Lime - Frequently asked questions

Hydraulic limes are made from limestone that contain clay and silicates. They offer faster setting times than non-hydraulic limes and are also more durable, making them suitable for external use including damp or exposed conditions.

The lower strength and greater permeability of natural hydraulic lime mortars compared to cement-based mortars makes them suitable for repairs to older buildings, helping to accommodate some of movement within the building, without placing additional stress on the brick or stone itself.

The more hydraulic a lime, the faster it sets and the higher it's final strength, but the less breathable and flexible it is. NHL5 is the most hydraulic, then NHL3.5, and NHL2 the least hydraulic lime.

Mortars made with natural hydraulic lime gain strength by a combination of hydraulic setting in the presence of water and carbonation from absorbing carbon dioxide.

The table below shows the ratio of hydraulic lime to sand for common lime mortars used for typical applications:

Mortar Mix Proportions
NHL 3.5: Sand	Typical Application
1.1	- Walls below DPC - Chimneys - Earth retaining walls
1.2	- External walls - Copings and cappings - Parapets and sills
1.3	- Facing to solid construction
1.4	- Internal Walls

Mortar Mix Proportions
NHL 5: Sand	Typical Application
1.1	- Submerged masonry
1.2	- Walls below DPC - Chimneys - Earth retaining walls
1.3	- Copings and cappings - Parapets and sills
1:3-4	- Walls above DPC

The table below shows the ratio of hydraulic lime to sand for common lime renders:

Render Mix Proportions NHL 3.5: Sand (By Volume)
Substrate	Basecost	Finish Coat
Weak or porous (soft brick)	1:2	1:2.5
Medium strength	1:2.5	1:2.5
Impervious or dense brick	1:2.5	1:2.5
Plasterwork	1:2	1:3

Render Mix Proportions NHL 5: Sand (By Volume)
Substrate	Basecost	Finish Coat
Weak or porous (soft brick)	-	-
Medium strength	1:3	1:3
Impervious or dense brick	1:2.5	1:3
Plasterwork	-	-

Note: As a general rule, each successive coat should be weaker and/or thinner as you move away from the substrate

Lime putty will hold up to three times its volume of clean well-graded sand. No further addition of water is required to make a mortar. We recommend 21/2 to 3 parts by volume of sand for mortar production. Through mixing for up to 20 minutes will ensure a workable product is achieved.

For repointing: Remove all mortar to a depth of 1 inch, ensure joints are dampened down, re-mix the mortar to ensure workability. Pack joints thoroughly, shape whilst it is in the green state. Ensure mortar dries slowly usually over 2 to 3 days. Dampen down if necessary with hot weather to prevent shrinkage.

Lime plaster undercoat/scratch coat can be made from 1 part lime putty to 2.5 parts grit sand. Before use, hair can be added for extra strength at an addition rate of 0.5kg per 100 litres of plaster.

Lime plaster float coat is made from 1 part lime putty to 2.5 parts well-graded sand. Hair can be added to the rate of 0.5kg per 100 litres of plaster.

Lime plaster Finishing coat is made by mixing 1 part lime putty with 1 part fine silica sand for a fine internal finish. For an external finish 1 part lime putty to 2 parts good quality building sand is suitable.

For full details on how to use lime plasters made with lime putty, visit the Lime Putty page.