Chimney DPCs and damp handling

[MortarThePoint]

I think this is where I have got to with the design now, a couple of options:. Is 75mm the right sort of height for the flaunching?

The pot is a 600mm Red Bank Canon:

 

[epsilonGreedy]

2 hours ago, Declan52 said:

You cut in into wedges from the centre of the hole and soften it and bend it up. It won't look as tidy as that welded one but no one will ever see it. 

 

 

Any risk that a chimney sweep brush will catch the wedges and bend them down?

[epsilonGreedy]

How much were the pots, the ones I am looking at are nearly £100 and I need 5.

[MortarThePoint]

2 minutes ago, epsilonGreedy said:

 

Any risk that a chimney sweep brush will catch the wedges and bend them down?

 

The wedges are up the outside of the flue liner which passes through them and on a little higher. That means the brush doesn't get near them

[MortarThePoint]

2 minutes ago, epsilonGreedy said:

How much were the pots, the ones I am looking at are nearly £100 and I need 5.

 

£55+VAT each I think from Civils and Lintels. I need to track down the receipt.

[MortarThePoint]

Need to be careful buying the Red Bank Canon as lots of places say they have it and it turns out to be the Contemporary Canon instead. Can't remember the codes, but essential to check before travelling.

[epsilonGreedy]

3 minutes ago, MortarThePoint said:

The wedges are up the outside of the flue liner which passes through them and on a little higher. That means the brush doesn't get near them

 

 

My feeling is that a chimney has so many inherent moisture ingress risks I would prefer to isolate the flu liners as a route for moisture as per the severe weather zone regs. Did you have a look at the video? It shows the first flu liner above the tray slotting over the lead tray upstand.

 

There is a forum member in South Lincolnshire who has had problems with damp blockwork around his chimney, not sure the cause was identified. His reports here prompted me to think more seriously about chimney design. Do you note @PeterW's caution about ensuring a firm base for the tray to prevent sag?

[MortarThePoint]

4 minutes ago, epsilonGreedy said:

 

My feeling is that a chimney has so many inherent moisture ingress risks I would prefer to isolate the flu liners as a route for moisture as per the severe weather zone regs. Did you have a look at the video? It shows the first flu liner above the tray slotting over the lead tray upstand.

 

There is a forum member in South Lincolnshire who has had problems with damp blockwork around his chimney, not sure the cause was identified. His reports here prompted me to think more seriously about chimney design. Do you note @PeterW's caution about ensuring a firm base for the tray to prevent sag?

 

I did watch the video and it is tucked inside.. If you are using Isokern, it's worth talking to their technical support. They said the pumice liner is semipermeable and so lets the moisture out. They recommend the tray goes  up the outside.

 

I did see @PeterW's caution and will take measures higher up. There's a lead tray in already that bridges the cavity and I'll take a look at whether it's sagging

 

 

[farm boy]

When my Brickie did mine he marked a line round outside of flue onto the lead tray then marked another line about 25mm further in then cut the round hole and dressed the lead up with using bits of scrap timber and pliers until liner fitting through the hole. The tray was then put on the chimney with the lead up stand  dressed round the out side of the flue liner.  Mine has two trays with a pot  with flaunching  and a Brewers stainless bird cage/cap . Any small amounts of water  that enters the  inside of the flue is quickly dried out by the draught  coming up the flue. It is important which way the lead trays are folding against the chimney. Depending on your situation, the bottom tray should be folded up on the inside of the roof and down on the outside to take any water in the brickwork to the outside. The peter scholey utube videos are very helpful. My flaunching had to Be done twice as the first time it was -4 and the excessive anti freeze chemical in the mortar made it go off too quick and crack.

 

 

 

 

 

[MortarThePoint]

I've had a response from my warranty provider to an email enquiring about this:

"The DPC at the top of the chimney is an example of this, in that it would be needed to comply with the current building regs, however, it is seen as best building practice by warranty providers and serves a valuable purpose in preventing the chimney becoming saturated from the top down and in extreme weather will help to protect the chimney from excessive wetting and potential frost damage in winter.

I can confirm that slate bedded to the top of the brickwork with adequate flaunching is one acceptable method of achieving the above protection."

So looks like I could use slate and no lead. I'll be buying some slate anyway and will have to see if I can get the lead reasonably, but good to have the fallback of just slate.

[PS: nobody here this morning so phoned brickie at 8am and he says he has an 'emergency on another job' so it will be tomorrow]

[epsilonGreedy]

1 hour ago, MortarThePoint said:

So looks like I could use slate and no lead. I'll be buying some slate anyway and will have to see if I can get the lead reasonably, but good to have the fallback of just slate.

 

 

I have loads of spare slate here including a few warped 500mm x 250mm slates. Are you in Cambridgeshire? How far south?

 

Your posts have got me thinking again about chimney flashing, I might need 3 trays because with my hipped roof there is about 1.6m of chimney exposed above the line of the back gutter flashing. This could mean two trays at the upper and lower roofline heights as identified in your initial post plus another under the flaunching at the top.

[MortarThePoint]

50 minutes ago, epsilonGreedy said:

 

I have loads of spare slate here including a few warped 500mm x 250mm slates. Are you in Cambridgeshire? How far south?

 

Your posts have got me thinking again about chimney flashing, I might need 3 trays because with my hipped roof there is about 1.6m of chimney exposed above the line of the back gutter flashing. This could mean two trays at the upper and lower roofline heights as identified in your initial post plus another under the flaunching at the top.

 

I'm south of Cambridge. Thanks for the offer, the local merchant has some which is cheap and nearby.

 

I've been staring at 3 trays which has felt weird. I have two as it passes through the roof line, then 1.8m higher is the top protection. My thinking has come down on the side of using slate for the following reasons:

1. Not much water actually goes into the pot itself and a proportion of that would run down the inside of the pot and possibly be caught by a lead tray, the rest would go into the flue liner
2. The pot sits snuggly on the pumice flue liner (could leave a gap but I'd prefer it snug) and Scheidel said the Isokern would absorb the water, so in reality any water draining down the inside of the pot will get absorbed by the pumice liner
3. Spoke to Schiedel and they said it's not their default to recommend a lead tray at the top 'any more' The installation video doesn't have one.
4. Building warranty are happy with slate
5. It's easier and doesn't require fabrication
6. I expect the mortar will key better to slate than lead(?)

7.  It's cheaper (but not if you consider the cost of a failed chimney ?)

Thanks for all the advice @PeterW and @Declan52 on doing it in lead. I hope I'm not making a mistake, still open to persuasion.

Edited March 22, 2021 by MortarThePoint

[Declan52]

Just don't put too much water in the mix as it will shrink more. 

[MortarThePoint]

4 minutes ago, Declan52 said:

Just don't put too much water in the mix as it will shrink more. 

 

The flaunching in general or if using with slate?

[Declan52]

Both.

[epsilonGreedy]

3 hours ago, MortarThePoint said:

The flaunching in general or if using with slate?

 

 

I would be interested to hear if your brickie supports the underside of slate + flaunching. I also noticed the half (header) bricks in the chimney bond design, is this just to continue a Flemish type bond pattern from the walls up to the chimney pots? 

[MortarThePoint]

5 minutes ago, epsilonGreedy said:

 

I would be interested to hear if your brickie supports the underside of slate + flaunching. I also noticed the half (header) bricks in the chimney bond design, is this just to continue a Flemish type bond pattern from the walls up to the chimney pots? 

 

I'll try to share some details. Correct about the Flemish bond.

[MortarThePoint]

3 hours ago, Declan52 said:

Both.

 

Do you think 75mm high enough for the flaunching on a 1.2m wide chimney stack with one pot?

[CC45]

if you are going to fit a WBS then I would not waste money on a pummice liner - its V V likely that your HETAS installer will want a SS liner for the WBS so the pummice is OTT - I know this since I wasted some of my cash on it.  125mm liner used in the end with vermiculite around it.  I would get it done before the scaffold comes down.

 

[MortarThePoint]

1 minute ago, CC45 said:

if you are going to fit a WBS then I would not waste money on a pummice liner - its V V likely that your HETAS installer will want a SS liner for the WBS so the pummice is OTT - I know this since I wasted some of my cash on it.  125mm liner used in the end with vermiculite around it.  I would get it done before the scaffold comes down.

 

 

If you just have vermiculite around a stainless steel liner what happens if/when he stainless steel liner has to be replaced?

 

You're correct, the stove installer is saying he wants to fit a stainless steel liner.

[CC45]

it ends up being someone elses problem!

Hold a bag open at the bottom and collect it all.  Reuse.

 

We used these register plate / adaptors + cement fibre board.

http://www.stoveandchimneydirect.co.uk/index.php?route=product/category&path=88_95

 

Tidy job.

 

[MortarThePoint]

I had wondered about just using a stainless steel liner some moons back:

 

[Declan52]

1 hour ago, MortarThePoint said:

 

Do you think 75mm high enough for the flaunching on a 1.2m wide chimney stack with one pot?

When you come to do it you will be able to gauge it much better when your up there. Might need some more to make such a heavy pot secure but without being able to see it from below.

[MortarThePoint]

1 minute ago, Declan52 said:

When you come to do it you will be able to gauge it much better when your up there. Might need some more to make such a heavy pot secure but without being able to see it from below.

 

I'll be at the mercy of the brickie so will specify "at least 75mm"

[epsilonGreedy]

28 minutes ago, MortarThePoint said:

If you just have vermiculite around a stainless steel liner what happens if/when he stainless steel liner has to be replaced?

 

 

Presumably the small amount of vermiculite between an 200mm solid flu and 125mm SS liner would be poured in loose without any cement or water in the mix. Just guessing...