Premium Torx drive Decking Screws For Hardwoods and Softwoods. Available in A2, A4 316 and hardened stainless steels.
Pilot drilling prevents the wood splitting
Pilot-drilling is always better with problematic woods. These are above all hardwood/tropical woods, but also some coniferous woods that tend to crack easily, such as eg Douglas fir.
Because of constantly occurring problems with the use of hardwood/tropical woods we want to point out some fundamental working guidelines that must be observed. However, we refer in general to the recommendations of your wood dealer, because there can be extreme fluctuations in the wood properties with the same wood type, above all with tropical woods. Bangkirai wood, for example, which is often used, can have very different properties, because the properties depend heavily on the source in each case. If the variety of wood properties within a range is ignored, this can lead to various problems with regard to screws breaking off.
With regard to the edge distances make sure that there is at least 6 cm clearance to the end of the board.
(Please note: because of the high internal stress the boards can also crack open later at the ends and in the middle. This also applies to thermally treated woods).
At a width of 140 mm, Bangkirai woods or other hardwood/tropical woods can swell or shrink by up to 7 mm, depending on the wood moisture. With direct screwing through the boards into the substructure we recommend using a pair of screws. If the board is fastened directly on the substructure and the board works from the centre by about 3.5 mm, this leads in some cases to the screws being sheared off. The hardwood/tropical wood does not allow the screw to absorb any movement because it can barely be compressed because of its own high density.
Although deck/wood construction screws today have a suitable deflection angle, hardwoods that are placed directly on top of each other function as shearing modules that shear the screws off if the wood swells or shrinks. (Per board half = 3.5 mm displacement, this conforms to about the inside diameter of a screw with a 5 mm thread, which is the minimum that should be used with tropical woods).
In certain circumstances, screwing in the centre of the board might be deducted from this. Unfortunately, tropical woods have an extremely high internal stress, which leads to the boards twisting (dishing), which in most cases requires pairs of screws.
However, using a spacer (eg distance strip or deck glider) between the substructure and deck board is very helpful here. This provides the screws with a possibility of bending in the direction of the working wood. The danger of shearing is greatly reduced. In addition, this clearance protects the wood from waterlogging at the support points. The ageing process is slowed down clearly.
A mistake that is frequently made is to have centre distances in the substructure that are too large. The most durable results are achieved if this clearance, and therefore the screw clearance in the lengthwise direction of the boards, is max. 60 cm.
Please note that the installation information provided here is merely a recommendation and does not constitute binding assembly instructions. Every assembly job is subject to different performance requirements, e.g. locally applicable building regulations, and the tradesman carrying out the installation is responsible for compliance with regulations.
Expert hints: Hazards in the construction of timber decks The various timber types differ from one another not only in their appearance but also in their technical properties:
• One particularly important property of wood with regard to deck construction is dimensional stability (also known as “resilience”). Experts use this term to refer to the property whereby wood changes shape in the course of use due to swelling or shrinkage.
The various timber types exhibit different degrees of dimensional stability. For this reason, special attention must be paid to the choice of timber type. For deck construction, we recommend using timber with high dimensional stability. Some timber types, including Massaranduba, exhibit lower-than-average dimensional stability, so we expressly advise against using these timber types for deck construction. Since, from an absolute perspective, the swelling and shrinkage behaviour increases as the width of the timber boards increases, we also recommend a maximum board width of 120 mm.
You can find details of the dimensional stability of some common timber types in the “Overview of timber types” on p. 35-38 of our catalogue, as well as on our website.
• Rift-sawn planks should always be used in preference to flat-sawn planks, as they have considerably better properties with respect to cracking, splintering, swelling and shrinkage, as well as dimensional stability, and therefore tend to distort and warp less. Often, so-called flat-sawn planks cannot be fastened permanently with either visible or hidden methods. In such cases, we cannot guarantee permanent fastening.
• Even fine particles of abraded metal can lead to dark spots of corrosion on the timber boards. Metalwork should not therefore be carried out in the direct vicinity of the deck.
• Constituent substances in the timber can cause contamination of adjacent surfaces; it is therefore important to take constructive precautions, such as maintaining sufficient distances from nearby components.
• As nature does not adhere to quality guidelines, the suitability of timber for deck construction does not depend solely on the timber type. Often, problems can occur even due to individual batches of a timber type that is normally harmless. Possible reasons for this include spiral grain and insufficient drying.
➔ Spiral grain refers to a wood grain that has grown in a spiral around the trunk axis; this becomes a problem if, in the course of use, the moisture contained in the wood deviates from the moisture level at installation. If this happens, internal tension in the wood is released and can therefore cause the deck boards to warp. The energy released in this process is so enormous that it often overwhelms even perfectly installed fastening systems.
➔ It is a property of every timber to be able to absorb and give out water. For the user, this property can primarily be perceived through the timber’s swelling and shrinking. One task of the timber trade is to bring timber to the correct state of dryness for the respective area of use. If timber is used that has an incorrect moisture content at installation, this can quickly lead to damage.
• Many properties of the timber vary strongly depending on the grade. It is therefore advisable to contractually stipulate all criteria in advance with your timber dealer!
• Particular care should be taken when purchasing Bangkirai. In the past, increased demand often meant that substitute timber from South East Asia was – knowingly or unknowingly – traded as Bangkirai. Most of these substitute timbers are considerably less suitable for deck construction. This results in cracking and strong warping and bending of the boards.
• It is essential to use identical timber types in order to ensure the durability of the deck – i.e. the upper deck and substructure must be made of the same material.
• A lot of damage to deck structures can be prevented in advance by thoroughly inspecting the timber that is to be installed. If, for example, the tradesman responsible already notices deformation in the deck boards before installation, none of these boards should be installed.