Local materials
In some situations it may be possible to use local materials, which will save on purchase and transport costs. Local timber supplies should be assessed carefully to decide whether this saving justifies a probable shorter useful working life, compared to purchased and pressure treated timber.
Uses of local materials
Timber: Bearers, main beams, stringers, decking
Stone Boulders: Bearers, abutments, piers, scour protection
Aggregate: Boardwalk approaches, concreting
Water: Concreting
Do not use sea sand or shingle for concreting, as the salt reduces the setting ability.
Timber
Timbers and preservatives lists different types of timber, with suggested uses and preservative treatments. In general, pressure treatment with creosote is more durable than pressure treatment with water-borne preservatives such as Tanalith.
Heavy-duty rubber gloves should be worn when handling preserved timber. Creosoted timber can stain and taint clothes, so protective or old clothing is recommended to be worn. Beware of burning any off-cuts of creosoted timber, as it will burn fiercely. Both Tanalised and creosoted timber give off noxious fumes. Save any extra or off-cuts for re-use elsewhere, or dispose of them at a refuse site.
Preservatives from pressure treated timber can leach out and damage surrounding vegetation. Creosote tends to ‘weep’ in warm weather, especially when newly applied. Order several months in advance if possible, and store outside where excess creosote can leach out harmlessly. There is evidence that the salts in Tanalised timber can leach in very acid conditions, such as peat bogs (Yorkshire Dales National Park, 1993). Scottish Natural Heritage (CCS, revised 1989) advise using larch rather than preserved timber for boardwalks on special sites. However, preserved timber has been used on many nature conservation sites for many years with no problems of vegetation damage.
For bridges, use Tanalised rather than creosoted timber for handrails and handrail posts, to avoid staining of hands.
With increasing management of local woodlands, good use can be made of local durable timber such as oak and chestnut, to make simple bridges and boardwalks.
Fixings
Nails
‘d’ in the illustrations represents the nail diameter
Use galvanised, sherardised or zinc or cadmium plated nails. To reduce the chance of splitting, preferably pre-drill for nails in hardwood and larch. Always pre-drill for nails 100mm or longer. Pre-drilled holes should be 0. 8 x diameter of the nail. Nails should be of the lengths shown below.
The minimum spacing of nails should be as shown below.
Screws
Use sheradised or galvanised screws. Minimum spacing should be as shown below.
The pilot hole should be half the screw length if fixing to softwood, and slightly deeper for hardwood. Make the pilot hole a smaller diameter in softwoods than in hardwoods. Dip screws in linseed oil or Vaseline before use.
Bolts
Electro-plated bolts are the best quality.
Coach bolt. Square collar locks in wood as nut is tightened. Up to 500mm long, 5 – 19mm diameter.
Rag bolt. Ragged end holds in concrete. 30 – 200mm long, 4 – 60mm diameter.
Length of bolt must equal the thickness of timbers, two washers and nut plus 5mm.
Washers
Use galvanised or sherardised washers.
Internal diameter should be slightly larger than bolt.
Timber connectors, used to strengthen grip between softwoods.
Dog clip, for joining adjacent timbers.
Timber joints
- Cut the joints accurately. Gaps will weaken the joint and collect water which hastens rot.
- Always nail lighter timber to heavy. Avoid nailing twice into the same grain line.
- Pre-drill for nails in hardwood or larch.
- Soak all joints with preservative before assembly.
T joint
Used for bridge and boardwalk decking. Nail obliquely, preferably with the deckboard overlapping the beam.
Lap joints
The full lap joint can be used in boardwalks to fix stringer to stob. Either joint is suitable for stile rails.
Mortise and tenon
The thickness of the tenon should not exceed one third the thickness of the rail. This is a strong joint, used for handrails on bridges and bars on stiles. It is however, difficult to mend if the rail is broken or vandalised. For extra strength, the joint can be dowelled.
Lengthening joints
These are used for handrails. If there is more than one rail, position the lengthening joints so there is only one on any handrail support.
Three-way joints
These are used for boardwalk corners and steps.
Finishing
To give a neat finish, chamfer the ends of the deck boards with a Surform.
Weather the tops of uprights to allow water to run off, and chamfer edges and corners to give a smooth and attractive top.
