Cladding / European Oak (Scottish/French)

Boat shed, Northern Scotland Oak Channelsiding Oak Cladding Dundee Oak board on board

European oak has a long tradition of use in the UK left unfinished and allowed to weather to a natural silver grey, in which state it has lasted for many centuries without maintenance. There are now extensive forests of oak all over Europe and a large and expanding resource in the British Isles.

Fixings should be designed to allow some movement to avoid splits developing, e.g. oversize screw holes etc, and be of non-ferrous or plated steel to avoid staining. The surface finish is likely to develop small checks and some moisture movement will take place in the sections.

Data sheet

  • Wood type - Hardwood
  • Environmental - Available from well-managed sources. FSC and PEFC.
    Distribution - UK, France.
  • Tree - Reaches a height of 18m to 30m. Diameters are about 1.2m to 2m.
  • Timber - The sapwood is 25mm to 50mm wide and lighter in colour than the heartwood which is yellowish-brown. Quarter-sawn surfaces show a distinct silver-grain figure due to the broad rays. The annual rings are clearly marked by alternating zones of early-wood consisting of large pores, and dense late-wood. Conditions of growth accordingly govern the character of the wood to a great extent; for example, in slowly grown wood the proportion of dense late-wood is reduced in each annual growth-ring, thus tending to make the wood soft and light in weight.
  • Drying - Oak dries very slowly with a marked tendency to split and check, particularly in the early stages of drying, and there is considerable risk of honeycombing if the drying is forced, especially in thick sizes. End and top protection must be provided to freshly sawn stock exposed to sun and drying winds, and sticker thickness should be reduced to about 12mm for stock piled in the open air during early spring and onwards until winter.
  • Strength - Oak has well known high strength properties.
  • Working Qualities - Medium to difficult.
  • Durability - Durable.
  • Treatability - Extremely difficult for vacuum pressure processes. (BS EN 350-2)
  • Moisture Movement - Medium. (Refers to the dimensional changes that occur when dried timber is subjected to changes in atmospheric conditions.)
  • Density (mean, kg/m³) 720Kg/m3 at 15% moisture content
  • Texture - Medium to coarse.
  • Use(s) - Heavy structural use, Cladding, Exterior joinery, Interior joinery, Furniture, Flooring
  • Colour(s) - Yellow brown.

Guidance on Fixing - Oak Hardwood Cladding

Hardwood boards are normally fixed by screwing to treated softwood fixing battens. Even for boards erected close to their in-use moisture content it is advisable to over drill the holes in the boards to provide a clearance around the shank of the screw to allow for seasonal variations in moisture content. The head of a countersunk screw will normally provide sufficient retention but using larger diameter screws with bigger heads is preferred for this reason. Restricting the board width to a maximum 150mm, and locating the screws on the quarter points of the board width, will limit the width of wood that will swell or shrink between the screws. A 2mm clearance between the shank of the screw and the hole in the board should be adequate for woods rated as small or medium movement if the boards are installed at about 18% moisture content.

Fixing to Green Oak Hardwood Cladding

However, if green Oak is to be used considerable shrinkage of the wood will take place between the screws. It is advisable in this case to pre-drill the boards to provide a 4-6mm clearance between the shank of the screw and the hole. As the head of the screw will not then be large enough to hold the board it will be necessary to fit washers under the screw heads in order to provide sufficient cover. The washers can be slotted to allow them to move with the wood, although care must be taken in aligning the slots and ensuring that the screws are in the outer ends of the slots initially.

Alternatively drilled washers can be set into recessed holes in the wood which are sufficiently oversized to allow movement between the washers and the wood. Roundhead or pan-head screws are normally used with washers, but there are also various sheeting screws available with built in washers and these have been successfully used for fixing green wood. They have the additional advantage of including a compressible rubber seal under the metal washer which will keep pressure on the boards even if they shrink in thickness. The use of large screws particularly in conjunction with metal washers, will have a strong impact on the appearance of the cladding. While this might often be desirable, sometimes concealing the fixings is preferred. One way of achieving this is to assemble the boards into panels by screwing through the battens into the back of the boards. It is possible to get sufficient screw retention in the thickness of a hardwood board but this is rarely feasible with a softwood board. In this case adequate clearance holes must be provided in the battens rather than the boards. The panels can then be fitted as a whole to the face of the building, either by screwing back the battens through open joints between the boards or screwing right through both the boards and battens and pelleting these holes after the panels are in place.

Screw fixings to Oak Hardwood Cladding

Screw fixings should be at least 55mm away from the ends of hardwood boards to avoid splits developing. At butt joints it is usually necessary to have separate fixing battens for the ends of each board. This is best accommodated by panelising the board layout so that the ends of boards always occur in line. All unfinished hardwood boards should be fixed with austenitic stainless steel screws and any washers should be of the same material. Even if the boards are to be stained or painted, stainless steel screws are preferable as any damage to a plated screw during fixing can result in corrosion and staining.

Company Registration Number SC92505. Website by Pelican Design Consultants.