What is the difference between lime putty and hydraulic lime?

Lime putty (non-hydraulic lime, CL90) sets by carbonation — it absorbs CO2 from the air over weeks or months. It is very soft and breathable, ideal for historic buildings. Hydraulic lime (NHL 3.5 or NHL 5) has a hydraulic set, meaning it cures through reaction with water, much like cement but slower. NHL 3.5 is the most common choice for internal lime plastering, while NHL 5 is harder and used for exposed or external work. Lime putty gives the most authentic finish for listed buildings.

What sand do I use with lime plaster?

Use well-graded sharp sand (also called plastering sand or coarse sand) for scratch and float coats. For the finish coat, use a finer grade of washed sand. Avoid builder's sand or soft sand as it contains too many fines and can cause shrinkage cracking. The sand should be clean and free from organic matter. Many suppliers sell lime-specific plastering sand that is already graded.

How thick should lime plaster coats be?

A scratch coat (first coat) is typically 8-12mm thick, applied in one pass and scratched to provide a key. The float coat (second coat) is usually 6-10mm, levelled with a darby or straight edge. The finish coat is 2-4mm, applied thinly and worked to a smooth or textured finish. Total thickness for a three-coat lime plaster system is typically 18-25mm. Always allow adequate drying time between coats — at least 3-7 days depending on conditions.

Can I use lime plaster on modern blockwork?

Yes, lime plaster can be applied to modern blockwork, but the blocks must be dampened thoroughly first as they are very absorbent. Some plasterers apply a lime slurry or spatterdash coat first to improve adhesion. Lime plaster is increasingly popular even on modern builds because it is breathable, reducing condensation and improving indoor air quality. However, lime plaster takes longer to cure than gypsum plaster and must be protected from frost during curing.

How long does lime plaster take to dry?

Lime plaster takes significantly longer to dry than gypsum plaster. Allow at least 5-7 days between coats for hydraulic lime (NHL), and 2-4 weeks for lime putty. The general rule is 1 week per 5mm of thickness for hydraulic lime. A full three-coat lime plaster system can take 4-8 weeks before it is ready for decoration. Keep the environment well-ventilated but avoid direct heat or draughts which can cause cracking.

Lime Plaster Calculator — How Much Lime Plaster Do I Need?

Calculate lime putty and hydraulic lime plaster quantities for traditional and period property restoration work. Includes sand ratios and bag counts.

Area to Plaster

Total wall or ceiling area

Lime Type

Lime putty for period properties; NHL for general use

Coat Type

Each coat has a different thickness and mix

Coat Thickness

Typical: scratch 10mm, float 8mm, finish 3mm

Number of Coats

Usually 1 coat per type

Include 10% wastage

Recommended — accounts for mixing waste, dropped material, and uneven substrates

Price Per 25kg Bag (optional)

Enter price per 25kg bag of lime for a cost estimate

How We Calculate This

This calculator estimates the quantity of lime and sharp sand needed for lime plastering based on the area to be covered, the type of lime, the coat being applied, and the thickness per coat.

The formula

Lime (kg) = Coverage rate (kg/m²/mm) × Area (m²) × Thickness (mm) × Number of coats × Wastage factor

Sand (kg) = Lime (kg) × Sand ratio

Coverage rates

Lime putty (CL90): ~2.5 kg per m² per mm thickness
NHL 3.5: ~1.8 kg per m² per mm thickness
NHL 5: ~2.0 kg per m² per mm thickness

Sand-to-lime ratios

The mix ratio varies by coat. Scratch coats use a 3:1 sand-to-lime ratio (by volume) for added strength, while float and finish coats use a 2.5:1 ratio for a finer, more workable mix. These ratios are approximate weight equivalents based on typical bulk densities.

Standards

Lime mortars and plasters should comply with BS 5262 (Code of practice for external renderings) and BS EN 459-1 (Building lime). For listed buildings, always consult with your conservation officer regarding appropriate lime types and finishes.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.