The short answer
NHL stands for Natural Hydraulic Lime — a breathable, traditional building lime that sets partly by reacting with water and partly by absorbing carbon dioxide from the air. It comes in three strengths: NHL 2, NHL 3.5 and NHL 5. The higher the number, the harder and faster-setting the lime. The whole skill of a good lime job is picking the softest grade that will still survive where it is used — because on historic sandstone, softer is almost always safer.
NHL 2, 3.5 and 5 are strength grades, not quality grades. A higher number is not "better" — it is just harder. The right choice depends entirely on how exposed the wall is and how soft the stone is.
What NHL actually stands for
Natural Hydraulic Lime is made by burning limestone that naturally contains clay and other impurities. Those impurities are what give the lime its "hydraulic" property — the ability to set in the presence of water, even without air. It is called natural because that hydraulic character comes from the raw stone itself, not from cement or chemical additives blended in afterwards.
This matters for old buildings. NHL keeps the two qualities that protect historic masonry — it stays breathable (vapour-permeable) and it stays flexible and relatively soft — while still being practical to work with and reliable to set. That combination is why it has become the standard binder for conservation work across Scotland.
How hydraulic lime sets and cures
NHL sets in two stages, and understanding this is the key to using it well:
- Initial set (hydraulic): When mixed with water, the lime reacts and hardens over hours to days — even in damp conditions or without air. This is what lets it be used on exposed walls and in the Scottish climate.
- Long-term cure (carbonation): Over months and years, the lime slowly absorbs carbon dioxide from the air and gradually turns back into a form of limestone. This is what gives lime its famous self-healing quality and its very long life.
Because of that slow second stage, lime work needs patience and protection while it cures — kept damp, shaded from strong sun, and shielded from frost. Rushing it, or letting it dry out too fast, is one of the most common reasons a lime repair fails prematurely.
NHL 2 vs 3.5 vs 5 — the three grades
The number after "NHL" refers to the minimum compressive strength of the set lime (measured in newtons per square millimetre). Here is what each grade is really for.
| Grade | Character | Typically used for |
|---|---|---|
| NHL 2 | Softest & most breathable. The gentlest on the stone, but the slowest to gain strength and least tolerant of exposure. | Internal plaster, sheltered walls, soft or delicate stone, fine pointing. |
| NHL 3.5 | The versatile middle ground. A good all-rounder that balances workability, breathability and durability. | General repointing, bedding stone, most external masonry in moderate exposure. |
| NHL 5 | Hardest & fastest-setting. The most durable, but the least breathable and least forgiving — closest to cement in behaviour. | Chimneys, copings, cills, below-ground work and severe weather exposure. |
Use the softest grade that will do the job. Reaching for NHL 5 "to be safe" is a classic mistake — too hard a lime brings back the very problems (trapped moisture, stone erosion) that lime is supposed to prevent.
Which grade for which job
There is no single "correct" NHL for a whole building. A good specification often uses more than one grade on the same property — a softer lime on the sheltered main walls and a harder lime on the parts that take a battering. As a rough guide:
- Soft, weathered or historic sandstone → lean towards NHL 2 or a gently specified NHL 3.5, so the mortar stays softer than the stone.
- Standard external repointing in the Central Belt → NHL 3.5 is the usual workhorse.
- Highly exposed details — chimney heads, copings, cills, string courses → NHL 5, where durability has to win.
The exposure to Scotland's driving rain, the softness of your particular stone, and even which way the wall faces all feed into the choice. This is exactly the kind of judgement that a proper site survey and mortar analysis is for.
NHL vs lime putty vs cement
NHL is not the only lime. It helps to know where it sits between the two extremes:
- Lime putty (non-hydraulic lime): The softest and most breathable of all, setting purely by carbonation (air). Superb for the most delicate historic work, but slow and needing very sheltered, patient conditions.
- NHL (hydraulic lime): The practical middle path — breathable and flexible like lime, but with a dependable set that suits the Scottish climate and exposed walls.
- Portland cement: Hard, rigid and effectively waterproof. Strong, but it traps moisture and erodes soft sandstone — the very damage covered in our cement vs lime guide.
For most historic sandstone buildings in central Scotland, NHL hits the sweet spot: breathable enough to protect the stone, robust enough to last for generations.
Common mistakes to avoid
- Defaulting to NHL 5. Harder is not safer — too strong a lime damages soft stone just like cement does.
- Ignoring the sand. The aggregate matters as much as the lime. The wrong sand ruins colour, texture and durability — which is why matching the original mix is worth doing properly.
- Letting it dry too fast. Lime cured in hot sun or drying wind, or hit by early frost, will be weak and crumbly. Protection during curing is not optional.
- Mixing lime with cement "for strength". Adding cement to a lime mix destroys the breathability that made lime the right choice in the first place.
Getting the grade, the sand and the curing right is the difference between a repair that lasts decades and one that fails in a few winters. If you would like it assessed and specified properly, a free on-site assessment is the best place to start.
