Unlike in Poland, in Western European countries, "economy method" construction is a niche part of the construction market. Even in the case of independent construction, packages of prefabricated elements (framing or panels), prefabricated and supplied with installation instructions, are often used in the West. In Poland, on the other hand, there is still a popular belief that a house built "by the economic method", without a complete project, without practical participation of professional labor and without a construction manager, and built with the cheapest(often not permitted to use materials) is the best and cheapest construction option. How erroneous such a belief is is evidenced by later complaints and often the need for costly repairs.
So what are prefabricated houses and what advantages do they have?
Prefabrication in the construction industry means the preparation under factory conditions of individual parts of a building in varying degrees of sophistication. Objects so produced are not only single-family houses, but also multifamily houses and public buildings. A growing area of prefabricated building use is various extensions and superstructures of existing buildings. An important feature of prefabricated buildings is that they can be assembled regardless of the season.
Prefabricated can be elements of the structure - in the form of skeletal structures or panels ready to be assembled at the construction site, prefabricated can be partitions (walls, ceilings), as well as whole modules in varying degrees of finishing. For both prefabricated partitions and modules, it is possible to have factory plumbing, insulation and single or double-sided finishes.
The most advanced option is finished modules with complete equipment for kitchens, bathrooms, etc. Modules are adapted in size to the transportability and individual conditions of the construction site.
Installation of prefabricated wall units (left) and prefabricated module (right), wooden structure
photo: archive of the EDG Association
Modular construction is beginning to gain popularity around the world, and it is worth noting that Polish companies also have their recognized place in the construction markets of Europe.
Regardless of the material used, prefabrication significantly speeds up the construction process - of course, the faster the implementation, the greater the degree of prefabrication. A prefabricated modular building with a small area (on the order of several dozen square meters) can be ready in the evening of the same day it was delivered to the construction site. We are, of course, talking about pre-fabricated and pre-furnished modules, mounted on a pre-prepared foundation slab.
Prefabrication uses include:
- solid wood, timber for wedge joints or glued laminated timber and steel in the case of frame structures, both for the simplest prefabrication, for assembly on site, and for factory-fitted finished wall and floor elements and module production,
- glued laminated timber or cross-laminated timber (CLT), expanded clay, concrete when producing massive wall elements,
- glued laminated timber or cross-laminated timber (CLT) and concrete in the production of massive prefabricated floor elements
Installation of a multi-family modular building with a wooden structure by a Polish company
photo: archive of EDG Association
Prefabricated wooden houses
The most popular type of prefabricated houses are timber-framed structures, so we will take a closer look at these types of houses. They owe their popularity not only to their lightness, speed of construction, warmth and good indoor climate, but also because they fit in with global ecological trends. In addition, high fire resistance (up to 120 minutes) can be achieved in wood construction without additional protection, only as a result of proper design. In discussions about the pros and cons of wooden-framed buildings in comparison with steel construction, there are always arguments about fire safety (non-flammable steel under the influence of high temperature "flows", losing its load-bearing properties, and combustible wood burns around the perimeter, maintaining load-bearing capacity for a long time and allowing safe evacuation). The problem of wooden structures is biological corrosion, steel structures - rust. But for this, wood is a 100% renewable material, and the primary producer is nature, man only carries out its processing and prefabrication.
Regardless of whether single- or multi-family houses or public buildings are to be built, similar design-production-execution regimes must be met. Since in Poland the knowledge of wooden structures is still insufficient - work and activities are being undertaken to popularize not only wooden construction, including prefabricated construction in itself, but also the requirements it must meet.
For many years, theAssociation of Energy-Efficient Prefabricated Houses has been actively promoting good practices in wood construction, conducting numerous training courses and participating in the development of publications in this field. Among other things, last year (2022) EDG developed a series of three publications on behalf of the Ministry of Climate and Environment: Timber Construction, a guide for the designer, for the contractor and for the investor (a collection of good practices in the field of timber construction with a focus on structures). The guides are free to download on the domzklimatem website.
Awell-designed house is one whose design includes calculations and drawings of all structural elements and connections. The construction project-because of the extremely poor required content-is not a blueprint according to which anything can be built. The basis of a durable and safe building is the structure, both in terms of load transfer, limit state of use, as well as in terms of airtightness and thermal performance.
The design of a wooden structure is carried out exclusively using the Polish version of Eurocode 5 (PN-EN 1995-1-1:2010+A1:2014). The old Polish standards (PN-B 03150) have not been applicable for many years (despite the theoretical permission to use them until the end of 2020). And in order for the construction to be durable and safe - it must be made using the right materials and in accordance with the design (and not - according to the imagination of "Mr. handyman").
The wood used must meet the requirements of the relevant harmonized standard and bear the CE marking. Thus:
- solid wood must comply with EN 14081-1+A1:2011,
- wood for wedge joints must comply with EN 15497:2014,
- glued laminated timber and glued solid timber must comply with EN 14080:2013,
- cross-glued timber must comply with ETA (European Technical Assessment), ultimately with EN 16351:2021.
Important - DIN (German) standards and related nomenclature and certifications do not apply in Poland, despite the misconception of some designers and dealers.
Since there is a lot of false information circulating on the market - it should be made clear that the commonly available class of solid wood is C24, and the assignment of wood to this class (as a result of visual or machine sorting) is linked to the guarantee of certain strength parameters. Class C24, on the other hand, does not mean that the wood is dry, planed or has any fire resistance. Wood is dry if it has been dry-sorted (and then the labeling on each piece must say "dry graded") and stored properly. So-called "general purpose lumber" must not be used for any structural elements (including battens) - i.e. wood not strength-sorted.
The wood will be planed, if such is ordered, and the fire resistance of the structure is due to the proper design of the elements and joints. Required by the Ordinance on Technical Conditions for most structures (except, for example, single-family houses), nonproliferation of fire (NRO) is achieved by impregnation with agents that meet the relevant requirements - the old ITB Instruction of 2004 does not apply.
Note that class C27 is available rarely, and only in the case of machine grading, class C30 in practice is very difficult to access, and higher classes are basically unavailable. The highest standard grade of laminated timber is GL32c or GL 32h, but in practice the highest grade used is GL 30c or GL 30h.
The wooden structure of prefabricated partitions or modules is filled with insulating material and clad with boards (e.g. OSB, gk board, gypsum-fiber board or cement-fiber board) with appropriate insulation. When selecting sheathing boards, attention should be paid to the nature of the room, including anticipated moisture, as well as the proper location of joints.
Brzechwa prefabricated house
© Brzechwa
Professional manufacturers of prefabricated houses have this knowledge, guaranteeing proper design, material and workmanship. Production is under strict control, with all prefabricated elements being made with great accuracy under factory conditions that ensure stable moisture content. Thanks to the reproducibility of the manufactured elements and constant control, the occurrence of errors is minimized, and if they occur - they are eliminated immediately. In order to choose a good manufacturer, it is worth reading references and user reviews.
Prefabricated house - regardless of the type of its construction, will be built much faster than built from scratch on the construction site and will guarantee a higher quality of workmanship. In the case of a wooden house, the wood, thanks to factory prefabrication and quick assembly, will not be exposed to long-term exposure to adverse weather conditions. Thus, prefabricated construction is the construction of the future if we focus on quality and speed of construction.
