10. Repair on plaster surfaces and plaster decorations on facades

The deterioration and damages on facades 
Insufficient understanding for and neglect of the gradual deterioration and damages on the facades are the causes of long term destruction or even loss of many historic buildings.

Visible damages

Some of the damages are immediate visible on the surface. For instance (Drawing no. 27):
1  Peeling paint layers
2  Leaching of the bonding agents of the plaster 
3  Peeling of layers of plaster
4  Small and larger cracks
5  Humid, moist or wet plaster from ground humidity

Causes of the damages
Most of the deterioration and damages has 5 causes:
1  Influence of water - and the consequences or followers of water: Moisture, ice, saltwater, acid rain. This causes dry rot and fungus in wood, salts in masonry and stones, frost damages, acid deterioration, algae, dirt etc.
2  Mechanical causes - from wear, ground decreases, insufficient carrying capacity and wind.
3  Technical faults - from  insufficient constructions: Leakages, insufficient adjoints between materials, too hard and moisture tight surface treatments or finishing coats, changes in the physical balance in the constructions.
4  Forced deterioration of the surface materials due to leakages in the roof, joints, watertight surface on iron etc.
5  Other causes - incorrect use and arrangements,  neglect of maintenance.

Technical survey on the masonry
But before the restoration of the facade, the masonry must be examined more closely. This should be executed preferably by an impartial authority, and not by the craft firm intended to be involved in the actual work. One or more of the following circumstances must be inspected depending on the type of construction:

1 Did any constructive circumstances lead to damages on the masonry like soilage on the facades? Are there constructions that should be altered?
2.  Are the flashings, trims, gutters and leaders in order?
3.  Is the masonry discoloured and is it necessary to clean it?
If so, an experimental cleaning must be executed well in advance of the renovation to find the suitable method of cleaning, for instance by water jetting etc.
The most considerate method towards both masonry and environment should be chosen.
4.  Is the masonry coated with an impermeable paint layer that has provoked the damages on the masonry?
If so, the paint-layer must be removed and replaced by a moisture penetrable type of paint.
5.  Is a moisture insulating layer induced in the footing, or is there any other obstacle for the soil dampness to enter the masonry?
6. Are there smaller or larger cracks in the masonry or the plaster? 
7. Are there cracks or gaps in the masonry, where water can enter? For instance in connection with mounted cast decorations, balconies, staircases, iron rails, iron anchors, wooden windows or doors, half-timbering, natural stone etc. If so, this can be dangerous for the facade. 
8. Are there evidence of corroded iron anchors, deeper in the masonry. 
9. Is the plaster deteriorating, peeling, flaking, constantly wet or dose it look "rotten"? If so, there are probably absorbed salts in the masonry. The plaster must be removed totally and the causes for the salts, ascending ground-moisture, salt storing in the building, thaw-salting of the pavement etc. must be removed or reduced and the salts drawn out of the masonry by packages of distilled water or a sacrificed layer of lime mortar. A new layer of lime mortar, constructed as two layers of increasing fineness, is added. These parts of the masonry cannot be surface-treated with lime wash or paint before all salts are removed.
10. What design/mixture has the existing plaster? The plaster repair must always have exactly the same design/mixture to ensure the same strength, elasticity, sough-ability - plus colour and grain-structure.

Plastic paint on facades
Frost-bursted joints and tiling often occur on lime washed facades successively coated with plastic emulsion paints, cement-based paints or modern silicone paints. Particularly old houses built with hand moulded bricks jointed with lime mortar are in the danger zone.
The cause of the damages is often due to the fact that the new materials used for surface treatment are much too impermeable which lead to a diminution of the transmission of the natural humidity and vapour. The humidity is accumulated behind the surfaces from which it slowly evaporates and crystals will deposit and keep growing.
This is shown by experience and it is documented by the so-called PAM numbers (value given for pressure against moisture). The PAM-values of the above mentioned materials are as follows:

Lime wash:     0,0 - 0,4
Paints based on cement powder    0,8 - 1,0
Plastic emulsion paints   2,0 - 7,0

These numbers emphasize that lime is the material with the highest diffusion without doubt. The importance of the open lime surfaces compared for instance to the more impermeable materials is revealed by all the damages that occurred during the last few years in connection with the new treatments.  Well-harden lime mortar is furthermore deteriorating if it is inclosed by a material with a high PAM-figure.
Other damages are provoked by the water that penetrates from the outside through fine cracks and accumulate behind the impermeable surface-treatment and does not have the possibility to evaporate quickly through the dense surface coating. The accumulation of water in the masonry leads to crystallisations and frost damages during winter which manifest itself as bursts of joints and formation of stone.

Cleaning for plastic paints etc.
The cleaning of masonry previously treated with plastic emulsion paints or materials based on cement must be done down to the firm ground.
Cleaning is done with low-pressure jetting, where it is possible. The work must be executed with great care in order not to damage the masonry additionally by admitting to much water. A possible mixture of sand and a successive water jetting should not be too hard as it will damage the fire coating of the tiles. In that case the surface of  the bricks will be damaged and consequently they will absorb much more humidity than before. It is recommended that experiments are made in every single instance until the correct method is found.
Building parts, windows, doors, cornices etc. that are not to be cleaned must be covered properly before the treatment is initiated.

Assessment of cracks

The crackings may appear both in an early state after the coating of facings, which is the most frequent situation, or it may appear much later, many years after the finishing of the building.
The crackings on plaster can, as shown on Drawing no. 29  be grouped in:
 A:   Surface crackings
 B:   Deeper crackings or slots 

Surface crackings 
Surface crackings do not overcome the thicknes of the single or multiple layers. In general they do not follow a specific direction. They may appear in any direction and they often give origin to closed cracking lines, such as large cells with big or small dimensions.
Surface crackings is generally due to wrong application of the coatings.
1 The use of mortars with unsuitable composition or mixture will cause crackings as shown in picture no. 1.
2 The application of mortars on too dry or hot surfaces causes too quick stiffening and the consequent laminar retraction, shown in picture no. 2
3 The application of renderings on dirty surfaces with any kind of fungi, powver, or under some active chemical decomposition, give orign to a kind of cracking followed by detachment, as seen on picture no. 3.

Deep crackings
Deep crackings attain deeper in the walls, overpassing the coating and attaining as well the composing elements of the wall supports, bricks, stone blocks, hydraulic binding and reinforced concrete.
Deep crackings also differ from others for their specific direction and location of specific points of the building architecture, as seen on picture no. 4.
Deep crackings are generally due to structural movements, mostly downwards, in the foundations of the facade.
The deep crackings and the surface crackings may have the same apperance when they are not coincident at the surface of the rough wall and in the coating. This happens when the wall detaches itself from the coating.
Another type of deep cracings is due to the expansion of corroded iron anchors, situated deeper in the masonry. Picture no. 5.

Examination
The method of examining the conditions of a cracked plaster/rendering is by knocking/percussion on the surface with a wooden shaft/handle of a hammer.
- a clear and open sound indicates adhesion of the plaster to the support 
- a hollow and deep sound indicates detachment from the support

Repair of cracks
If the following conditions occur in a new rendering, it will be sufficient to fill up the crackings with a thin past of the binder, used in the rendering:
- the affected area is limited
- the crackings are small
- the dimensional stability has been checked
-  a finishing layer, limewash or painting is to be applied.
If the affected area is wider or if the crackings overpass 2 mm, the new rendering should be taken of and a new layer applied, with the right composition or mixture and the right construction.

Graphic diagnose sheet of the deterioration and damages 

In order to produce a documentation of the technical state of the facade, and at the same time get an overview, it is recommended to work out a Graphic diagnose sheet, as shown at Drawing no. 30.
She sheet can be made with colours or with graphic symbols, covering the most common types of damages on plaster on facades.

Photoserie III: Damages on plaster decorations:

A Flaking of plaster layers
B Detail of flaking
C Gaps in joints between cement elements

Historic research on the masonry
Besides these technical examinations of the masonry it will be relevant to investigate the history of the facade, that is the previous colours, previous repairs, previous alterations previous plaster coats - or other surface-treatments.
The sources for this can be written documents or records in archives, old drawings, paintings or photographs, but primary, careful investigations and observations on the facade of traces of colours etc.

Restoration attitudes
1 First of all, the original parts of the building must be preserved and protected by restoring the building as carefully as possible. Repair should be preferred to replacement.
2 The maintenance and restoration of the building must be carried out with the same traditional materials and crafts techniques that were used when the building was originally built
3 We must have a thorough understanding of the materials and constructions, regarding both the weak and the strong points, in relations to what is required of them. Therefore  the restoration work must be carried out by multi-dicipli-nary ex-per-tise and specially trained and experienced craft skills.
4 It is important, through the repair- and restoration work to keep up or improve the architectural entirety of the facade.
5 When a restoration implicates severe alterations a brief and concise docu-mentation is required including for instance descriptions, photos, drawings etc., in case information about the constructions one day should be needed. The documentation must be available in a public archive.

The traditional building- and crafts techniques
The traditional building- and crafts techniques involve a number of carefully prepared preventive elements to avoid these life-threatening damages of the buildings. They represent a kind of natural physical balance, which will be destroyed and causes more damages, if unconsidered changes are being made.
The traditional building constructions have 5 preventive elements, which will diminish the chances and cause later damages:
1 Quality-improvement of the materials and micro-constructions by the crafts-techniques. Examples: Hand-wrought iron, quartersawn wood, waterlogged wood, mixing and application of lime mortar, mixing and application of paint, etc.
2 3-double protection of critical constructions and elements. Examples: The joint between window and masonry, the construction of the wooden windows, rust protection of iron, the tile roof, the plinth/socket-construction, the building of wooden balks in masonry etc. 
3 Sacrificial layers: On very critical points, the traditional constructions operate with sacrificial layers or elements, which are easier to change or repair than the vital elements. Examples: The plinth-mortar, the mortar-joint on windows, the linseed oil putty on windows, the traditional paints on exterior wood, the horizontal bottom- or top boards in wooden constructions, etc.
4 Maintenance-signals: The traditional materials and constructions send out distinct ìmaintenance-signalsî, when they need to be maintained. Examples: Linseed oil paint and other traditional paints, lime wash, tile roofs, windows, exterior wooden boards etc. It is very important to know these distinct "signals" and to read them right. Uninformed people often interprete these as serious damages, needing to be renewed - and not just maintained.
5 Repairable elements: The traditional materials and constructions are ìrepairableî, which is a very important quality. If something happens, you can repair parts or whole elements. This gives these elements, for instance wooden windows, a life-long durability. There are thousands of examples of wooden windows lasting more than 200 years - and still in an excellent state of preservation!

There is a lot of examples of how more knowledge and awareness of these 5 important, but often forgotten qualities of the traditional building materials, building constructions and crafts techniques could save both the house-owners and the different countries quite a lot of money.

Repair methods
After a cleaning has taken place at the facade a repair of defective and frost bursted bricks has to be executed: Defective stones are cut out and new hand moulded or sandfaced tiles are bricked in which lead to the best adherence to the following lime wash. Frost bursted joints are scraped out until firm mortar, min. 3 - 4 cm, and a new joint filler is applied. 
Any parts with loose plaster, cracks etc, are cut of. Plaster or mortar joints repaired with cement or mortars containing cement must also be cut out and replaced by lime mortar, as the lime does not adhere sufficiently to cement-based surfaces.
The surface of cement-based mortar is tight sheets of silicates absorbing humidity behind the surface and a humidity absorption that slowly evaporates. The more open lime mortars quickly gives off the moisture again. Lime washed surfaces on which repairs are made with cement mortar will appear very stained in damp weather. Cuttings up to the existing original, but appropriate plaster shall always be made with right lines and right angled corners.
After a cutting out of bricks and joints down to a firm ground the masonry is carefully cleaned of remains from the mortar, dust etc. and water is sprinkled on until the surface is saturated as much as the bricks and joints will be able to absorb a little of the mortar water. The joints are completely filled and a backfill may be necessary too and eventually all joints are compressed with a brick jointer.

Technical principles for plaster repair

When carrying out repairs on existing plaster on masonry or making a new layer of plaster on old buildings, there are 5 main principles, which have to be followed:
1 All repairs on existing plaster on masonry or new coats of plaster should use an air-lime-mortar, without hydraulic additives or cement, or, on certain extreme positions,  a slightly hydraulic mortar. This goes also for repairs of old cement-plaster.
2 The plaster shall be an exact copy of the existing plaster, regarding roughness, colour, surface-character, traces of the tools etc. The mason has to make a sample for a critical comparison, before the whole work is started. Certain research-institutes can analyse the old plaster and determine the type of the binding agents, the sand and other additives.
3 Before adding new plaster, the masonry has to be cleaned of dust, dirt and detached parts by brushing and subsequently the surface is watered carefully with plain tap-water. 
4 Layers of plaster shall be carried out in three coatings or layers: 

A: A rough but thin ground layer of air lime mortar 1:3 (or hydraulic lime-mortar 2:1:9, 1:1:6 or 1:2:9 - depending of the deterioration, weak- or hardness of the background masonry) with quite rough gravel, thrown on the masonry, and left fairly uneven. 
B: After hardening: a 2-3 cm second backing coat with coarse gravel, thrown on and then drawn to an even layer, and after that allowed to harden for one week. 
C: Then followed by a very fine finishing layer/coat with fine grained sand, also thrown up and drawn to an even coating.

Demands for weather and temperature
Mortar-repairs must under ideal circumstances take place during a relatively humidity in the air at 75-95 %. The temperature should be minimum +5 Celsius and maximum +18-20 Celsius. A total absence of wind is also recommendable.

Materials
See chapter 2

Index