Lime render, thatch and traditional material conditions

When a listed building consent comes back with a traditional materials listed building condition, it usually says something close to this: no work shall begin until a sample panel of the render, or a sample of the mortar, roofing material or ridge detail, has been approved in writing by the local planning authority. It is not box-ticking. The condition exists because the wrong material can damage the fabric of an old building within a few winters, and because the character of a listed building often lives in its surfaces rather than its shape.

The practical effect is that you cannot leave the specification to your builder to sort out on site. It has to be decided, drawn or described, and approved, before anyone mixes anything.

Why materials are conditioned so tightly

Section 16 of the Planning (Listed Buildings and Conservation Areas) Act 1990 requires the decision maker to have special regard to the desirability of preserving the building and any features of special architectural or historic interest it possesses. Section 66 applies a similar duty to planning decisions affecting a listed building or its setting, which our note on sections 66 and 72 and what they require of decisions works through.

A conservation officer granting consent for, say, re-rendering a cob or rubble stone wall is satisfied that re-rendering is acceptable in principle. What they cannot judge from a drawing is whether the render will be a lime mix that lets the wall breathe or a cement mix that traps water behind it. So they condition it, and they ask for a sample rather than a written specification, because samples cannot be misread.

The same logic runs through mortar colour, pointing profile, thatch ridge type and the width of a glazing bar. These are the details a reader of the building notices without knowing why.

Lime against cement, and what an officer is looking for

Traditional solid walls of soft brick, stone, cob or clay lump were built to get wet and dry out again. Lime mortars and renders are softer than the masonry around them, slightly porous, and sacrificial, which means moisture moves through the joint and evaporates rather than sitting in the masonry face. Cement is harder and much less permeable. On a soft wall it pushes water into the brick or stone, and the frost damage appears in the face of the material rather than in the joint, where it cannot be repaired by repointing.

There are three families of lime an officer will expect to see named:

• Non-hydraulic lime putty, classified as CL 90 under BS EN 459-1, which sets by carbonation and is the softest option.
• Natural hydraulic lime, graded NHL 2, NHL 3.5 and NHL 5 in increasing strength, which sets partly by hydration and suits more exposed elevations.
• Hot-mixed lime mortars, made on site from quicklime, which have returned to favour for bedding and pointing soft masonry.

A specification that will survive scrutiny gives the binder and its classification, the aggregate by source and grading, the mix proportions by volume, and the finish. Mortar for pointing is usually described as a proportion of binder to well-graded sharp sand, in the range of one to two and a half or one to three, with the aggregate chosen to match the colour and texture of the original. Where the original mortar can still be found in a sheltered joint, send a sample for analysis and let the result drive the mix.

And avoid one common mistake. Adding cement to a lime mix to make it "go off" faster defeats the point of using lime at all.

Thatch, ridges and fire

Re-thatching a listed building in the same material, to the same detail, can sometimes be treated as repair rather than alteration. Changing the material or the ridge almost always needs listed building consent, because it changes the way the roof reads.

The three coat materials used in England are long straw, combed wheat reed and water reed, and they are not interchangeable. Long straw has a shaggy, eaves-and-gable-wrapped appearance with liggers across the ridge. Combed wheat reed looks crisper, with cut ends showing. Water reed is stiffer, lasts longest, and gives a harder line that looks wrong on a building that has always been strawed. A condition will normally fix the material, the fixing method, the ridge type, whether the ridge is flush or block cut, and the pattern of any decorative work.

Fire is the other half of the conversation. Thatch fires are most often linked to the flue serving a wood burner rather than to sparks landing on the roof from outside. Guidance developed by fire services and the thatching trade points at the same set of measures: a properly lined and insulated flue, a chimney raised well clear of the ridge, with a figure of about 1.8 metres commonly cited, a spark arrestor kept clean, and a fire barrier board or membrane beneath new thatch. Talk to building control and your insurer about these at the same time as the conservation officer, because insurance requirements sometimes go further than the consent does.

Sample panels and how approval actually works

The sequence is simple and it is worth following in order.

1. Agree with the conservation officer what the sample needs to show and where it can go, ideally on a discreet elevation that will remain part of the finished work.
2. Have the panel prepared by the person who will do the whole job, using the actual materials, at around a square metre for render or a metre run for pointing.
3. Let it cure. Lime changes colour as it carbonates, and a panel judged wet will be judged wrong.
4. Submit photographs, the written specification and the panel location on a plan as an application to discharge the condition, and ask the officer to inspect.
5. Keep the written approval. It is the document that proves the work was authorised.

Allow time for this. Discharging a condition has a statutory determination period, and adding curing time on top means the specification work needs to start weeks before the scaffold does.

Sourcing and lead times

Traditional materials do not come off a builders' merchant shelf, and the programme has to reflect that. Hand-made bricks to a particular size, lime putty matured for months, water reed, a specific pantile, or leadwork to a historic profile all have lead times measured in weeks or longer, and thatchers in many counties are booked well ahead.

That has two consequences. Order once the sample is approved, not before, or you risk buying the wrong thing. And build the lead time into the contract programme, because a contractor under pressure at the end of a job is exactly the person most likely to substitute.

Recording what was used

Keep a materials record for the building. It costs almost nothing and it is worth a great deal at the next application.

A simple file holding the approved specification, the photographs of the sample panel, the delivery notes, the mortar analysis if one was done, and photographs of the work in progress does three jobs. It proves compliance if a query arises. It tells the next owner's builder what is on the wall. And where the work involved opening up historic fabric, the photographic record feeds into any Historic Building Recording the authority asked for, and into the heritage evidence for future proposals, which our note on internal alterations that harm significance touches on.

When a contractor substitutes

This is the failure mode that causes real trouble. The sample is approved, the scaffold goes up, and the render arrives as a bagged cement-based product because the merchant was out of the specified lime.

Carrying out works to a listed building that need consent, without that consent, is an offence under section 9 of the 1990 Act, and failing to comply with a condition attached to a consent is an offence under the same section. The authority can serve a listed building enforcement notice requiring the work to be undone. Hacking off cement render from a cob or rubble wall causes more damage than the render did, so the remedy does more harm than getting it right first time.

Two protections are worth putting in place. Write the approved specification into the building contract as a document, not as a note. And ask to be told before any substitution, in writing, with the officer copied in.

A Heritage Statement prepared at application stage should already have set out why the material matters to the significance of the building, which makes the later condition easier to discharge. If you are drafting one, the Heritage Statement page explains what it needs to cover.

This is general guidance, not legal advice. Check the position with your local planning authority's conservation officer or a suitably qualified building conservation professional before relying on it.

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