- Practical advice
- Lime plaster in three layers
- Essential records
- Start cleanly
- Three layers, three functions
- Prepare the mortar
- The preparation: tedious but unavoidable
- Promote the catch
- Water masonry
- The application, by spans of up to one meter fifty
- Project gobetis
- Scrape the mortar
- Apply the plaster body
- Equalize the mortar
- Lay the mortar
- The finishing layer: all the finesse... or almost
- Project finish mortar
- Tighten and equalize the mortar
- Scrape with a piece of cleat
- Remove the cleats
- Apply the topcoat
- Equalize the mortar
- Make the finish smoothed
To preserve the character of this house, the restoration of the facade required the application of a traditional coating in three layers, expertise mastered... and dry weather.
Cost: about 80 € / m2 (including all three layers, materials - sand and lime -, equipment and scaffolding)
Time: 4 to 6 weeks
Equipment: trowels, floats, trowels, buckets, wheelbarrow, pulley, concrete mixer, shovel, scaffolding...
The rules to follow
- The control of the thickness of the different layers avoids any risk of impasto. It is enough to provide at the edge of the walls and in the splay windows vertical slats (or "guides"). After controlling their plumb, drag a Mason ruler over each layer (especially the 2nd and 3rd), scrape off the excess mortar and set it straight.
- The dosage of the mortar depends on the indications given by the manufacturer and the nature of the support. Each of the three layers must offer decreasing resistance compatible with that of masonry to avoid microcracks.
Lime plaster in three layers
Repeatedly, this lime plaster in three layers protects and beautifies for decades this house style running around the city.
The choice of binder focused on a natural hydraulic lime, easier to implement than the plaster originally used. The moldings (cornices, frames, corner chains...) are "manufactured" identically (in plaster), from profiles previously recorded.
Lime is the only binder that (with plaster) lets the masonry breathe and allows expansion movements.
Its decorative aspect is well established: it catches the light, gives life and relief to the facades, and leaves beautiful ocher shades.
Before starting the staking of the old plaster, it is strongly recommended to take pictures to preserve the views of the moldings and to make an overall survey (facades and gables). This is to determine their respective measurements (length, width and thickness) as well as their locations (distance between two window frames, height with respect to the ground of the cornice strips...).
It is also essential to take fragments of moldings and to raise the profile to guarantee a restitution to the identical
The first step is to pick the front with a chisel and chisel to expose the masonry.
However, the large area to be deposited here requires the use of a perforator-burineur.
This done, fragments of plaster and friable mortar joints still in place are removed using a piochon.
The exposed masonry is washed with a water jet (or pressure washer set to a minimum), before the application of the first layer of the plaster.
A hexagonal wire mesh is fixed using galvanized steel tips.
The interest of this lattice: facilitate the grip of the first layer (thick 8 to 12 mm instead of 5 to 8 mm usually).
This extra thickness is made necessary by the irregular surface of the facings.
The humidification of the masonry depends on the good behavior of the coating.
Its purpose is to prevent the rubble from excessively pumping the water contained in the plaster.
It is therefore necessary to water until the surface of the stones is saturated.
In case of big heat, it may be necessary to renew the operation.
Three layers, three functions
The new coating is done in three layers.
Each has a precise function and dosage.
The bond coat (gobetis) is a mortar rich in lime, very fluid, and sprayed vigorously by hand.
It is integral with the support and serves as a support surface for the next layer.
Its thickness is here a little higher than normal, masonry a little heterogeneous can not be as rigorously plumb as a brick wall!
Its composition is about 35 kg of lime for 80 kg of sand.
The second layer (plaster body) is the thickness of the coating and ensures flatness.
It gives it its final shape by erasing surface unevenness.
It favors especially the impermeability of the masonry.
It is compact and homogeneous, with a less rich dosage than the gobetis (about 35 kg of lime per 110 kg of sand) and it forms a thickness of 6 to 8 mm.
It is applied about a week after the first coat.
The third layer (finish) brings the final color.
Less binder-rich than the plaster body (about 35 kg of lime per 140 kg of sand), it is also thinner (6 mm thick).
The application is troweled two to five days after hardening of the plaster body.
It is finalized with the trowel, during the beginning of the grip (to be tested by pressure with the tip of a tool).
Finally, circular passages with a wet sponge allow to slightly highlight the grains of sand.
Prepare the mortar
The binder used for the first two layers is natural hydraulic lime (pure white lime "Cesa NHL 3,5" from Saint-Astier).
In finishing, the choice fell on a lime of the same nature but colored ("Colorchaussable" of Saint-Astier).
This ready-to-use binder is formulated from pure lime, very fine sand and dyes.
It is therefore wasted without sand, only by adding water (about 7 liters per bag).
Added with sand, it is first streaked dry before adding water.
Concrete mixing allows the site to be supplied in regular quantities (homogeneous paste).
The sand and lime are loaded dry in the rotating tank, using the same container or tool (bucket or shovel), so as not to distort the dosages.
The water is added gradually (here with the help of a pipe).
To facilitate mixing, it is advisable to aim the walls rather than the center.
The mortar must be creamy and hold without running on the trowel.
A mortar that is too dry is weak and tends to crumble.
On the other hand, if it is too wet, it causes a significant withdrawal.
The preparation: tedious but unavoidable
A hammer-hammer facilitates the exposure.
Otherwise, the old plaster is removed with a chisel and a mallet.
Remnants of plaster are eliminated with the piochon (small peak).
Promote the catch
A medium mesh, fixed with galvanized nails, favors the attachment of the first layer (thick enough to cover the rubble).
The day before the first layer, the masonry is watered with large water, until saturation.
Just before the start of the work, it will be rewetted quickly.
The application, by spans of up to one meter fifty
In the concrete mixer, 30 kg of lime and 50 kg of sand are mixed dry for about five minutes.
The mixture is progressively moistened until a fluid paste is obtained.
The gobetis (1st layer) is projected with a dry and precise gesture to adhere well to the support.
The operation always starts at the top of the wall, in spans about 1.5 m wide.
Scrape the mortar
Using a board with a handle, like a large trowel, the mortar is scraped from bottom to top.
Then it is tightened with the back of the trowel.
Apply the plaster body
The plaster body (2nd layer) forms the thickness of the coating.
It erases the surface inequalities.
It is applied by spraying or by plating the mortar with the back of the trowel.
Equalize the mortar
Circular and vertical movements (from bottom to top) with the trowel allow to equalize the mortar.
This operation facilitates the training and plumbing to the rule.
Lay the mortar
Leaning on the cleat frames between the bays, the mason ruler slides over the surface of the mortar to set it straight up.
The finishing layer: all the finesse... or almost
Cleats (6 mm thick), placed between the windows and held by mortar pads, will serve as a guide for preparing the finishing layer.
They must all be perfectly level.
Project finish mortar
The finishing mortar (3rd coat) is troweled or pressed against the surface with the back of the tool or with a trowel to a thickness of about 6 mm.
Tighten and equalize the mortar
During the beginning of the setting, the mortar is tightened and equalized with the edge of the trowel.
It is held inclined to raise the mortar upwards.
Scrape with a piece of cleat
When the surface is too narrow to use the large aluminum ruler, a piece of cleat with flat edges is used to scrape and level the mortar.
Remove the cleats
Once the finish is applied and leveled, the guide bars are removed, taking care not to damage the surface.
The void is filled with mortar and the excess scraped with a ruler.
Apply the topcoat
The site at the mid-point of the application of the finishing layer, carried out from top to bottom, moving forward with successive vertical bays.
Equalize the mortar
The mortar is leveled and tightened with a metal smoother dipped regularly in water to keep a clean surface, and handled in circular motions.
Make the finish smoothed
The smooth finish is achieved with a wide stainless steel spatula.
It slides on the surface to eliminate small asperities and equalize all.