# Complete Guide to NHL Lime Mortar for Heritage Restoration
Natural Hydraulic Lime (NHL) mortars are the material of choice for heritage restoration, conservation projects, and traditional masonry. Unlike cement, which traps moisture and damages old stone, NHL lime mortar is breathable, flexible, and compatible with historic substrates. This calculator uses the proportional system codified by the Roman architect Vitruvius in 25 BC — a system still valid today.Why lime mortar compatibility matters
Using cement mortar in historic buildings causes severe damage. Cement is harder than the surrounding stone, meaning thermal movement cracks the stone instead of the joint. It also traps moisture behind impermeable render, causing spalling and frost damage. Lime mortar is always sacrificial: it crumbles before the stone does.
# The Three Vitruvian Layers: Trullissatio, Arenato, Marmorato
Vitruvius described a three-coat render system that remains the standard for lime plasterwork. Each coat has a specific function and aggregate size: Trullissatio (scratch coat, 1:3, 30mm) bonds to the substrate; Arenato (float coat, 1:2, 16mm) creates a flat, even surface; Marmorato (finish coat, 1:1, 4mm) provides the final texture, traditionally with marble dust.# NHL Grades and Their Applications
# Aggregate Selection for Lime Mortar
The aggregate is not a filler — it is 60-75% of your mortar by volume. Sand quality directly determines mortar quality. Sharp washed sand with angular grains creates stronger mechanical keying. Rounded river sand gives a more workable mix but lower strength. For heritage pointing, always try to match the colour and texture of the original mortar through petrographic analysis.Vitruvius, De Architectura Book VII, Ch. 2
"When the lime is slaked, let it be beaten with iron tools [...] if it stick to the tool and come away clean, it shows the lime is rich and well slaked; if it is soft and heavy, it is deficient in lime."
Two thousand years of empirical testing, condensed into a single field test that still works today.