Why lime, not cement, on old roofs
Walk around the older parts of Horsham — The Causeway, the Denne Road conservation area, the timber-framed buildings of the town centre — and almost every historic roof and chimney you see was built with lime mortar. It is not a stylistic preference. Lime and cement behave in fundamentally different ways, and using the wrong one on a historic building causes slow, expensive damage that often takes years to become visible.
Lime mortar is comparatively soft, flexible and, critically, breathable. It allows moisture that gets into a wall or chimney to evaporate back out, and it flexes with the small seasonal movements that every old building makes. Modern Portland cement does the opposite: it is hard, rigid and largely impermeable. On a soft historic brick or a Horsham Stone chimney, cement traps moisture behind and within the masonry. Because the water cannot escape through the hard cement joint, it escapes through the softer brick or stone faces instead — and as it freezes and thaws over successive West Sussex winters, it spalls those faces away.
Historic England and the Society for the Protection of Ancient Buildings (SPAB) both advise that lime mortar should be used in preference to cement on pre-1919 buildings, precisely because cement traps moisture and accelerates decay of historic fabric. — Historic England / SPAB guidance
The tragedy of cement pointing on an old building is that it looks like a repair while quietly destroying the very fabric it was meant to protect. A chimney repointed in hard grey cement can lose the faces of its bricks within a decade, turning a simple repointing job into a rebuild. This is why we never use cement on pre-1930 masonry in Horsham — and why a roofer who does should be a warning sign on any period property.
What natural hydraulic lime actually is
The lime used in roofing and masonry repair today is almost always natural hydraulic lime, abbreviated NHL. It is graded by strength: NHL 2 (feeble, for the softest historic work), NHL 3.5 (moderately hydraulic, the workhorse grade for most roofing and chimney repointing), and NHL 5 (eminently hydraulic, for more exposed or structural situations). The grade matters. Too strong a lime on a soft old brick recreates the same trapping problem as cement; too weak a lime on an exposed chimney washes out before it cures.
For the great majority of Horsham conservation work — repointing a Victorian chimney stack, bedding reclaimed peg tiles, renewing flaunching on a period roof — NHL 3.5 is the correct specification. It is strong enough to weather a Sussex winter on an exposed stack, soft enough to protect the historic brick, and breathable enough to let the building dry out. On the softest, oldest fabric we may drop to NHL 2, and on the most exposed positions step up to NHL 5, but 3.5 is the default a competent heritage roofer reaches for.
Natural hydraulic lime is graded NHL 2, NHL 3.5 and NHL 5 by increasing strength; NHL 3.5 is the standard grade for most historic roofing and chimney repointing. — BS EN 459-1 (building lime classification)
Where lime mortar is used on a roof
Lime mortar appears in more places on a traditional roof than most homeowners realise. The most common is chimney repointing: raking out the failed old joints and repointing the stack in matching lime. Then there is flaunching — the sloping mortar bed on top of the stack that seats the pots and sheds rainwater — which is one of the most exposed pieces of mortar on the whole building and a very common source of leaks when it cracks.
Lime is also used for bedding ridge and hip tiles on traditional mortar-bedded roofs, for pointing the verge (the gable edge) on period properties where a modern dry-verge system would look wrong, and for bedding reclaimed clay peg tiles and Horsham Stone slabs. On a heritage or Horsham Stone roof, getting the lime specification and the joint finish right is as much a part of doing the job properly as sourcing the reclaimed stone itself.
Conservation areas and listed buildings in Horsham
Much of Horsham’s historic core falls within a conservation area, and a significant number of individual buildings are listed. In both cases, the use of lime mortar is frequently not just best practice but a requirement. Conservation officers at Horsham District Council will expect repointing and repairs to a historic building to be carried out in an appropriate lime mortar with a matching joint finish, and specifying cement can be grounds for refusing or reversing consent.
On a listed building, work affecting the character of the roof or chimney — including the mortar — can require listed building consent, and carrying it out without consent is a criminal offence, not merely a planning breach. This is covered in detail in our guide to listed building consent for roofing, but the short version is: on a listed or conservation-area property, the mortar specification is part of what the authority controls, and lime is almost always the answer.
On listed buildings, unauthorised alteration — including inappropriate cement repointing of a historic chimney — is a criminal offence under the Planning (Listed Buildings and Conservation Areas) Act 1990. — Planning (Listed Buildings and Conservation Areas) Act 1990
Matching the joint and the colour
Using lime is only half the job; matching it is the other half. A historic building has a particular mortar colour — governed by the sand used originally — and a particular joint finish, whether flush, recessed, struck or a traditional Sussex finish. A repair in the correct lime but the wrong colour, or with a modern flush joint on a building that was originally recessed, still looks wrong and can still draw objection from a conservation officer.
We match the sand and the mix to the existing mortar so the repair blends rather than stands out, and we finish the joint to match the original. On a prominent chimney or a front elevation in the conservation area, this matters enormously: a well-executed lime repair should be almost invisible, looking as though the building was always that way rather than announcing itself as new work.
Why lime work costs more — and lasts
Lime is slower to work with than cement. It cures on its own timescale, needs protecting from rain and frost while it sets, and demands more skill to mix, apply and finish correctly. That makes lime work more expensive per metre than cement pointing — but the comparison is misleading, because cement on a historic building is not a cheaper version of the same repair. It is a different, damaging intervention that will cost far more to put right later when it has spalled the brick faces.
Done properly, lime repointing lasts for decades and protects the historic fabric it sits within, which can last for centuries. It is the difference between spending once, correctly, and spending twice — once on the wrong repair and again on undoing the damage it caused. On the irreplaceable historic stock of Horsham, that is not a close call.