The right temperature for repointing?

[michael]

I need to do some lime mortar brickwork pointing in my loft. It’s a smallish area and so I intend using a sand and hydrated lime mix (powder) which I have to hand rather than sand and lime putty.
I understand that lime pointing shouldn’t be attempted when the temperature falls below 5 centigrade. Dose this mean during application when the mix is moist or over the whole period of carbonation. The temperatures here at the moment are staying at 5 and above during the day but falling to 2 or 3 during the night.
The underlying problem here is that I need to replace a rotten lintel over an internal door. The brickwork above this door (up in the loft) really needs repointing to add strength and stability before propping and removing the lintel.

 

[Flyfisher]

I suspect there is no definitive answer and it'll be one of those things where experienced lime users have learned what works and doesn't work. I can't claim to be experienced enough with cold-weather lime work to know, although my understanding is that the 5 degreeC 'rule' is more of a rule-of-thumb than a hard scientific rule. Keep above and you'll almost certainly be OK, drop below and you might be OK but it's less certain. I realise none of that is of much help, I'm just filling in time until someone more knowledgeable comes along.

What I really wanted to say was that if the work really can't wait, you could consider using a fan-heater on a low thermostat setting to keep the attic space above 5C overnight, preferably in conjunction with covering the work with damp hessian to prevent it drying out too quickly. Due care and attention to fire risks must, of course, be carefully thought through.

 

[michael]

I suppose I could just keep the loft hatch open overnight so that the ‘warmish’ air in the rooms below circulates into the loft but I’ll no doubt be met by a flood of condensation water in the morning which I would like to avoid.

 

[chuckey]

I think that 5 degrees makes sure that there is no freezing of the excess moisture content before it evaporates. How about using a switched on lead light/ office lamp to keep the area warm or rather frost free?
Frank

 

[middi]

All the suggestions on other posts are good.

If the temp is kept at absolute minimums ( say 3 to 5 deg C ) the lime will take a long time to carbonate and set the mixture due to it's continued high moisture content retention.
If moisture is still present in mortar that has had no chance to evaporate, because of lower temperatures, and frost occurs then the mixture will become " powdery" and weak.

The time limit is variable with prevailing weather conditions.
From few days in summer months conditions to as much as few weeks in cold damp conditions.

Middi

 

[michael]

Thanks for your responses.
I guess that I will have to avoid freezing temperatures ‘0 degrees and below’ where the water in the mortar would freeze but you also mention frost which can occur above 0 degrees.
I wonder if it’s possible for frost to get into a loft? The reason I ask is that my outdoor pot plants can be protected from frost with a simple fleece or plastic covering and my loft is covered with slate and roofing felt. Should I only be worrying about freezing temperatures in this situation?

 

[middi]

I would refrain from doing any plasterwork unless I could guaranty a frost free environment.

The responsability of this is entirely within the applicators power. ie;- yours.

Once lime plaster ( or pointing) has been done it would be preferable to keep temp above 5 deg. C. for speedier and guaranteed results

Middi

 

[Flyfisher]

I think there are two things to consider in the cold;

1. The obvious risk of freezing, causing physical disruption of the mortar

2. The less definable slowing down of the carbonation reaction by which the mortar actually 'sets'.

I would guess (and I am guessing!) that slowing down the carbonation is not, in itself, a fundamental problem - as long as the structure remains stable during the prolonged setting time of course - but that the water in the mortar mix will still continue to evaporate, even at very low temperatures, so that when the temperature rises again and carbonation can progress, the mortar mix may have dried out to such an extent as to disrupt the carbonation reaction.

In other words, the mortar may look set because it has dried out but it will not have had a chance to develop its usual strength. This may or may not be a problem, depending on the application.

I have experienced lime mortar failing even when the work was done in warm weather, and I have assumed this was because I did not protect the work from drying out too quickly. My very amateur view is that there seems to be a good case for wrapping new lime mortar in hessian no matter what the weather. This should keep the mortar sufficiently damp to facilitate good carbonation.