing completion. Although I had a temporary roof covering the works I took the decision to repair and relay the lead in stages leaving the old lead in place until I needed to remove it. This meant there was very little risk of water getting into the church below. I started form the East end of the roof so that the overcloaks would be against the prevailing weather from the South West.





One of the two rainwater heads taking water from the parapet gutter of the new roof behind. MM Mickey Mouse?? :roll:



All good fun...

Doug

 

[Flyfisher]

Impressive work there, as always, Doug.

I suppose it's heresy to even think it, but given that the leadwork would not be visible from the ground and given it's allure to metal-stealing low-lifes, was an alternative solution ever considered for this repair work?

 

[DJH]

There's always thought given to that sort of problem FF and although scrap prices are always attractive to 'low lifes' in 2000 they hadn't reached anywhere near today's prices. The church is in a prominent position in the town and the roof is very high. Alternative materials such as copper, s/s steel, zinc and aluminium etc tend to be very noisy during downpours and could even drown out the lusty singing below...

As it happens they did strip the lead from the Vestry roof which I did in 1995. :twisted: :evil: I took this personally as, if I say it myself, it was a tour-de-force of leadwork design overcoming many obstacles to good design. Grrr...

Oh well. Mange tout...

Doug

 

[Flyfisher]

I was actually thinking about a non-metal roof as, presumably, any metal could be a target for theft.

Is something like bitumen or fibreglass never considered for such areas of flat(ish) roof?

 

[DJH]

I think low value roof coverings are being used more and more but there are often historical reasons for replacing like-for-like especially if the building has availed of an English Heritage grant in the past when from there on the integrity of the building must be maintained. Another reason is of course that there isn't the longevity in non-ferrous roof coverings against metal roofs (unless they've been nicked of course!).

A properly installed lead roof should last the life of he building or at least a hundred years.

Doug

 

[Flyfisher]

I quite agree that alternatives would spoil the integrity of the building and it's a great shame that such things even have to be considered these days (or perhaps metal theft has always been a problem?), but I'm a bit surprised that there are no modern alternatives with the longevity of lead.

On the other hand, it's reassuring to know that even my own clumsy leadwork should outlive me!

 

[vicky whitworth]

Glad to hear the new hip is coming along, Doug. Thanks for asking about my leg. It's doing OK. But it's taking ages. I still can't run on it but I can walk 5-7 miles so I can't complain.

Where's the Somerset church? Illminster? Martock? It's got a hammy look about that stone? Vicky

 

[DJH]

Yes it's the Minster Vicky.I was lucky enough to win several contracts to work on it over the years.

5-7 miles?? I can't drive that far without being out of breath...

Doug

 

[Keithj]

Fabulous photos!

Our roof here (at the house, not the church) has some lead work like that. Beautifully done, by a Suffolk craftsman. Neighbourhood watch is pretty good round here.

One of the other churches in the benefice had the lead nicked off the porch roof twice in three months - and that's in a residential street with neighbours all round! It now has a sheet plastic roof. The DAC wasn't delighted, but the insurers and the PCC weren't prepared to stump up for a third lot of lead.

 

[DJH]

but I'm a bit surprised that there are no modern alternatives with the longevity of lead.

There are quite a few modern alternatives to metal covered roofs but I suppose none of them have been around quite long enough to lay claim to longevity yet. I worked on an historic building many years back where the decision was taken (I wasn't involved) to use a single skin plastic/fabric (Troka or something like that?) to line the parapet gutters as it was not possible to alter the falls of the gutter to allow for the required hight of drips (steps) if lead was used. I can't say whether it was a success or not.

Many large fbuildings both old and new are covered with similar materials, a lot of DIY sheds included and they are designed and fitted to resemble metal roofs with false longitudinal joints.

Time will tell I suppose but it's not all the world if the roof fails on a shed but it can be disastrous if it does over an historic building with ancient finishes below...

Doug

I suppose I'd have a hard job nowadays selling the lead garden ornament I used to make.

 

[Flyfisher]

but I'm a bit surprised that there are no modern alternatives with the longevity of lead.

There are quite a few modern alternatives to metal covered roofs but I suppose none of them have been around quite long enough to lay claim to longevity yet.

Yes that's a fair point . . .

I suppose I'd have a hard job nowadays selling the lead garden ornament I used to make.

. . . and that's a sad one.

 

[DJH]

Yes I suppose it is FF but even sadder is the amount of lead statuary and ornament such as vases, planters and miscellaneous pieces that are stolen, melted down and sold on as scrap where actually the original piece is more valuable as it is but not so easily sold on I suppose.

I used to be able to say that most things I made would actually increase in value overtime.

Here's some examples of a mixture of decorative architectural and garden ornament which demonstrates my point...







Doug