Why Pure NHL Mortar is Often Too Hard for Historic Buildings (And What We Use Instead)

If you are researching restoration work for your period home, you likely already know the golden rule: never use modern cement on historic masonry. Cement traps moisture and destroys old stone and brick.

Because of this, many homeowners explicitly ask their builders to use "lime mortar." However, a common and dangerous misconception has taken hold in the modern building industry: the belief that all lime is safe for historic properties.

Today, most general contractors use a product called NHL (Naturally Hydraulic Lime) straight out of a bag. While pure NHL is better than cement, applying it directly to soft historic fabric can actually cause the exact same damage we are trying to prevent.

Here is why pure NHL is often too hard for older properties, and why Hughes Heritage uses a traditional, bespoke gauged mix of freshly slaked lime putty and NHL to protect your home.

The Problem with Pure NHL

Naturally Hydraulic Lime (NHL) is sold in bags and sets via a chemical reaction with water—very similarly to modern cement. It is categorized by its strength, usually NHL 2, NHL 3.5, and NHL 5.

Because it sets relatively quickly and is easy to use, modern builders love it. However, the higher grades of NHL (especially 3.5 and 5) cure incredibly hard over time.

If you use pure NHL 3.5 to repoint a delicate Regency brick wall in Cheltenham or a soft Cotswold stone cottage, you create a fundamental structural imbalance. The mortar should never be harder than the stone or brick it holds together. If the NHL mortar cures too hard, it loses its breathability and flexibility. When the building naturally moves, or when moisture freezes inside the wall, the hard NHL forces the stress into the soft historic masonry, causing the face of your beautiful stone or brick to shatter and blow off—exactly like cement does.

The Pure Traditional Alternative: Lime Putty

Before NHL became commercially popular, true historic buildings were constructed using Lime Putty (Non-Hydraulic Lime).

Unlike NHL, lime putty does not set with water. Instead, it cures through carbonation—it slowly absorbs carbon dioxide from the air, turning back into limestone over a period of months. Lime putty is incredibly soft, highly flexible, and offers maximum breathability. It is the absolute perfect material for period homes.

So, why don’t builders use pure lime putty for everything? Because its slow setting time makes it highly vulnerable to the unpredictable British weather. If a heavy Cotswold rainstorm hits pure lime putty before it has carbonated, it can simply wash out of the wall.

The Master Craftsman’s Secret: The "Gauged Mix"

At Hughes Heritage, we do not compromise the historic fabric of your building for the sake of a quick, modern fix. Instead, we use a traditional technique known as "gauging."

We create a bespoke mortar by blending freshly slaked lime putty with a carefully measured fraction of low-strength NHL. This gauged mix gives your period property the absolute best of both worlds:

• The Protection of NHL: The small addition of hydraulic lime gives the mortar an "initial set." This protects the fresh pointing from rain, frost, and wind during the crucial early stages of curing, ensuring the work remains pristine regardless of the weather.

• The Softness of Lime Putty: Because the bulk of the mix is pure lime putty, the final cured mortar remains exceptionally soft, flexible, and vapor-permeable. It acts exactly as it should—as a sacrificial, breathable joint that allows moisture to safely evaporate, perfectly preserving your historic Cotswold stone or Cheltenham brickwork.

True Heritage Craftsmanship

Successfully restoring a period property requires more than just buying a bag of "lime" off a shelf. It requires an intimate understanding of building chemistry, local masonry, and traditional techniques.

By using a custom-gauged mix of lime putty and NHL, we guarantee a flawless, historically accurate finish that actively protects the structural integrity of your home for generations to come..