External timber cladding has been a well established feature of buildings in the UK & Ireland and particularly Northern Europe for many centuries.
Traditionally, timber cladding has either been left unpainted and allowed to naturally weather; or been lovingly repainted every spring, as evidenced in the many picturesque towns and villages around the shores of the Baltic.
Today, timber cladding and facades are becoming increasingly fashionable in modern buildings, but the requirements of modern styles and building techniques demand a more adventurous colour palette combined with simpler and less frequent maintenance.
Factory finished cladding, allowing machine application of high build coatings and protective treatments under controlled conditions, can offer a significant extension of maintenance cycles; but cladding exposed on the face of a building presents major challenges for even the toughest coating.
The selection of a stable timber species; the design, dimensions and grain orientation of the board; and the choice and selection of fixings all play a major part in determining maintenance frequency and ultimate service life: whilst the quality of site installation and fitting work is also crucial in ensuring a successful project outcome.
This information sheet examines some of the key design and installation factors involved in specifying a successful and durable factory finished cladding system; provides typical specifications for factory painted cladding; gives some indications of expected service life for a range of timber species and finishing systems; and finally offers practical maintenance guidelines.
External Timber Cladding Coating Systems
All coating systems require maintenance to maintain their decorative appearance and it is important to recognise this when specifying coated cladding.
Opaque finishes have the longest maintenance intervals and are the easiest to repair and maintain.
Translucent finishes maintain the natural aesthetic appearance of the timber substrate but allow more UV light to penetrate to the timber surface. This disrupts the surface, increasing stress on the coating film, which results in earlier coating breakdown and requires more frequent maintenance.
If maintenance is not carried out early enough, the timber surface will begin to discolour and degrade (in the same way that untreated timber cladding does) and this can make maintenance more time consuming and restoring the original appearance of translucent finishes difficult.
Consistent application of the coating system is vital for long term performance. Although wet film measurements are a useful process control test, checking and maintaining uniform dry film thickness across the cladding board is the best assurance of in service performance.
The specifed average dry film thickness for a Teknos factory applied coating system is 100µm, with an absolute minimum dry film thickness at any individual point on the surface of 80µm.
The sealing of all exposed end grain is critical to prevent in service moisture ingress. Moisture ingress will result in significantly increased localised dimensional movement, mould growth, staining of the substrate under the coating system and coating adhesion failure. If left unrectified, substrate failure will occur.
External Timber Cladding Materials & Design
Timbers should be selected to minimise defects; chemically modified and heat treated timbers, which can offer improved stability, can also be considered. Timber selection should be guided by BS EN 942 Standards with, in the case of European Redwood, use of J2 graded section which minimises knot and other natural defects not covered by the Teknos warranty.
Timber selection must be in accordance with the durability, use, and exposure conditions described in BS EN 350 and BS EN 335. Where the natural durability of the timber falls outside the requirements of BS EN 335-2, it must be preservative treated as per BS EN 599-1.
When using double vacuum preservative impregnation, particularly with solvent based materials, the manufacturer’s recommended drying times (typically 2-14 days) must be followed before coating. If using a water based, surface applied, preservative, such as Teknol Aqua 1410 or Aqua Primer 2907, the boards must be factory coated, on all surfaces, to a minimum dry film thickness of 80 µm before site exposure to comply with BS EN 599-1.
Modified timber, such as Accoya meets durability of Class 1, but treatment with either Teknol Aqua 1410 or Aqua Primer 2907 is recommended to provide additional resistance to staining by non destructive Blue Stain fungi.
Timber Cladding Profile Design
All exposed edges and internal and external mould details must have a minimum 3mm radius, to avoid thinning of the coating.
All non vertical surfaces must allow efficient water shedding, with a minimum slope angle of not less than 15o.
All component profiles to follow best practice design recommendations and for cladding as described in the relevant Trada, Napier and BRE publications. Key points to note:
- Installation and design must include air gaps at the top and bottom of the facade to allow ventilation of the back of the boards and preclude moisture ingress.
- A chalk line or jig block should be used to obtain 3mm clearance between board joints, allowing for board expansion and contraction in service
- Profile design should allow for individual boards to be removed and replaced without damaging adjacent boards or incurring significant maintenance cost.
- Cladding profile design should maximise water shedding to encourage rainwater run off and eliminate traps for standing water.
- Stress grooves must be machined into the back face of the boards to minimise warping and twisting.
Reducing exposure stress will significantly reduce maintenance and increase the durability of a coating system. Where possible building design should give consideration to roof overhangs and recessing clad sections, normal design features in European countries where clad buildings are common. Avoiding or limiting cladding features on elevations exposed to direct weathering should also be considered.
The design should incorporate concealed fixing where possible. Moisture ingress through surface fixings will cause localised saturation and discolouration of the timber, creating a weak spot for Stain development and leading to localised failure.
If factory finished Cladding boards are fixed through the face, caps and fillers will help protect the fixing and a final site applied finish is required to ensure the integrity of the coating system.
During processing and factory coating, cladding moisture content should be controlled following the guidelines set out in BE EN 942. As a broad rule of thumb, the moisture content of cladding during final fit should approximate to the equilibrium moisture content of the cladding in service.
Most modern factory finished cladding coating systems use water based acrylic resins because of their durability and flexibility. These systems dry much faster than traditional solvent paints, allowing for faster handling, but require a period of time to fully cure before developing full water resistance. A minimum temperature of 15oC is recommended during coating and drying operations and Teknos’ recommended overcoating and drying schedules must be followed at all times.
When using vacuum or brush coaters, ensure the coating is uniformly applied to achieve the specified dry film thickness across all weathered surfaces. Uneven film application will reduce service life and require more frequent maintenance.
Non exposed surfaces should be coated to a minimum 60µ dry film thickness (80µ dft where a surface applied preservative is used) to provide protection and reduce stresses on the board.
All end grain must be sealed with 2 coats of Teknoseal 4000 in accordance with Teknos’ cladding system specifications. This is especially important on finished boards cut during installation.
Transport and site storage
Boards must be protected from the elements during transport.
Site storage areas should be well ventilated and not subject to extremes of temperature. Remove any airtight packaging before storage to ensure free ventilation and prevent condensation forming. Store the boards off the ground on suitable bearers.
If stacking finished boards face to face, always interleave with a protective paper or “Jiffy Foam”. Avoid films that contain Plasticisers as these will adhere to the coating surface.
Under no circumstances should boards get wet before installation.
Installation, Storage and Wall Construction
Important Installer Responsibilities:
- Seal all site cuts with 2 coats of Teknoseal 4000 end grain sealer
- Use a chalk line story pole or jig block to obtain 3mm clearance between board joints
- Nails must penetrate solid wood (sheathing and stud) by 30mm. Nailing to sheathing alone does not properly secure the cladding
- Claddings must not be installed over wet sheathing. Use kiln dried sheathing and strapping. Allow rain soaked materials to dry prior to installation.
- Butt joints must be tight, treated with 2 coats of end grain sealer, and made over solid wood to provide secure nailing.
- Do not lift pieces to allow for alignment. Cut along the top and touch up the cut. Raising one end may result in unlocking.
Making Walls Waterproof
Weatherproofing exterior walls requires the proper application of an approved sheathing membrane under the cladding. The purpose of the sheathing membrane is to provide a continuous barrier to prevent drafts and the entry of wind driven rain into the wall cavity.
Joints in the cladding are not designed to prevent passage of wind and rain. Passage of wind and moisture into the wall may occur, with sustained exposure to strong winds. Moisture may be driven through nail penetrations and overlap joints of sheathing membrane. In such exposures, improved resistance against moisture penetration may be obtained by a modified construction technique known as the Rain Screen Method in which the cladding is fastened to vertical wood strapping placed over the sheathing membrane and attached to the wall studs. This construction technique provides an air space for wind driven moisture to flow by gravity down the back face of the cladding to vents at the bottom of the wall. To maintain the Teknos warranty, cladding must be installed on strapping.
Installing over Rigid Insulation
Timber cladding should not be applied directly over rigid insulation. Rigid foam sheathing can cause moisture to accumulate on the back of cladding, causing staining, buckling, and damage to finish coats. Application over rigid foam sheathing must meet the following conditions:
- Cladding must be applied to strapping, creating an air space between cladding and rigid foam or fibreglass.
- Strapping must be a full 30mm thick and kiln dried. The air space allows for the venting of accumulated moisture.
- Use thicker cladding patterns in widths of 150mm or less. Thick, narrow cladding is more stable than thinner, wider patterns and better able to resist dimensional changes.
- Lighter opaque colours will maximise heat reflection and reduce dimensional movement.
If Caulk is used where cladding meets corners, windows, doors and trim, use colour matched or clear caulking. Take care to avoid creating water traps or inhibiting ventilation of the back of the boards and, as an alternative in critical areas, consider the use of cover moulds in jointing details.
Teknos UK & Ireland
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