Just had my chimney rebuilt with lime mortar by a local builder and replaced the pots myself yesterday. today a lot of the the mortar used to flaunch the pots in is cracking any ideas as to why?used a three sand to one lime mix also any ways of stopping it cracking or am i going to have to start again?
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lime mortar
- Thread starter k99mn
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stuart45
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- Somerset, England
Have you built the pots into the stack, or are you using the flaunching alone to hold them ?
The reason why he mortar has cracked is because it has dryed too fast and the fast shrinkage had made it crak.
This is a charachterisc of lime mortar especially when it is applied quite thick in dry warm weather.
To avoid the above the mix must be shaded ( and kept damp if possible), as the shrinkage " cracks" start to appear the mortar must be "trowelled" and more ( thinner skin) similar mortar to be used to close the gaps ( cracks).
This may have to be repeated a couple of times untill most of the moisture has evaporated from the mortar.
This is why it is quite common to apply thick mortar in stages in order to eliminate as much as possible the initial cracking problem.
You can certainly apply another layer of mortar, as long as you keep an eye on it, apply the mortar late afternoon or early evening as the evaporation rate is not as fast during darkness hours.
Damp hession, or old blankets are good to slow down the evaporation process.
Needless to say that the best time to use lime mortar is during those cool, foggy, autumn days when the temp is about 8-10 deg.
Middi
This is a charachterisc of lime mortar especially when it is applied quite thick in dry warm weather.
To avoid the above the mix must be shaded ( and kept damp if possible), as the shrinkage " cracks" start to appear the mortar must be "trowelled" and more ( thinner skin) similar mortar to be used to close the gaps ( cracks).
This may have to be repeated a couple of times untill most of the moisture has evaporated from the mortar.
This is why it is quite common to apply thick mortar in stages in order to eliminate as much as possible the initial cracking problem.
You can certainly apply another layer of mortar, as long as you keep an eye on it, apply the mortar late afternoon or early evening as the evaporation rate is not as fast during darkness hours.
Damp hession, or old blankets are good to slow down the evaporation process.
Needless to say that the best time to use lime mortar is during those cool, foggy, autumn days when the temp is about 8-10 deg.
Middi
Lime shrinks as it dries. if it has been fitted the way cement is normally used to flaunch a chimney then you will have a large thickness of lime which will crack as it shrinks. To stop the cracking requires repeated compression of the lime during its drying period combined with frequency dampening to slow the drying rate.
However...
Flaunching needs to be strong so as to be able to support wind loading on your pot and you can also consider it to be a roof element and hence wants to be waterproof. I would therefore suggest that lime isn't the best material to use and that this is one position where cement should be used. I would be interested in other peoples views on this though it is a bit late for ours which was flaunched with cement yesterday. All the rest of the repair work was done with NHL though.
However...
Flaunching needs to be strong so as to be able to support wind loading on your pot and you can also consider it to be a roof element and hence wants to be waterproof. I would therefore suggest that lime isn't the best material to use and that this is one position where cement should be used. I would be interested in other peoples views on this though it is a bit late for ours which was flaunched with cement yesterday. All the rest of the repair work was done with NHL though.
stuart45
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- 738
- Location
- Somerset, England
Many builders use the flaunching to hold the pot in place, meaning that it has to be quite thick. The correct method is to build about 25% of the pot into the stack. The flaunching should be a weatherproofing and not structural. With a 3 ft pot it should sit on the course before the 2 oversailing courses, and then one more on top.
NHBC guidelines for the mix are 3/1/.25 sand/OPC/lime.
I would agree with MdB that a cement mortar is the best solution here, as being weatherproof is the main idea of the flaunching.
NHBC guidelines for the mix are 3/1/.25 sand/OPC/lime.
I would agree with MdB that a cement mortar is the best solution here, as being weatherproof is the main idea of the flaunching.
Whilst it is quite correct that by using cement the mix would be much stronger, and to a certain extent waterproof, if this cement capping is used on soft red bricks the bricks immediately below the pot will suffer considerably in frosty weather leading to spalled bricks.
To overcome any possible instability the pot should be " sunk" an extra brick depth as opposed to a cement application to set the pot on soft red bricks.
Middi
To overcome any possible instability the pot should be " sunk" an extra brick depth as opposed to a cement application to set the pot on soft red bricks.
Middi
many thanks for all the advice certainly answers the question, i am now slightly concerned however because my pots aren't built into the stack...however they aren't overly tall at 24 inches and i dont really fancy taking the stack apart again to build them in also with reference to the weatherproofing i read the instructions on a can of thompsons weatherseal which states that it is micro porous so consequently lets the material breathe would this be a good alternative to put on afterwards?
I wouldn't worry too much about your pot falling off, As long as you have at least 2-3 inches of mortar all around the bottom it should stay put for a good few years. It is quite a heavy thing, even if it is just resting on the bricks with no mortar around the bottom it will need a hell of a lot of wind to move it.
As to the " microporous" statement, unfortunately microporous in not good enough as far as lime mortar, lime render or soft red brick is the subject to be applied on, these building materials require free-flowing air movement to do the job they were designed for
Middi
As to the " microporous" statement, unfortunately microporous in not good enough as far as lime mortar, lime render or soft red brick is the subject to be applied on, these building materials require free-flowing air movement to do the job they were designed for
Middi
thanks for putting my mind at rest, i had a feeling the microporous thing wasn't any good but good to have it confirmed,re the cracking in my original post it was suggested that i had put too much mortar on at once and should have put several thinner layers, with this in mind would it be a solution to now add another layer of mortar on top of my original, bearing in mind it has now been done for three days,would the new stuff still adhere to the older stuff?
stuart45
Member
- Messages
- 738
- Location
- Somerset, England
As you have held it with the flaunching you might as well leave now, as probably half of the pots in the country are put on incorrectly.
However it is something to bear in mind for the future if you ever have a stack re-built. It's easy enough to do the job properly.
Being a bricklayer for over 40 years I have had to replace a number of pots over the years due mainly to them leaning over at a dangerous angle. Even a slight lean will tend to crack the flaunching allowing water penetration. I wouldn't underestimate the power of the wind, especially in exposed areas. It won't usually bring them down overnight, but the damage is usually done over a period of time. You only have to look at the bowing of cavity walls where the ties have failed and negative wind pressure has moved them out over time.
In some countries an expansion gap is left between the pot and the flaunching to allow for movement and filled with mastic.
However it is something to bear in mind for the future if you ever have a stack re-built. It's easy enough to do the job properly.
Being a bricklayer for over 40 years I have had to replace a number of pots over the years due mainly to them leaning over at a dangerous angle. Even a slight lean will tend to crack the flaunching allowing water penetration. I wouldn't underestimate the power of the wind, especially in exposed areas. It won't usually bring them down overnight, but the damage is usually done over a period of time. You only have to look at the bowing of cavity walls where the ties have failed and negative wind pressure has moved them out over time.
In some countries an expansion gap is left between the pot and the flaunching to allow for movement and filled with mastic.