- Features for facade coatings to take into account
- Which coating for which support?
- Respect the implementation of coatings
- 4 points to know to apply a façade coating
- The opinion of the expert on facelift
- Mistakes not to be made when laying a facade
- How to project the coating to the projecting machine?
The first visible element of a house is its facade which must be impeccable. With a wide range of colors and finishes, the exterior coatings combine protection and decoration.
The facade coating allowsstandardize, protect and decorate exterior walls. Composed generally of cement mortar or lime, it is applied in one to three layers directly on the support. In addition to the aesthetic aspect, its role consists mainly in waterproof masonry while letting it breathe. If some prefabricated or slab concretes can be left in the rough, masonry of concrete blocks, hollow bricks, perforated blocks or cellular concrete must be coated; indeed, these materials do not have sufficient impermeability. The water absorption by capillarity, coded in three classes (W0, W1 and W2), partly defines the level of waterproofing. This protection is higher or lower depending on the number of layers and the final thickness of the coating. For example, on a facade heavily exposed to rain, it is better to opt for a coating with low capillarity (W2).
Features for facade coatings to take into account
But the exposure of the house is not the only criterion of choice. The nature and the intrinsic characteristics of the support (concrete, concrete block, cellular concrete, brick, stone...) are the elements which determine the type of coating to be used; the latter must necessarily be compatible with its support to have a guarantee of holding in time. The price per square meter depends on various factors: type of facade to be coated, surface, height (the costs of scaffolding are greater), protections to bring (masking films, window frames...), etc.
Which coating for which support?
Coatings on new support come in two families: traditional coating or monolayer. The traditional coating and multilayer plaster, includes industrial mortars and mortars prepared on site. It is characterized by an application in three layers: the gobetis serves as a hook, the body of plaster (in one or two passes) ensures the flatness, the waterproofing of the support and the decorative finish. This coating can also be done in two layers (without gobetis) if it is sprayed mechanically.
Themonolayer coating, much faster to implement, is formulated to ensure both dressing, waterproofing and finishing. Its compatibility with the support is determined according to the standard NF EN 1015-21regardless of the declared strength class of the coating. CS IV * single-layer coatings and industrial mortars are not not suitable for the renovation of old façades (rubble, stone, brick terracotta...).
For these substrates, there are lime plasters (aerial or hydraulic), bastard coatings, plaster and lime plaster renders (MPC) and plaster coatings. Lastly, there are also specific mortars for old earthen facades (rammed earth, mud, cob,...) and for buildings classified as historic monuments that benefit from special regulations. The application is done manually (trowel pier) or mechanically (pump to project, zip line). It is recommended to wet the day before and to moisten the same day, especially during hot weather.
* To define the compatibility between the traditional plaster (multilayer) and its support, it is necessary to take into account the three classes of resistance to tearing (Rt) of the support and the four classes of resistance of the plaster (CS) according to the nature of the support to be coated. CS IV = compressive strength> 6 N / mm2.
Respect the implementation of coatings
Each type of coating corresponds to precise implementation. It is therefore imperative to respect the dosages and mixing times indicated, but also the application thicknesses to guarantee the waterproofing. Professional rules for the application of mineral masonry waterproofing coatings are defined by the standard NF DTU 26.1. The regulation specifies, through various tables, the dosages for each layer to be made, the drying times between the different passes and the minimum thicknesses to be respected. These indications can also be obtained from the manufacturer.
4 points to know to apply a façade coating
To avoid possible disorders, the standard defines the coatings and their compatibility with different facade materials.
- Through its unified technical document (NF DTU 26.1 P1-1), façade plasters in cement mortar, hydraulic and aerated lime, mix of plaster and aerial lime are applied to the following substrates: new or old stone masonry, terracotta brick, rubble stone, stone seen, mounted with weak mortar or plaster, concrete (block, cavernous, lightweight aggregates...), autoclaved aerated concrete.
- The work can be carried out between +5 and +30° C for mortars containing a hydraulic binder (cement or lime and cement) or between +8 and +30° C for lime-based mortars and colorful coated decorative finish.
- The coating that is not composed of a hydraulic binder base or capillary must be stopped at least 15 cm above the ground. This value differs according to whether the capillary cutoff of the masonry is higher or lower. As for the high dose coating with hydraulic binders or low capillarity, it can be applied to the finished ground level.
- The final thickness of a monolayer plaster should be 12 to 25 mm on a neat masonry and 15 to 18 mm on a common masonry. In no case may the final thickness of the coating be less than 10 mm. As for the multilayer coating, the average thickness of its body (with gobetis) is between 15 and 20 mm. Depending on the desired final appearance, the thickness of the topcoat is generally between 5 and 8 mm.
The customization of facades has been in fashion for several years so that manufacturers have developed complementary accessories to achieve various finishes.
It is now possible to reproduce a design in the cool of the coating thanks to dies and other stencils imitating different materials.
To avoid mistakes, manufacturers specify on their packaging the strength class of the coating, the type of support for which it is intended, the method of implementation, the color, etc. Example: OC 1 means that the coating is compatible with all substrates.
The opinion of the expert on facelift
"The facades have to be cleaned every ten years, but given the cost of the operation, every fifteen years an average wall is cleaned up, so it can be interesting to invest a little more for example in an ITE ** to optimize the insulation of his house to reduce his energy bills. "
* Romain Girard, Front / ITE VPI Product Manager at Vicat
** Thermal insulation from the outside
Some manufacturers make available the data sheets of their products.
Downloadable for free, they provide information to make informed choices.
Depending on the application method and the finishing techniques, it is possible to obtain an infinity of results ranging from "raw" to "fine float".
Mistakes not to be made when laying a facade
Preparation, implementation, drying... each coating has its own specifications which must be respected. Otherwise, the protection against bad weather becomes ineffective and the final rendering very unsightly.
PreparationExcess mixing water
Increased shrinkage, risk of cracking.
Increased stiffness, risk of cracking.
Risk of crazing and / or shading.
Risk of separation.
Weather too cold and wet
Risk of efflorescence especially on decorative plasters and lime renders.
Weather too hot and wind dry
Risk of "roasting" (the coating becomes brittle).
Risk of cracking.
Risk of "spectrum" phenomenon and cracking.
Mortar too rigid
Risk of separation on the support.
Risk of shading and cracking due to the rise of laitance on the surface of the coating.
Risk of shading due to poor dressing of the coating or irregularities of the substrate. Sensitive to dirt because of its high roughness.
Risk of shading due to variations of angle, distance, air pressure...
For more information: Construction Quality Agency (qualiteconstruction.com).
How to project the coating to the projecting machine?
- The coating is prepared in a bucket.
- Water is added gradually when the mixer is in action.
- The consistency of the mixture should be liquid.
- The casting machine is made of stainless steel or galvanized.
- A crank turns a hedgehog, placed inside and equipped with slats that eject the material.
- The wall must be moistened a few hours before the projection of the first plaster run.
- The machine is placed parallel to the support.
- The two following layers are realized by projecting the coating according to a 45° angle compared to the plane of the front wall.