Simple bridges and boardwalks

Bridges on public rights of way in England and Wales which cross streams or other natural obstacles are the responsibility of the Highway Authority, who should be consulted over design and construction. Bridges which are not on public rights of way will normally be the responsibility of the landowner.

The site must be carefully checked to choose the best crossing point where banks are stable and unlikely to erode. The likelihood of flooding should be assessed, and the design of abutments to support and secure the bridge should be considered. See Footpaths – Abutments and end seats for further details.

Consider using material from within the woodland for bridge and boardwalk construction. Durable hardwoods and some softwoods (see below) can provide bridge beams. Durable hardwoods can also be used for handrails and posts, and where planking machinery is available, for decking.

The abutments, which support the beams of the bridge, can be made from mortared stone from the stream bed, or gathered from nearby as available. Log cribs can be constructed as shown. Small sleeper bridges can be supported by sleepers, as shown below.

Alternatively, concrete abutments will be needed. See Footpaths – Abutments and end seats for further details.

Footbridge with handrail

Where available in the woodland, the main beams of the bridge can be made from straight trunks of Douglas fir, larch or Scots pine. For short spans, oak or sweet chestnut could also be used, where they are available in sufficiently straight lengths. Mature ‘overstood’ sweet chestnut coppice may yield suitable material. The table gives a guide to dimensions required for different spans.

Table 13d: Bridge beams

Logs must be straight, and of the diameter specified for the middle third of the log length. Remove bark and branches, but do not trim flush. For softwood logs, soak all cut areas with at least three coats of water-based wood preservative, as soon as possible after felling. Although this will not soak in far, it eliminates fungal growth on the cut ends, and reduces end cracking by slowing the rate of drying out. Wear protective clothing when applying wood preservative, and take care not to allow any leakage into the soil or stream.

Logs should not be used if they have knots larger than 100mm (4”), signs of fungal decay, or a marked spiral grain slope. Also reject logs that have surface cracks of 0.2mm width at time of felling, or 3mm after seasoning.

The total depth of cracks in any section must not exceed one third the diameter of the log. A working life of no more than 10 years should be expected.

Decking should be of sawn softwood, pressure treated with preservative. Durable hardwoods, such as oak or sweet chestnut could also be used if available in the woodland. These can be converted to planks with a mobile bandsaw operated by a trained operator, and then carefully stacked and dried in the woodland before use.

Decking should overlap the outer edge of the main beam by about 50mm. Pre-drill the decking, and attach using no.12 x 90mm sherardised screws. A strip of bituminous felt 20mm wider than the main beam, laid between decking and beam, will help protect the beam from rot.

In low risk situations, where there is a drop of less than about a metre to the bottom of a narrow stream bed, ditch or gully, handrails are not necessary. Most people will have no problem in crossing a bridge of, for example, 2m length and 1.2m width. Handrails should be provided in situations where there is a drop greater than about 1 metre, where there are likely to be strong cross-winds, or where the pathway is in a popular location which is likely to be used by older or infirm walkers, who may require a handrail. If a handrail is provided, it must be strong enough to withstand people leaning on or falling against it. A weak handrail is more hazardous than no handrail.

The handrail posts are supported by a raker, attached to two deckboards, which are extended to a distance equivalent to half the height of the handrail post. The handrail posts, of treated softwood, should be spaced 1m apart, on one side only in normal situations. A single handrail 100 x 50mm is sufficient for normal situations, joined by a half-lap joint as necessary at posts. The toeboards are attached on either side as shown, using no. 12 x 90mm sherardised screws. In hazardous situations there should be handrails on both sides of the bridge, 1.4m high, and with three rails on each side.

Materials:

Beams
Logs of suitable length and diameter (see Table 13d)

Decking
150 x 50 x 1125 (4 per metre length of bridge), treated softwood
150 x 50 x 1650 (2 per metre length of bridge), treated softwood or 100 x 36 for hardwood

Toeboards
75 x 50 x twice length of bridge

Handrail posts
75 x 75 x 1.3m

Handrail
100 x 50mm x length of bridge

A winch and chain block, operated by suitably experienced workers, may be required to lift the beams into position.

Sleeper bridge

Reclaimed railway sleepers and telegraph poles are useful materials for bridge building, on sites where vehicle access to the site is possible. Sleepers are available from reclamation companies in the following sizes:

250 x 125mm x 2.6m

300 x 150mm x 2.4m-6m (crossing sleepers)

Sleepers can be used for simple pedestrian or light traffic bridges up to 2.6m span, where the sleepers are lain as shown across the gap. Alternatively, they can be used as beams for bridges with decking and handrails attached. Note that the attachment for the handrail requires placing the sleepers on edge, to give sufficient depth of timber for attaching the handrail posts. Where handrails are not essential, it is easier and probably cheaper to build a wide bridge even if only for pedestrian use, rather than a narrower bridge where handrails then become necessary.

Where flooding is possible, the sleepers should be wired together and attached to the sleeper abutment, to prevent them being dislodged by flood. For ditches and dry gullies which do not flood, attachment is not essential as the weight of the sleepers holds them in position. A minimum 20 year life can be expected from a bridge of good quality sleepers.

For a narrow pedestrian bridge, or for spans over 2.6m using crossing timbers, a structure with handrails and decking becomes necessary. Use a cantilever attachment for the handrail posts as shown, with a 20mm mild steel tie rod through the width of the structure. The main beams should be not less than 150mm deep and strutted apart, with spacers and a bottom strut as shown.

For further details on bridges, see Footbridges in the Countryside (CCS, 1988) or Footpaths – Footbridges.

Boardwalks

Boardwalks can be useful for crossing ground that would be damaged by uncontrolled trampling. This may include:

• Damp ground, where the vegetation breaks up under trampling, turning soil to mud or damp peat to a morass.
• Dry or sandy ground, with thin soils and sparse vegetation. Some woodlands have a delicate flora of mosses and lichens, which trampling would destroy.

Various designs of boardwalk are given in Footpaths – Boardwalk designs. The design must be robust, and create a surface that is more comfortable to walk on than the ground it traverses, otherwise the boardwalk won’t be used.

Railway sleepers are used for many boardwalks, as they are durable and simple to use. On soils or peat which are damp but without areas of standing water, it should be sufficient to lay the sleepers on the ground as bearers, with half or full-length sleepers forming the decking. The decking sleepers are held in place by their own weight, but a wire stapled as shown will discourage vandalism.

For short lengths of boardwalk, those that are ‘one way’ or only receive limited use, sleepers can be laid lengthways as shown.

Boardwalks can be constructed from materials available in the wood. These are suitable for crossing damp ground, but are unlikely to be successful on dry ground, because their uneven surface makes them less attractive for walking than the surrounding ground. They are suitable for more remote woodland locations, where an uneven walking surface may be appropriate. Sweet chestnut or oak are the most durable, and can be used for bearers, stringers and decking as shown. The boardwalk should be checked annually and any weak parts replaced.

A very ancient design of wooden trackway is shown below. This is the Sweet Track in the Somerset Levels, which is a 6,000-year-old, 2km length of wooden trackway found buried in peat deposits. The crossed poles are made of oak, ash and lime, and the trackway itself is made of oak. Although not a practical proposition for today’s needs, a replica section could be an interesting project and a good test of skills in riving oak!