What is NHL 3.5 lime mortar used for?

NHL 3.5 is the most common hydraulic lime for UK heritage buildings. It's used for repointing brickwork, rendering external walls, and internal plastering on pre-1919 solid-wall buildings. It's strong enough for most applications but softer than cement, so it protects soft bricks and allows breathability. Use NHL 3.5 for general heritage work unless exposure or substrate requires NHL 2 or NHL 5.

What is the mix ratio for NHL 3.5?

For pointing and rendering: 1 part NHL 3.5 to 2.5-3 parts sand by volume. Use coarser sand (up to 5mm) for base coats, finer sand (up to 3mm) for finish coats. For plastering: 1:2.5 to 1:3. Add water gradually until the mix is thick and sticky like porridge. Too wet and it will slump. Too dry and it won't bond.

How long does NHL 3.5 take to set?

NHL 3.5 is touch-dry in 24-48 hours and ready for the next coat in 7-14 days. Full carbonation (hardening) takes 4-6 weeks. Do not paint or seal NHL 3.5 until it has fully carbonated. Lime putty (non-hydraulic) takes 3-6 months to fully set by comparison.

NHL 3.5 lime mortar: properties, mix ratios, when to use

What is NHL 3.5?

NHL 3.5 stands for Natural Hydraulic Lime 3.5. It's made by heating limestone that contains natural clay impurities (silicates). The silicates allow the lime to set partially by reaction with water (hydraulic set) and partially by carbonation (absorbing carbon dioxide from the air).

The "3.5" refers to the compressive strength classification under BS EN 459: 3.5-10 MPa at 28 days. This makes it stronger than NHL 2 (2-7 MPa) but weaker than NHL 5 (5-15 MPa).

NHL 3.5 is the most commonly used hydraulic lime in the UK because it balances strength, workability, and compatibility with heritage masonry.

Properties of NHL 3.5

Strength

NHL 3.5 develops compressive strength of 3.5-10 MPa at 28 days. This is strong enough for most repointing and rendering work but softer than cement mortar (15-25 MPa), so it acts as a sacrificial layer, protecting the masonry.

Setting time

NHL 3.5 is touch-dry in 24-48 hours. It's ready for the next coat (rendering or plastering) in 7-14 days. Full carbonation takes 4-6 weeks. The hydraulic set provides early strength, while carbonation continues to harden the mortar over months and years.

Breathability

NHL 3.5 is highly vapour permeable (SD value 0.1-0.3m), allowing moisture to move through the mortar. This is essential for solid-wall buildings, which rely on breathability to manage moisture.

Flexibility

NHL 3.5 is flexible compared to cement. It absorbs minor building movement without cracking. If cracks do form, they self-heal over time as carbonation continues.

Workability

NHL 3.5 has good workability (easier to trowel and point than NHL 5 or cement). It stays workable for 2-3 hours after mixing (longer than gypsum plaster but shorter than lime putty).

Mix ratios for NHL 3.5

Repointing (brickwork or stonework)

1 part NHL 3.5 to 2.5-3 parts sand by volume. Use well-graded sharp sand (mix of particle sizes). Add water gradually until the mix is stiff but workable (like thick butter).

Rendering (external walls)

Base coat: 1:2.5 to 1:3 (NHL 3.5:coarse sand up to 5mm). Apply 8-12mm thick, scratch for key.
Top coat: 1:2.5 (NHL 3.5:finer sand up to 3mm). Apply 6-8mm thick, floated smooth or textured.

Plastering (internal walls)

Base coat: 1:2.5 to 1:3 (NHL 3.5:coarse sand). Apply 8-12mm thick, scratched.
Finish coat: 1:2 to 1:2.5 (NHL 3.5:fine sand or lime putty with fine aggregate). Apply 2-4mm thick, troweled smooth.

Adding fibre

For thick coats (over 15mm) or over lath, add animal hair or synthetic fibre (2-3 handfuls per 25kg bag of NHL 3.5). This adds tensile strength and reduces cracking.

When to use NHL 3.5

General heritage work

NHL 3.5 is suitable for most UK heritage buildings (pre-1919 solid walls, listed properties). Use it for repointing, rendering, and plastering unless the substrate or exposure requires a softer or harder lime.

Moderate exposure

NHL 3.5 is suitable for sheltered to moderately exposed locations (urban, suburban, rural inland). For very exposed locations (coastal, hilltop, driving rain), use NHL 5.

Soft to medium-strength masonry

NHL 3.5 is compatible with soft bricks (pre-1900), soft stone (limestone, sandstone), and lime-built walls. For very soft substrates (cob, very soft brick), use NHL 2 or lime putty. For hard stone (granite, hard sandstone), use NHL 5.

NHL 3.5 vs NHL 2

Strength

NHL 2 is softer (2-7 MPa vs 3.5-10 MPa for NHL 3.5). Use NHL 2 for very soft substrates (cob, rammed earth, very soft brick) where NHL 3.5 would be too hard.

Setting time

NHL 2 sets slower than NHL 3.5. It's touch-dry in 48-72 hours and ready for the next coat in 14-21 days. This makes it less practical for commercial work but better for conservation projects where the substrate is extremely soft.

Availability

NHL 3.5 is widely available from builders' merchants. NHL 2 is less common and may need to be ordered from specialist lime suppliers.

NHL 3.5 vs NHL 5

Strength

NHL 5 is harder (5-15 MPa vs 3.5-10 MPa for NHL 3.5). Use NHL 5 for exposed locations (coastal, hilltop) or hard substrates (hard stone, engineering brick).

Durability

NHL 5 is more weather-resistant than NHL 3.5. It's the best choice for external rendering in exposed locations or for chimney repointing.

Workability

NHL 5 is stiffer and harder to work than NHL 3.5. It also sets faster, giving less working time. For internal plastering or repointing in sheltered locations, NHL 3.5 is easier to use.

How to mix NHL 3.5

By hand (small batches)

Measure NHL 3.5 and sand by volume (use a bucket). Mix dry ingredients thoroughly with a shovel or hoe. Add water gradually, mixing until the consistency is right. For repointing, aim for stiff but workable (like thick butter). For plastering, aim for sticky and porridge-like.

By machine (larger batches)

Use a cement mixer (not a forced-action mixer, which can damage the lime). Add sand and NHL 3.5 dry, mix for 1 minute, then add water gradually while mixing. Mix for 3-5 minutes total. Do not over-mix (weakens the mortar).

Fattening overnight

For best workability, mix NHL 3.5 the day before use and leave it covered overnight. This allows the lime to fully hydrate, improving plasticity. Remix briefly before use. This step is optional but recommended for plastering and rendering.

Application tips

Damp the substrate

Always damp the substrate (brick, stone, plaster) 10-15 minutes before applying NHL 3.5 mortar. Dry substrates suck moisture out of the mortar, preventing proper set and carbonation.

Apply in thin layers

For rendering and plastering, apply NHL 3.5 in layers no thicker than 12mm per coat. Thicker coats crack as they dry. If you need more thickness, apply multiple coats with 7-14 days between coats.

Protect while curing

Mist NHL 3.5 mortar with water daily for 7 days after application to slow drying. Protect from frost (cover with hessian or tarpaulin), strong sun (shade netting), and wind (windbreak). NHL 3.5 that dries too fast will crack and remain weak.

Cost of NHL 3.5

NHL 3.5 costs £10-15 per 25kg bag (covers roughly 2-3m² of repointing or 1-1.5m² of rendering per coat, depending on joint depth or thickness). Sand costs £3-5 per 25kg bag.

For comparison, cement costs £5-7 per 25kg bag. The extra cost of NHL 3.5 is offset by its breathability, flexibility, and compatibility with heritage masonry.

Summary

NHL 3.5 is the most common hydraulic lime for UK heritage work. It has moderate strength (3.5-10 MPa), sets in days (vs months for lime putty), and is breathable and flexible. Use it for repointing, rendering, and plastering on most pre-1919 solid-wall buildings.

Mix 1:2.5 to 1:3 (NHL 3.5:sand by volume). Damp the substrate before application. Protect while curing. Allow 7-14 days between coats. Full carbonation takes 4-6 weeks.

Use NHL 2 for very soft substrates. Use NHL 5 for exposed locations or hard substrates. For more detail, see the full guide to hydraulic lime mortar types.