Chimney render / lead questions

[Zebra]

I have just rendered the chimney up to the part where it narrows, thus:

It was cement rendered up to the same level before, and the brickwork underneath was too spalled to leave uncovered, so I've rerendered in NHL2.
There are different substrates under the render, and it now appears patchy when wet.

On the far left there is a piece of woodwool board covering a void, whereas everywhere else is the brickwork of the chimney itself. There is a damp patch which appears after rain. The scratch marks on the scratch coat of render are even appearing through, after four coats of limewash. What if anything should I do to prevent these wet patches appearing?
The second question is about leadwork: I made a piece of lead to cover the join between the flat roof and the chimney, and left the ends long until the rendering was complete. They overlap thus:

How should I cut the ends now to look neat? Nothing too fancy - it's not that sort of house.
Thanks!

 

[tobydog]

Zebra - I'm no expert with lime render but I have used it and try to ensure that I understand the pro's and con's of how to use it. Looking at your case here, was NHL 2 really the correct strength to use on an exposed chimney stack? NHL 2 is much weaker than NHL 3.5 and therefore (I believe) more porous. Won't NHL 2 simply act as a sponge in this situation? NHL3.5 or 5 may have been a better choice.

 

[Zebra]

Ah, now you surprise me. I had thought I might get told off for not using putty. I used NHL 3.5 for the pointing on the chimney, which I believe was the right choice. Indeed, I could have used it for the rendering also. However, elsewhere on the house I have putty based render, as the bricks are very soft, and I'm sure that in a moment, someone else will say I should have used putty here.
I should emphasise, it's not the entire chimney stack that's been rendered, just the bottom bit.

 

[malcolm]

NHL 2.5 sounds fine to me. Is the render wet because of rain from outside or water evaporating through from the chimney? Seems odd a vertical chimney wall would be wetter than a vertical house wall. Ahh - just rendered I notice - that's why it is damp!

I've been lime washing all day and have noticed the first coat from a couple of days ago still goes completely transparent when wet (it is white when dry). I hope this isn't a normal thing for lime wash and will happen less with more coats and some carbonisation? How long has yours had to dry?

 

[Zebra]

I finished the rendering around 10 days ago, and then gave it a coat of limewash a day for four days, or thereabouts. Hmmm, yes, I suppose it could still be the bricks underneath drying through the render...... but these bricks are so quick-drying, I have to keep soaking them every half-hour when rendering, or they dry out. Surely they're dry by now. Unless the render is slowing their drying out. Which could account for why it's particularly the bottom part which is wet - the top dries out first. Hmmm...... yes, thank you malcolm. Time will tell in that case......

 

[malcolm]

It's a good point - if the bricks were cement rendered they will be soaked inside and could take a year to completely dry. The white looks like it could be salts coming through which would mean the render and limewash are working. Could be something to put off worrying about until next year.

I would go a straight line on the lead, sloping to approximately match the roof slope, and meeting the other bit of lead low enough so water dripping along the line falls down the front of it. But your other details look good so you might come up with something better.

 

[DJH]

Hmm...pity you cut the lead Zebra it should have been reasonably easy to fold it around the corner. Anyway it might be best if you lift the top flap and cut the two lower pieces vertically and then fold the top flap in under the lower ones. This will help prevent wind lift. I'd then round off the corner of the lower piece.

How do you use a mastic gun with hooves...

Doug

 

[Zebra]

Ah Doug, I was hoping you'd pop up soon. Fold it? I never even thought of that. Oh well, if my efforts in lead are a little amateurish ( like most other stuff I've done here ), then at least by God its better than what was there before - in my opinion anyway.
So, I think I know what you mean. I might just post a little sketch tomorrow to be sure I've understood. Thanks,
Deborah

 

[plasticpigeon]

Zeborah?

 

[DJH]

I wanted to do a little sketch Zebra but that's beyond my computer skills....

 

[Zebra]

Mine too, actually. This however, I can do:
What do you think?

 

[DJH]

Yep...that's it. The top piece is bent in under the other two pieces?

 

[Zebra]

Yes. Thanks.

 

[a twig]

Thanks,
Deborah

This part of this post has lead to me doing an entire days work on the roof with Pulp - Year 2000 stuck in my head, which certainly didn't help things when the flashing wasn't going to plan and frustration was kicking in!

 

[formerly known as]

I think that maybe that that brownish staining is from sulphates in the stack. You can't do anything about it - the lime is acting as a poultice and drawing it out.

 

[Zebra]

OK. That's good to know. I suppose I can keep freshening it up with more Iimewash but it will keep coming back. That's OK. I can live with that.

 

[formerly known as]

just beware that the sulphate will weaken the render a wee bit. nothing you can do once the internal parging has gone and the bricks are taking the brunt. I have an internal breast that does it - worry not.

 

[King of the Monkeys]

I have to agree with a previous post that impurities will be drawn from the flue through the chimney into the render. How ever unfortunately the top coat of render appears to be ( sorry !) too thin as the scratch of the previous coat is showing through.

 

[Zebra]

Yes, you're right King of the Monkeys, the top coat of render is a little thin just there, unfortunately. It isn't all like that, it's just a bit uneven underneath. Oh well, I suppose I could just try applying many many coats of limewash, see if that improves it.
Out of interest, will these sulphates continue to leach out even though the chimney is now lined? It's over 40 years, I think, since an open fire was last lit in it (with the exception of a small one-off experiment of mine), as I have a woodburner with a lined flue, and the last lady had a gas fire.

Thanks

 

[formerly known as]

for the sulphates to show on the exterior of the brick, the brick through its section will be contaminated. In my experience, this will continue, no matter how much is drawn out. It *might* be the case that lining of the flue will concentrate the exit to the exterior of the stack. Don't panic - you can do nothing about it.