Green oak staining

[Zebra]

I'm making a small porch canopy over each of my external doors, to protect the new oak doors. I've enough green/part seasoned oak for the frames, but some oak I've used externally before has stained the render. Brown stains - I guess the tannins have run down the lime render and stained it. How could I prevent this again?

 

[Keithj]

Been there (still there). We were "required" to use green oak in our repairs. Three years on we still get tannin stains (outside and in ) whenever there is heavy rain (like now).

I've been told that it should improve, gradually, but with no indication of when it will stop staining. The old oak (16th century or so) doesn't do it, so there is light at the end of the tunnel.

I don't worry too much about the tannin on the lime, because it washes off fairly easily and a coat or two of limewash covers it. I spend hours carefully rubbing it off the lead, on the advice of the roofers who said it will rot through the lead.

As for how to prevent it: if it's outdoors and in the rain, it's going to do that until all the tannin is leached out. I suppose you COULD paint it with something waterproof, but I wouldn't!

 

[Zebra]

I know what I think I'll do. It'll look ugly in the short term, but I think I'll stick a little strip of say roofing membrane, or dpc, under the porch supports where they're attached to the wall, for the tannin to leak onto. They'd be long enough to reach the ground, and I could hold them down with bricks or stones. Leave them there for a year or two - hmm, how many years did you say Keith, three? Oh well, it's not like I'll have the garden all done and pretty in three years, is it?!
Thanks Keith, your answer has me convinced that I have to do something, and not just ignore it!

 

[Me!]

Following on from Keith's comments, I was trying to find out some more details about lead and green oak together and I found this article where it specifically says -

"Green oak can cause corrosion as tannin (tannic acid) contained within the oak attacks the lead. If the lead is laid directly onto green oak a separating membrane should be used such as a Class A building paper or, in certain circumstances, a bitumen paint may be suitable, particularly under lead flashings."

but then it also says in the next sentence -
"If green oak is fixed above leadwork, tannic acid can leech onto the lead, and while in this situation the tannic acid does not harm the lead, it can cause unsightly staining and discolouration."

How can it rot the lead one way and not the other?

 

[LadyArowana]

How can it rot the lead one way and not the other?

Another of life's great mysteries, like how those insulated flasks keep hot things hot and cold things cold, how do they know? ( I expect Doug knows, about the lead, and maybe about the flasks too :lol: )

 

[Penners]

Another of life's great mysteries, like how those insulated flasks keep hot things hot and cold things cold

I can assure that this doesn't work. I filled my Thermos with some coffee and a choc ice, and they were both ruined.

 

[DJH]

I know that flasks keep things warm as well as hot and cold Lady A. :wink:

The article you refer to Me! is primarily about lead sheet used to line gutters. I know the author and was a founder member of the Lead Contractors Association and served with him on the Council and vetting Committee of the LCA back in the day!

The situations he's referring to are when lead sheet is laid over new oak (although old oak can still cause a problem) and no allowance is made for separating direct contact between the oak boards and lead sheet. While lead flashings may not come into direct contact with the sole boards of a gutter there may well be oak used in construction of the walls of the gutter so a bitumen paint will isolate the lead from these timbers. Another reason for coating the rear side of the flashings is that fumes from oak below can permeate past the upstand of the lead gutter against the parapet wall and attack the back of the flashings in the process.


Any green oak such as windows or other details over or above and not in direct contact with lead sheet could have tannin leeching causing an unsightly run off stain but it does not have such a deleterious effect as it would if trapped in damp conditions under or between oak and lead sheet.

Wherever lead is built into and used in situations where it is subject to sustained damp conditions and is in contact with Portland cement, lime, oak etc it should receive at least two coats of a hard bitumen paint with, where masonry is to be built over it, a sprinkle of sand when the top coat is just tacky to give some grip to the mortar.

Doug

 

[Flyfisher]

Wherever lead is built into and used in situations where it is subject to sustained damp conditions and is in contact with Portland cement, lime, oak etc it should receive at least two coats of a hard bitumen paint with, where masonry is to be built over it, a sprinkle of sand when the top coat is just tacky to give some grip to the mortar.

Would that apply where lead flashing is mortared in between brickwork as well?

 

[DJH]

No not really FF because it's only tucked into about an inch and a half of mortar and the mortar sets or the free lime carbonates quickly. If it's built in any deeper then it's worth doing it. You can still get a run off from mortar if it's had a shower of rain on it before it cures.
Lead flashings that cover brickwork or masonry where the lead is not in direct contact but just close with walls below, don't normally need protection although, saying that, there will be exceptions.

Doug

 

[Flyfisher]

OK, that's good. It was your comment about "subject to sustained damp conditions and is in contact with Portland cement, lime, oak etc" that made me wonder. I guess pointing that secures such leadwork is not in 'sustained damp conditions' as it will quickly dry out when any rain stops.

As for lead staining, I've found that patination oil does a pretty job of preventing that as far as plain rainfall is concerned, though I don't know if it would help protect against anything leached out of green oak.

 

[DJH]

Patination oil can only help but remember that it is only a temporary protection for lead and is formulated to degrade slowly in uv light from the sun to allow the natural silver grey patination to form on it's own. Tannin will leech out for a good few years I suspect...

Doug

 

[FamilyWiggs]

Another of life's great mysteries, like how those insulated flasks keep hot things hot and cold things cold

I can assure that this doesn't work. I filled my Thermos with some coffee and a choc ice, and they were both ruined.

Ah, as with so much in life, method is important.

Did you add coffee to choc-ice or choc-ice to coffee?

 

[Penners]

Well, howdy, stranger! Where the hell have you been? This forum is the poorer without your incisive wit and wisdom.

I've tried both methods, but after a lengthy ride in my saddlebag they both seem to end up the same.

 

[LadyArowana]

Well, howdy, stranger! Where the hell have you been? This forum is the poorer without your incisive wit and wisdom.

I've tried both methods, but after a lengthy ride in my saddlebag they both seem to end up the same.

Oh well, that's yer problem right there then. A coffee float is a beverage to be consumed immediately, and unsuitable for transport :lol:

 

[Penners]

I don't see any chocolate on that choc-ice.

I should send it back to the supplier and demand a refund.

 

[LadyArowana]

I don't see any chocolate on that choc-ice.

I should send it back to the supplier and demand a refund.

As always "Your actual product may differ slightly from that illustrated"

 

[FamilyWiggs]

Well, howdy, stranger! Where the hell have you been? This forum is the poorer without your incisive wit and wisdom.

I've tried both methods, but after a lengthy ride in my saddlebag they both seem to end up the same.

Oh well, that's yer problem right there then. A coffee float is a beverage to be consumed immediately, and unsuitable for transport :lol:

Good grief I may have been gone a while, but that photo is a coffee from a Berni Inn in the 1970s surely? Did you have the whitebait starter, surf and turf, and black forest gateaux for desert too?

 

[Penners]

Answer the question, Wiggs. Did you have a chitty from sir for your absence?

 

[Flyfisher]

Good heavens - a cola float! I remember those from my dim and distant youth. I wonder if there was any room amongst all the E numbers for anything vaguely nutritional?

 

[Penners]

Cola floats always had a slightly odd, metallic taste, but lemonade floats were fantastic! They gave me the sophisticated palate that I have today. :|