1) Basic principle: Compatible material, long life

In restoration the aim is not to replace the old entirely but to produce solutions that behave like the existing material.

A material may be "durable"; but if it is incompatible with the rest of the building, it accelerates decay. So the first question in restoration is: "Will this material work with the building?"

2) Material selection in stone structures

Why is stone sensitive? Stone is porous despite appearing hard; it absorbs and releases moisture and responds to temperature changes.

Most common mistake: Using cement-based pointing and plaster on stone walls. Results: the wall cannot breathe, moisture is trapped inside the stone, salt efflorescence begins and the stone surface eventually spalls.

Correct approach:

  • Existing stone type is analysed (limestone, sandstone, etc.)
  • Porosity and hardness compatibility are considered
  • Stone from the same quarry or with similar properties is chosen if needed
  • Lime-based mortars are preferred for pointing and repairs

3) Material selection in brick structures

Historic bricks differ from modern industrial bricks: they have a lower firing temperature, are more flexible and distribute moisture in a balanced way.

Wrong approach: Using hard, high-strength modern brick and "locking" the old brick with cement-based joints. The old brick cannot take the stress and cracking and surface loss begin.

Correct approach:

  • Analysis of the existing brick's size, colour and texture
  • Specially made restoration brick used if needed
  • Joints designed to be weaker than the brick (this is intentional)

4) Timber elements

Timber is used in historic buildings as structure (beams, binders), finish (joinery, cladding) and as an aesthetic element.

Biggest misconception: "Decayed timber must always be fully replaced." In many cases only certain parts of the timber are damaged; complete removal destroys authenticity.

Correct approach:

  • Timber type is identified (pine, oak, etc.)
  • Source of decay (moisture, insect) is removed
  • Local consolidation is carried out
  • If needed, splicing (prosthesis) is applied
  • Chemical preservatives are used in a controlled way

With timber the aim is to keep as much of the original in place as possible.

5) Mortar and plaster selection: The most critical issue

In historic buildings mortar and plaster are not just "binders"; they are the building's breathing system.

Why is lime mortar preferred? It is flexible, vapour-permeable, manages salt movement and is weaker than stone and brick (intentionally). So damage goes to the mortar, not the wall; repair is easy and reversible.

Why is cement problematic? It is rigid, not flexible, blocks breathing and damages the main material in the long term.

How is the right mortar determined? Existing mortar is analysed; lime type (air lime, hydraulic lime) is chosen; sand particle size and colour are chosen for compatibility; sample application is done and observed.

6) Using new material: When and how?

New material is not completely forbidden in historic buildings; but there are conditions. New additions: are not imitated exactly as the old; but are compatible and simple; can be distinguished on close inspection; and do not harm the building. This is one of the basic ethical rules of restoration.

7) The most common material mistakes on site

  • Using hard mortar "to make it stronger"
  • Moving to implementation without material trials
  • Ignoring colour and texture compatibility
  • Putting craftspeople's habits above the project
  • Using chemical products without control

Most of these mistakes lead to results that are hard to reverse.

8) Practical checklist for the client

In a restoration job these questions should be asked:

  • Is the mortar used lime-based?
  • Was the existing material analysed?
  • Was sample application done?
  • Could new materials harm the old?
  • Is the implementation process being supervised?

If there is no clear answer to these questions, there is risk.

9) Conclusion: Material is like the language of restoration

You cannot form a correct sentence with the wrong words. You cannot do correct restoration with the wrong material.

In historic buildings material selection is as much a cultural and ethical responsibility as it is a technical decision.