Many red brick houses were originally built using lime mortar and the need to continue this trend is essential to the health of a property as older houses move and need to breath.
If cement mortar is used in repairs, the movement inherent in older properties results in damage to brickwork. It is always difficult to repair a property where cement mortar has been trowelled over existing lime mortar as the cement mortar is rock hard and can stick to the sides of the bricks making it often impossible to rake out cleanly and without damage.
When cement mortar is used in the repair process it also waterproofs joints making it difficult for moisture to escape. It then becomes trapped and the result is damp and rotten bricks. The use of cement mortar in modern builds is not problematic as the structure is built to accommodate it.
One can always tell a good lime mortar finish as – although it may be trowelled in – it will have been brush finished. This is in direct contrast to the way some contractors attempt to trowel finished joints – its consistency simply does not lend itself to this. When lime mortar is correctly brush finished, the aggregates are brought to the surface; this always looks better than trowelling which pushes the aggregates into the mortar.
Freezing temperatures or not conducive to brickwork repairs, but neither are baking hot summers. In either case, the newly pointed brickwork needs to be protected with hessian if temperatures are extreme.
Advice: there are unscrupulous contracters who will pretend they are using lime mortar when in fact they are using white cement. This enables them to complete the work quicker whilst saving money on materials. Buyers beware.
Some companies say they undertake lime mortar repointing but use powered guns. However, many would argue that a standard lime mortar mix has too many large pieces of aggregate for it to easily pass through the end of one of these nozzles. In order for this to happen the mix is often bastardised to include cement, hydrated lime, and a smidgen of hydraulic lime. This is added to soft sand (which has much smaller aggregates) and it is this which enables the ease of application which unfortunately results in houses not gaining the full benefit that a pure natural hydraulic lime mortar has to offer.
In addition, the mortar mix is soft which means it can then be finished with a trowel which often results in a property looking like it has had its joints filled with a creamy cement mortar.