Mortar mix for chimney

[liam546]

I own a Victorian property which was built around 1900 I believe. The brickwork has a lime mortar. I am looking to repoint the chimney stack and rebed the pots but would like advice on the best mix to use.

I recently bricked up a back door and used

X1 hydraulic 3.5 lime
X2 sharp sand
X1 building sand

I didn't use any cement as from what I understand the brickwork needs to breathe and cement isn't used in the original mortor. I was going to use the same for the chimney stack and to re bed the pots but wondered as it's the stack and not the exterior wall as such whether I should use a cement mix for additional strength given the exposure of the chimney. Or perhaps just a sand and lime mix again but a stronger lime such as 5 rather than 3.5.

Any advice would be much appreciated?

Untitled

• liam546
• 25 Sep 2013
• 1

 

[stuart45]

Use a NHL 5 for an exposed stack. You can use cement on the flaunching.

 

[liam546]

So you wouldn't use any cement in mix?

Just x3 sand and x1 5.0 hydraulic lime for re bedding chimney pots and re pointing stack

 

[stuart45]

You can for bedding the pot, but lime is probably better for the pointing.

 

[liam546]

Ok - well I'll go with a sand/hydraulic lime mix then.

X2 sharpe
X1 building sand
X1 lime.

Just one last question. I have a NHL 3.5 bag left over and in finding my local suppliers don't stock NHL 5.0. Do you think NHL will be ok for bedding pots and pointing chimney?

 

[liam546]

I also have a spare bag of white rugby cement and hydrated lime. But I want to make sure I get it right.

 

[stuart45]

Use the NHL for the pointing, but add cement for the pots. Years ago they used to put tallow in the lime for the pot flaunching. The pots need to be watertight, whereas the pointing needs to breathe.

 

[liam546]

Thank you Stuart. I've read that when doing the crown I should use cement instead of a mortar. What do you think?

Also I've read its advisable to create a drip edge off the crown so that the drips don't run down the chimney brick faces. I'm comfortable on the mortar to use and the pointing. But have just read a few conflicting views on what to aye fir the crown. i.e a mortar or cement and also whether and how to incorporate a drip edge.

 

[stuart45]

A crown is normally used in the USA, and overhangs the stack. To form one you need formwork round the stack. In the UK the pots are normally just flaunched in with the mortar flush with the stack.
Use cement mortar here.

 

[liam546]

Now you say it Stuart, all the videos I have watched regarding crowns overhanging and drip edges have been from the USA. They seem a good idea. How come they construct these on the states but not here? Is it because they have more extreme weather changes?

Also the US seem to use cement but as you have suggested and as I've read a few times on UK sites a cement mortar is better? Again any reason in the differences?

 

[stuart45]

Weather is more severe in places, and also here the tradition is to flaunch,
Concrete is used for the crown as it is stronger, but for flaunching cement mortar is OK.

 

[liam546]

Thank you Stuart. I'll go with that then. Thank you so much for your expert advice. It's greatly appreciated.

I'll be repointing stack, re bedding the 4 pots and putting some mesh over the pots to stop birds and leaves. Is there anything else I need to be aware of?

I plan to literally remove old flaunching, brush down and re bed pots in about 3 inch of mortar before tapering off to edge.

 

[stuart45]

Sounds like you have the job worked out.

 

[liam546]

Thanks Stuart, would you recommend adding some fibres to the mix to prevent cracking of the flaunching over time? Or wire mesh?

 

[catlad]

I have started adding fibres to the mix you will also need some bits of broken slates to sit the pots on while your flaunching it, I would also not recommend doing it without scaffolding.