Table of contents:
- Stability and beauty
- Quality classes and factors that affect the wood quality
- Avoid structural damage through product samples
- Quality sorting of conifers
- This is how planks and boards are checked
- Structural components
- Sorting classes
- Automatic sorting
- Sorting classes
- Chemical wood protection: the solution?
- Alternative: constructive wood protection
- Buy the right wood
- Different levels of quality
- Buy good wood
- A strong piece of nature
- Steady growth
- Fresh wood works
- Ideal wood moisture
- Dry wood
- Avoid construction damage through care and knowledge
- Possible construction damage
Video: Buy wood: recognize quality and prevent construction damage
The best way to avoid construction damage is to use high-quality wood for house building. With the help of quality and sorting classes, everyone can assess the right quality and then buy the right wood for the respective purpose.
Stability and beauty
Wood is a building material, just as nature lets it grow - it has different properties and qualities, depending on the type and location of the tree as well as on the climate, soil conditions and water supply, tree distance and neighboring plants. Wood of various strengths can also be found within a trunk, and it is more resilient in the longitudinal direction than across the wood fiber. If you process a trunk with branches or twigs, you inevitably cut the course of wood fibers, the stability fades. The load-bearing capacity of components and construction products should, however, be predictable, because they have to meet certain quality requirements. That is why there are quality and sorting regulations for timber construction: Standards or quality descriptions provide tradesmen and builders with reliable criteria for the constructive and / or aesthetic value of load-bearing and non-load-bearing timber - and avoid complaints.
Quality classes and factors that affect the wood quality
Lumber was divided into four quality classes I, II, III and IV according to the old standard. Dimensions, trunk position of the wood (cut from the near-earth part, the middle or near the crown) and natural defects are taken into account. The lower the number, the higher the quality of the wood. Grade IV shows the greatest number of wood defects, they complicate the processing of the wood and reduce its value. Lumber defects are:
Knots: Depending on the number, size and location, knots reduce the bending and tensile strength of timber and attract mushrooms. Well-overgrown branches can also inspire you to create attractive designs for wall and ceiling coverings or furniture.
Tree edges: A tree trunk is cut into lumber in standard dimensions. By adjusting the saw, the saw blade cuts the bark in a straight line, you get "trimmed" wood. If parts of the trunk surface remain on the cut, it is not sharp-edged, but remains stable. Only badly edged timber is often difficult to connect or use as a support and is therefore considered to be less qualitative.
Rotational growth: If the wood fibers run parallel, but at an angle to the tree axis, experts speak of simple rotary growth. It depends on the inclination of the fibers whether the wood is suitable for building.
Printing wood: wind bends trees, the side of the trunk facing away from the wind comes under pressure. There, the cell walls thicken by storing lignin, the wood-based “adhesive”. Such wood is brittle and tends to warp. You can recognize it by its reddish color, which is why the wood defect is also called red hardness.
Harzgallen: There are resin-filled cavities in softwoods (except fir and yew), they reduce the stability of the wood and make the surface treatment more difficult.
Insect infestation: feeding troughs of larvae reduce the wood quality - how far depends on the number of feeding tunnels per meter of sawn timber.
Mistletoe infestation: Mistletoes lower their roots in deciduous tree crowns and softwood trunks and remove nutrients from the host plant. Dead roots leave small holes.
Fungus infestation: Harmless fungi only discolor the wood, dangerous fungi also destroy it - blueness, for example, does not reduce strength, as does discoloration in orange, yellow, pink or light violet. Brownish and whitish stains, on the other hand, indicate to the specialist that the wood is of inferior quality.
Cracks: On the standing tree, as a result of felling, transport or drying, tensions can arise that cause the wood to tear at the expense of its strength.
Various types of solid wood can be found within a trunk.
Photo: iStock / Lepro
Avoid structural damage through product samples
Beauty arises in the eye of the beholder - regardless of good quality. Product samples help the builder and the wood professional to communicate. Non-load-bearing components such as stairs and doors, floor coverings, wall panels or facade profiles are sorted according to various criteria, they take aesthetic and structural requirements into account. A number of standards define quality classes - however, it is difficult to say whether they meet the client's expectations and wishes. In addition, manufacturers often label their products for interior and fitting out with individual sorting or quality classes - they are similar to the names in the standards, but make it difficult to compare them. Clarity is created by material samples, on the basis of which builders and renovators can determine the desired quality without any doubt - the basis for satisfaction with carpenters and carpenters, parquet layers and drywallers.
Quality sorting of conifers
A distinction is made between A and B sorting for profiles made of softwood. If profiled wood does not meet the requirements of B sorting, it belongs to the committee. If you want to save, you can order cheaper B-goods for ancillary and hobby rooms.
|feature||permissible in A sorting||permitted in B sorting|
|Tree branches||unusual branches up to 5 mm Ø, tightly overgrown branches up to 20 mm Ø, small unusual edge branches, small damage to edge breakouts, as far as the coverage is not affected.||unusual and heavily chipped branches up to 20 mm Ø|
|Cracks||End cracks as long as a board width and hairline cracks||larger continuous cracks up to 300 mm in length in the direction of the grain|
|Pith||up to 4 mm wide, 20% of the number may have marrow tubes in a maximum of half the board length||Marrow tube of any width and length|
|Printing wood||To a small extent||as far as laying the profiles is possible|
|Rotary growth||only if not visually recognizable||as far as laying the profiles is possible|
|Harzgallen||isolated up to 2.5 cm2 area, max. 3 pieces, each 1.5 m board length||allowed|
|Insect feeding||up to 5% of the number of pieces may have holes|
|features||slight discoloration up to 10% of the surface at 10% of the quantity||Discolouration and solid colored stripes without blue areas|
|Editing||occasional dug out places and small planing errors like slight roughening next to branches; Tree edge on the lower groove cheek of a profile board over a total length of up to 500 mm - the tongue must not be weakened by the tree edge||Plane errors, as far as laying is possible; pitted areas; Tree edge, as far as the laying and stability are not significantly affected|
This is how planks and boards are checked
Binding standards apply to load-bearing and stiffening components made of wood. They define minimum qualities that wooden building products must have for their load-bearing capacity and ensure the safety of wooden structures.
Standards apply to load-bearing wooden components made of wood.
Photo: iStock / northlightimages
Squared timbers : For economic reasons, crossbars in wooden buildings are increasingly slimmer than in the past - they are considered boards or planks according to their dimensions, but have to withstand bending stress like squared timbers and are sorted according to their criteria. They are marked with the suffix "K", the full names are S 7K, S 10K or S 13K. Boards and planks installed in an upright position must withstand bending loads and therefore have to be labeled with a sorting class - a “K” after the number indicates the sorting as squared timber, for example, the wood is labeled S 7K.
Slats: Identical criteria are used for the grading of slats as for other sawn timber, but there are only the two sorting classes S 10 and S 13 - slats of the higher class must not contain any marrow tubes, in sorting class 10 it is allowed on spruce slats.
Glued laminated timber: Glued laminated timber is classified according to its strength in classes BS 11, BS 14, BS 16 and BS 18, the European names are GL 24, GL 28, GL 32 and GL 36.
Laminated timber: Not all solid wood products are standardized, for the strength value of laminated timber you need individual approval notices or individual proof.
Wood-based materials If wood-based panels are used as load-bearing components, they too are subject to standards - European product standards assign technical classes to the various wood-based materials and regulate their area of application depending on the product and manufacturing process.
Today, sawn timber is divided into sorting classes - this work is carried out by skilled people by visual sorting, it is done like the division into quality classes
• branches, • tree edges, Width of annual rings, • pressure wood, • fiber inclination, • insect caused damage, • curvature, • mistletoe infestation, • cracks, • discoloration.
Sorting is often left to automatic sorting machines with different processes. In addition to knottiness and fiber deviation, they rate:
Elasticity: The wood is bent - and resiliently springs back to its original shape. If permanent deformation occurs due to bending, the wood has reached its elastic limit.
Bulk density: The density indicates the ratio of wood mass to volume without wood pores - it is 1.56 grams per cubic centimeter for all types of wood. The bulk density measures the individual ratio including the wood pores and provides information about the strength, hardness, abrasion and workability of the wood.
Marrow tube: It forms the middle strand of the first annual ring in the trunk and is often cracked. It must therefore be cut out of planks and boards.
After visual sorting, softwood is divided into the three classes S 7, S 10 or S 13, and after mechanical sorting into the four classes MS 7, MS 10, MS 13 or MS 17. The sorting classes of hardwoods are preceded by an "L", they These are, for example, LS 7 or LMS 7. Wood is sorted with a wood moisture content of less than 20 percent - experts then call it “dry”. Standard components made of softwood are given the additional designation TS for dry sorting. The load-bearing capacity of an identical wooden cross-section increases with the height of the number - however, a larger cross-section of a lower class can be just as stable as a smaller cross-section of a higher class. As a rule, wood of sorting class 10 is used for standard components.
Solid construction timber KVH is used for building like sawn timber, but sorted according to more stringent criteria and is therefore considered to be of higher quality.
Deciduous and coniferous trees are sorted into different classes.
Photo: iStock / nzphotonz
Chemical wood protection: the solution?
Only wood with a moisture content of 20 percent and more offers destructive fungi the necessary living conditions, and most insect pests only lay their eggs in fresh wood. In the past, wood pests on structural components were generally prevented using chemical agents. But builders were concerned about health and the environment. Today, wood preservatives are only used for endangered wood, such as building materials with constant contact with the earth or water, and timber with a moisture content of over 20 percent. Technically dried solid construction timber KVH does not need chemical wood protection in rooms with a normal living environment, nor does it need wooden components that are covered by clothing or that can be controlled towards the room. Chemical wood protection is only essential for structures that are accessible to insects and cannot be controlled. Where wood is more at risk of moisture, naturally more resistant types of wood such as larch, Douglas fir and oak are used.
Alternative: constructive wood protection
Architects and craftsmen found their way back to old knowledge and protect wood with clever planning. Constructional protection keeps moisture out or lets it drain away quickly. A wide roof overhang protects horizontally installed wood, such as the balcony, as well as the wall construction and facade. Barrier or air layers prevent moisture from neighboring components of other building materials from entering the wood. Drip edges on the lower end of vertical wooden parts allow water to drain off completely - bevel the wood downwards at a 15-degree angle. Purlin heads are covered with boards because cut surfaces absorb water particularly well. Stainless metals are used for nails, screws and support shoes, they prevent discolouration.
Buy the right wood
Builders protect their property with dry solid construction timber KVH and carpenters and timber construction companies protect their good reputation. Finger-jointed solid construction timber KVH is technically dried to 15 +/- 3 percent wood moisture and leveled to size - this way, even delicate components can be manufactured. Solid construction timber KVH is planed instead of rough sawn, connections are easy to make. These are approved construction products, they comply with the applicable building law and all technical regulations, including the new European timber construction standard.
Different levels of quality
Sawn timber (freshly sawn (over 30% moisture)) may only be used as auxiliary construction timber, semi-dry (between 20 and 30% moisture content) for subordinate structures, dry (20% moisture and less) for general timber construction, e.g. roof structures in expanded attics.
Solid construction timber KVH is installed when dimensional accuracy is important for invisible constructions. Solid construction timber KVH is suitable for closed structures in timber house construction and conventional timber structures.
Duobalken are installed in visible constructions, for example as a ceiling beam or rafter layer with increased requirements for dimensional accuracy.
Glulam is installed in visible and / or highly stressed constructions or with particularly high demands on dimensional accuracy.
Buy good wood
Good wood takes time: it takes almost 100 years for this natural product to mature into a building material with excellent properties. Building with wood is as topical as ever today, for centuries and in the future, builders will benefit from solid construction and a healthy living environment.
In nature, trees often wear crowns that are heavy in size - in the house you wear heavy blankets.
Photo: iStock / piovesempre
A strong piece of nature
Strong as a tree - this is how builders and modernizers want the load-bearing parts of their house, such as the roof structure, ceilings and walls. In nature, the trunk bears its own weight and an often heavy crown, bulky branches and an abundance of leaves or needles. Year in, year out, heat and cold, wet and drought, snow load and wind pressure add to the trunk. Nevertheless: the tree holds up.
Wood consists of three different cell types, each with a different task.
• Narrow, thick-walled strength cells along the trunk give the wood stability.
• Long, thin-walled lead cells also transport water and minerals in the longitudinal direction from the root to the crown.
• Thin-walled storage cells are located mainly across the trunk, hoarding nutrients and building materials and storing metabolic products that process soft sapwood into hard heartwood.
A tree grows by dividing its cells. This creates soft early wood in spring and firm late wood in late year - both together form an annual ring.
Fresh wood works
Fresh wood releases water to its surroundings if there is too great a gradient between the wood and the surrounding moisture. Volume and shape change with moisture, wood shrinks, warps or tears - the wood “works”.
Ideal wood moisture
Depending on the type of wood, location and age of the tree, freshly cut wood contains up to 80 percent moisture - too much to build. Ideal timber is only as moist as the air in its later surroundings, experts speak of equilibrium moisture. It is in accordance with the DIN 1052 standard
• For buildings closed on all sides with heating at 9 +/- 3 percent
• for buildings closed on all sides without heating at 12 +/- 3 percent
• For covered open structures at 15 +/- 3 percent
• For structures that are exposed to the weather, at 18 +/- 3 percent.
Construction wood for walls, ceilings and roof trusses as well as the expansion of residential buildings should have a maximum of 18 to 20 percent wood moisture content in accordance with the standards. Fresh wood must therefore be dried so that it can be used as construction timber.
Architects, planners and carpenters recommend dry timber: It has been shown that a moisture content of 15 to 18 percent is sufficient for the dimensional stability of timber. Dry timber with this ideal moisture content is offered under the design solid construction timber KVH.
Avoid construction damage through care and knowledge
Builders usually conclude contracts in accordance with the VOB contract specification - it regulates the rights and obligations between the client and the craftsman in a balanced manner. By 2004, the VOB timber moisture content had also been more than 20 percent, the revised version from January 2005 contains formulations for the use of dry construction timber up to 20 percent. An estimated 70 percent of all roof trusses are not built in accordance with standards because architects write inaccurate tenders, craftsmen bow to the time pressure of planners and builders skimp on the wrong end. The carpenter uses an average of five cubic meters of timber for a roof structure - dry timber for the entire roof structure costs around 700 euros more than sawn timber. Damage to construction can be prevented with dry solid KVH construction timber.
Possible construction damage
• The building sits overall.
• Excessive cracks appear on visible structures, they can lead to heat loss - tearing wall connections open the building envelope.
• Structural components deform unevenly.
• The rafters of roof trusses twist. This not only looks ugly, but can also lift the roof.
• Wet timbers shrink, causing clothing to tear off.
• Wet wood is susceptible to insect attack, which affects the statics.
• Wet timber is not suitable for closed structures. The moisture cannot go out, but condenses on the outer clothing - risk of mold.