Chimney DPCs and damp handling

[epsilonGreedy]

6 hours ago, MortarThePoint said:

The perspective of the photo makes the pot look massively off centre, but it is in the centre.

 

 

A good BuildHub forum thread on chimneys has been missing. Thank you for all these details.

 

Did you start with 600mm tall pots?

 

I ask because the original architect diagrams for my house have tall Dickensian pots. Think I will need  700mm pots more more to achieve the overall profile shown below.

 

 

[epsilonGreedy]

21 minutes ago, MortarThePoint said:

So that's £4k per chimney before even connecting a stove to it. Probably looking at £5-6k all finished.

 

 

Your chimneys look very high, I guess that was needed because of the steep roof pitch (50 degrees?) and the need to get the pot top just over ridge height?

 

Was it necessary to call your scaffolder back to create an extra platform part way up the chimney?

[MortarThePoint]

2 hours ago, epsilonGreedy said:

Did you start with 600mm tall pots?

 

No the pots go on last at the top ?

 

Just kidding. The architect's drawing included a 450mm projection above flaunching. With 225mm of embedment, that left us either going with 600mm or 750mm as there are no 675mm pots of our type. In hindsight, I think we could have had just 150mm of embedment, but I'm happy with how it looks.

 

2 hours ago, epsilonGreedy said:

I ask because the original architect diagrams for my house have tall Dickensian pots. Think I will need  700mm pots more more to achieve the overall profile shown below.

 

 

 

Your design looks nice and I think suites the taller pot It may be a function of the aspect ratio of the chimney itself above the roof line.

[MortarThePoint]

2 hours ago, epsilonGreedy said:

 

Your chimneys look very high, I guess that was needed because of the steep roof pitch (50 degrees?) and the need to get the pot top just over ridge height?

 

Was it necessary to call your scaffolder back to create an extra platform part way up the chimney?

 

Roof pitch is 48 degrees and your correct that pushes up the chimney. That said, the edge of flue liner to weathered surface horizontally is 2.7m, so well beyond the 2.3m minimum. That suggests we could have had it about half a metre lower. I guess it's just one of those things that when I looked at the architect's drawings it didn't occur to me to change it so it must have looked right. There may be some architect's wisdom I am lacking.

 

One scaffold visit for the chimneys. The brickies didn't go up as far as I had hoped before the lift so they had to 'nose bleed' it to start with on the scaffold laying ~450mm below the boards.

Edited March 27, 2021 by MortarThePoint

[epsilonGreedy]

4 minutes ago, MortarThePoint said:

Roof pitch is 48 degrees and your correct that pushes up the chimney. That said, the edge of flue liner to weathered surface horizontally is 2.7m, so well beyond the 2.3m minimum. That suggests we could have had it about half a metre lower. I guess it's just one of those things that when I looked at the architect's drawings it didn't occur to me to change it so it must have looked right. There may be some architect's wisdom I am lacking.

 

 

My comment about the chimneys being high was only from an engineering and construction perspective. When I discussed chimney height with my architectural technician who produced the building control diagrams he cautioned against lowering the height from an aesthetic point of view.

 

They engineering bricks look good, have you had a chance to view the effect from a distance?

[MortarThePoint]

Yes I think they work well. They provide a bit of interest and I think they do accentuate the corbelling.

[MortarThePoint]

If you haven't chosen your flue liner yet I'd suggest considering a 200mm one rather than the 175mm we went with. It doesn't cost more (liner sections tad bit more, but less granular fill around it) and allows more flexibility as to what is connected to it (e.g. could change to an open fire down the line).

[MortarThePoint]

Well stacks are pretty much finished now, 4 courses left on the smaller one. I think we have followed good practice in terms of chimney damp handling, but I like belt and braces so was wondering about applying StormDry since the scaffold is there and it will never be easier.

 

For the main stack the area is 30 courses x 4.6m round + 1.2m2 flaunch = 11.6m2

Smaller stack ~4m2

 

It's not cheap stuff running at about £4/m2 (5m2/l and £20/l) but feels worth it the £60 to me. 

 

Has anyone used StormDry and has any comments? Does it affect the appearance of the masonry? Looks a bit strange when the rest of the wall is wet below, but that's just showing it's doing its job.

 

Here are some interesting likes:

• General StormDry promo video: 
• StormDry vapour test: 
• StormDry watering can test: 
• StormDry flood test: not really relevant but interesting: 

[Mr Punter]

23 minutes ago, MortarThePoint said:

Has anyone used StormDry

 

Yes.  I used some on a freestanding wall that was prone to getting saturated.  It has been very efffective.   I also intend to use it on a new build South facing brick flank wall on first and second floors.

[MortarThePoint]

Other than the cost and minimal effort involved I can't see a downside

[MortarThePoint]

Has anyone got a recommendation for something to put on the flaunching to help waterproof that. There are a couple of spots where water stands, so I'd rather get  sealed. I don't know if the likes of StormDry can cope with standing water.

[Brickie]

On 31/03/2021 at 17:52, MortarThePoint said:

Other than the cost and minimal effort involved I can't see a downside

I don’t have a scientific background,but a product which claims to inhibit the ingress of moisture in one direction whilst simultaneously allowing egress in the other direction seems too good to be true. 

[MortarThePoint]

44 minutes ago, Brickie said:

I don’t have a scientific background,but a product which claims to inhibit the ingress of moisture in one direction whilst simultaneously allowing egress in the other direction seems too good to be true. 

 

It will be highly hydrophobic (like car wax makes beads of water on the surface) and so will stop liquid phase water from passing through. Capillary action is all about the contact angle of water with the sides of the capillary. If greater than 90 degrees the surface tension of the water can't pull it in to the capillary. If less than 90 degrees, the surface tension pulls the water further in.

 

In the gas phase (i.e. water vapour) the water isn't interacting with the surface or relying on capillary action. I expect this is how a Gortex coat works as well.

[MortarThePoint]

4 hours ago, Brickie said:

I don’t have a scientific background,but a product which claims to inhibit the ingress of moisture in one direction whilst simultaneously allowing egress in the other direction seems too good to be true. 

 

I expect the hydrophobic properties would stop water wicking (capillary action) out of the brickwork wherever it pools, but hopefully there are weeps there to deep with that.. 

[Paul_c]

On 21/03/2021 at 20:17, MortarThePoint said:

 

I don't understand, the upstand is about 100mm high

 

How does the next flue liner sit on this (how would it bond and not leak smoke) when you still have more sections to go up?

[MortarThePoint]

13 minutes ago, Paul_c said:

How does the next flue liner sit on this (how would it bond and not leak smoke) when you still have more sections to go up?

 

For mine, the flue liner just passed through the hole unabated (i.e. the lead all sits outside the liner). In severe weather areas it is supposed to come inside the liner though.

[Gary009]

Is the whole lead dpc not a weak point? Like a plastic dpc in a garden wall? 

[MortarThePoint]

I guess, depends how well the mortar sticks to the tray. The weakness is in tension or shear I suppose. It's how it's typically done though.