- What is a dry screed?
- What are the advantages of a dry screed?
- How to make a dry screed?
- Some preparatory work
- We make separating coatings
- Making a substrate
- We mount sheet material
- Drawing conclusions
When it comes to floor screed, a picture immediately comes to mind when a team of construction workers of several people, accompanied by the noise of a constantly rotating concrete mixer, carries and levels the cement mortar. People are exhausted by hard work, everything is dirty, wet … Is there an alternative? How about dry prefabricated floors?
What is a dry screed?
“The space between the walls, where it is supposed to arrange an artificial floor, is covered with thin layers of dry earth, which should be tamped tightly, and the top layer should be leveled under the spirit level. On top of this layer, along the width of the floor, are placed wooden blocks 2 inches thick squared, at a distance of 1.5 to 2 yards from one another; the space between them is filled with a layer of crushed brick, pieces of which should be from 1.5 to 2 inches in volume. When leveling this layer, one must observe that the pieces do not come out in the least from the upper plane of the bars, for which it is better to level them with an iron rake and slightly kill with a wooden rammer, which is done partly and in order to force the sharp ends of the rubble to ascend into the upper layer of the earth. When the whole space is so filled up, they take out the wooden blocks and the space they occupy, they fill it up with the same brick rubble, equalizing it with the rest of the space, as mentioned above, with a rake and a rammer … “.
So in the 11th issue of the magazine “Rural Builder” for 1852, one of the flooring options is described, which has much in common with modern dry screeds. Here we see backfill, and beacons, and ramming – although the crushed stone is large, and instead of sheets of gypsum fiber board, the author talks about the technology of pouring a plaster coating right at the facility. Of course, it is far from the systems of prefabricated bases presented in our time, but these differences relate more to the material part than to technological processes. Dry floors made of cement-bonded, chipboard and gypsum-fiber boards were actively used in Soviet times, however, this technology reached a new quality level after the appearance of Knauf on the domestic market with their complete system “Superpol”.
Basically, a dry screed is a floating subfloor that is not rigidly tied to the ceiling and load-bearing walls. This method of flooring is based on the fact that all materials used are placed in their places dry, without any processing and addition of water, wet processes are completely excluded. The main elements of the technological cake of such floors are a leveling backfill or a layer of insulation, as well as sheet material laid on top and finishing. In certain cases, a film hydro and vapor barrier is added to them..
What are the advantages of a dry screed?
This technology is indispensable when the repair time is tight. Such floors are installed very quickly, in addition, the topcoat can be laid almost immediately after the installation of the prefabricated base, while after pouring the traditional cement-sand screed for the rest of the work, it is necessary to wait for it to dry completely – and this is about 28 days. The absence of wet processes allows making dry screeds even at low temperatures, from -5 ° C to + 5 ° C.
Prefabricated subfloors, thanks to the floating structure, provide the floors with excellent sound insulation characteristics. Dry screed has a very low thermal conductivity.
The low weight of such floors significantly reduces the load on all load-bearing elements, which allows the use of lighter, correspondingly cheaper building structures. They are successfully used on wooden floors in the construction of new cottages, even when repairing floors with a large difference in height, as, for example, in houses of the Stalinist period.
The dry screed is well suited for the restoration of floors. It can minimize the effects of building shrinkage. It is arranged on movable bases, without the danger of cracks, breaks, creaks. It is not necessary to remove old coverings such as floorboards or parquet floors. Due to the fact that during the installation of prefabricated floors there is no dust, dirt, as well as a large amount of moisture from the solution, they can be produced even with clean walls, for example, in a residential apartment or a functioning office.
A dry screed cake can easily accommodate various engineering and technical communications, floor heating elements.
The prefabricated base can withstand high operational loads, both distributed and point (up to 1000 and 500 kg / m2 respectively). Plasterboard partitions can be installed on top of the dry screed.
If we consider the economic component according to the principle “cost of materials + cost of work”, then in comparison with floors on logs and cement-sand screeds, prefabricated floors are much cheaper.
The dry screed installation system is not as time consuming as cement grouting. It is technically quite simple, therefore it is available not only to professionals.
How to make a dry screed?
Some preparatory work
Work on the installation of a dry screed always begins with a thorough preparation of the base, this is especially important during repair and restoration. The first step is to completely remove the old floors if they are completely worn out or if the level in the room needs to be lowered / restored. The overlap must be freed from construction debris, on its surface there should be no sharp protruding parts that could damage the moisture-proof film. All cracks, large potholes and irregularities are sealed with a strong, fast-setting mortar, special attention is paid to the places where the floor joins the load-bearing walls, slab joints.
If a decision is made to make a prefabricated base on top of old floors, then they should be cleaned of moving parts, large gaps should be sealed with materials suitable in each case.
Attention! To avoid the appearance of squeaks, solid wood flooring and parquet should be strengthened with nails or self-tapping screws..
The next step will be to determine the floor level and, accordingly, the height of the technological cake. Geodetic level marks are applied to enclosing structures in the area of corners, both internal and external. If work is carried out in several rooms, the floors of which will have the same level, then in each of them marks should be placed at the same height. For the convenience of taking measurements, they usually take a height of about 1.3 – 1.6 meters from the base. Connecting the control points with a chopping cord, along the perimeter of each room we get a solid starting line, from which, using a tape measure, you can very easily set beacons and check the horizontalness of the planes.
When calculating the possible height of a dry screed, one should start from either the level of the finished floors to which it is necessary to adjoin, or from the technological features of the prefabricated floor, such as:
- the need to lay communications,
- the presence of a layer of insulation,
- the degree of unevenness of the base.
In any case, the starting point will be the highest place in the room or group of rooms being renovated. Note that the minimum thickness of the entire structure of the prefabricated base is considered to be 35 mm, of which 20 mm are the finishing sheets, and 15 mm are backfill or porous-fibrous, foam materials.
For temperature and humidity adaptation, all elements of the system should be brought into the installation area in advance, at least one day in advance..
Attention! Sheets should only be stored horizontally on a very level surface.
Before the start of the prefabricated flooring works, wet works, such as plastering, must be completed, all communications laid.
We make separating coatings
The dry screed technology does not require the obligatory laying of roll materials on the base, however, sometimes there is a need to protect the floor cake from moisture entering the ceiling from adjacent structures, from not completely dried concrete, or if a room with a high level of humidity is located on the floor below. Dense polyethylene with a thickness of 200 microns is used as a waterproofing over floor slabs, monoliths, cement screeds. The film is rolled out with an overlap of about 20 cm, the joints are glued with packing tape. Each strip of polyethylene is cut with a margin so that you can wrap it on the walls along the height of the precast floor.
Wooden floors and old plank floors are covered with impregnated construction paper, sometimes in combination with vapor barrier membranes. In this case, the task is to prevent fine-grained materials from spilling into the gap between the boards and blowing through the backfill layer. Like plastic wrap, the paper is laid with an overlap; with significant unevenness in the base, the joints are glued.
In order to take full advantage of all the advantages of a floating base in terms of sound insulation and create a completely independent system that can withstand thermal expansion without deformations, it is necessary to leave a technological gap of 10 mm between the prefabricated floor base and the enclosing structures, be it load-bearing walls, pipes, columns. To ensure such a gap, an edging tape made of foamed PVC is attached to the walls; strips of mineral wool, polypropylene foam and other elastic materials can also be used. The width of the gasket should be no less than the thickness of the floor pie, the excess can be cut off at the end of the installation.
Making a substrate
It should be said that the technology of dry floors does not imply the obligatory use of bulk materials. On a flat or leveled base, mineral wool with a density of at least 130 kg / m can be used as a substrate2, as well as expanded polystyrene, extruded polystyrene foam.
If there is a need to improve the sound and heat insulation properties of the floors, then foam and porous fibrous materials can appear in a combined version with traditional backfill, if, of course, the allowable design height of the floor is allowed. With such a system, the surface is first leveled with bulk materials, then an interlayer of one sheet of gypsum fiber board is laid, then insulation and finishing sheets.
Practice shows that the most common substrate for prefabricated bases is still backfill, especially since it is not only convenient for leveling the floor, but also has certain sound insulation properties and reduces the thermal conductivity of the floor..
The backfill for dry screeds must meet certain building codes. These can be bulk materials with a granule size of no more than 5 mm, which ensures their minimum shrinkage. They must have a density of not more than 500 kg / m3, compressive strength not less than 2.5 MPa and humidity not more than 1%. For these purposes, mineral materials are used with high porosity, but at the same time low hygroscopicity:
- crushed stone from blast furnace slag,
- expanded vermiculite,
- slag pumice,
- expanded sand perlite,
- expanded clay screening.
Leveling of the mineral backfill is carried out according to the rule according to the beacons, set in accordance with the design level of the finished floor. The most convenient and cheapest way to make lighthouses is from a galvanized CD profile..
Attention! Any beacons after filling must be removed, since the shrinkage of the granular material can be up to 5%, due to which the finishing sheets will rest on the rigid edges of the guides, and not on the flat area of the substrate, which inevitably leads to deformation and destruction of the coating.
It is recommended to compact the bulk material step by step with hand rammers, especially in the presence of large layers and in places with the greatest traffic, for example, in doorways.
Recall that the minimum layer of a free-flowing substrate at the highest point of the base can be at least 15 mm. With a backfill height of more than 60 mm, it is necessary to separate the layer with an expansion GVL board. For the lower layer in such cases, you can use a backfill material of a larger fraction..
If the installation of a dry floor is carried out in stages, then in order to prevent spilling of granular material, subsidence of planes in places where work is interrupted and in doorways, it is necessary to install bulkheads from scraps of sheets of gypsum fiber board, boards.
We mount sheet material
For these purposes, asbestos-cement plates, OSB, chipboard, moisture-resistant plywood, moisture-resistant gypsum fiber sheets (GVLV) are used.
Lay the sheets of the prefabricated base on top of the leveled backfill or insulation plates as tightly as possible to each other, in the direction from the front door to the opposite wall. They are mounted in two layers with bandaging of the joints of the lower sheets with the upper ones, the ready-made Knauf-superpol elements do not require this, since they already have a two-layer structure with an offset of 5 cm.
Attention! The joints of the sheets should not coincide with the joints of the insulation boards.
Each row of the first layer is installed with bandaging the joints of the previous one by at least 250 mm. It is recommended to lay the sheets of the second layer perpendicular to the direction of the sheets of the first layer.
We do the trimming of the finishing sheets in such a way that the damper tape installed around the perimeter of the room is not squeezed, that is, with a 10 mm gap near the walls.
Sheet material of different layers is fastened with PVA, or other adhesives and mastics, which are applied with a notched trowel with a comb height of 2-3 mm. As additional fasteners, stainless self-tapping screws are used, with which sheets of different layers are screwed to each other with an interval of 100-150 mm. In most cases it is necessary to pre-drill through holes and sweep recesses. To work with GVLV and finished floor elements, special self-tapping screws with a double thread are used.
It is recommended to scroll the cover under load. You just need to stand on the top sheet near the entry point of each self-tapping screw.
All movements along the backfill should be limited as much as possible. As a last resort, during installation it is necessary to move along the islands of drywall scraps.
Before laying each sheet, the loose substrate should be additionally compacted with a metal trowel. The quality of installation of dry floor elements must be monitored during operation with a rack level or a rule with a length of 2 meters.
It is not recommended to join the prefabricated base sheets along the line of doorways. To do this, in such places, with bandaging and careful scrolling with self-tapping screws, an insert is made, which at least 250-300 mm goes into adjacent rooms.
If thin-layer coatings are to be used over the dry screed, then the joints of the sheet material, as well as the heads of the self-tapping screws, should be putty and sanded. In a room with high humidity, prefabricated floors must be covered with a coating waterproofing, the abutment of the dry screed to the walls is sealed with elastic sealants, such as silicone, acrylic, polyurethane.
So, as you can see, prefabricated floors are easy enough to assemble. The technology of their device is really simple and does not require complex design solutions, the use of special tools and mechanization means. Not only is it indispensable under certain conditions, it is also as accessible as possible in terms of financial and labor costs.
In fairness, we note that the dry screed has some disadvantages and restrictions on the use. Heavy partitions made of bricks, gypsum blocks and similar materials cannot be built on such bases. Prefabricated floors do not tolerate flooding very well, water penetrates into the layers of the substrate and the appearance of mold, unpleasant odors is possible – in other words, not only the topcoat suffers. A dry screed cannot be made at an angle due to the danger of the bulk layer slipping and subsequent deformation. Do not use such floors in rooms with active dynamic and vibration load..