Boardwalks have the advantage that they can be constructed across a habitat without interfering with natural drainage patterns, and with little ground disturbance compared to other methods of path construction. However, they are generally expensive to construct, require maintenance, and can look obtrusive in open landscapes such as moorland and mountain.
Boardwalks are usually constructed for either of the two following reasons, which in turn affect their design.
- To provide a safe path across otherwise impassable terrain such as deep marsh or alongside streams. These boardwalks are obligatory, and there should be no difficulty in keeping users on the boardwalk. Handrails may be needed for safety.
- To protect a fragile habitat, such as a bog, marsh, sand dune or woodland floor. Except in very wet marshes and bogs there is often no physical reason for walkers to stay on the boardwalk and the line and width must be such that walkers are not tempted off it.
Width
The required width depends on whether the boardwalk is designed to be one- or two-way, obligatory or voluntary, and on the numbers of people likely to be using it at any one time. Suggested widths are between 750mm and 900mm for one-way use, and between 900mm and 1200mm for two-way use. A boardwalk receiving occasional use need only be 750mm wide, whereas a popular one-way nature trail should be at least 900mm, to allow people to stop or overtake. Heavily used boardwalks near information centres, or linking car-parks and beaches must be at least 1200mm wide. The widths for wheelchair use are given here.
Boardwalks that are designed mainly as nature trails will require large ‘passing places’, maybe with information boards, where groups can gather.
The following design has been used at Hardwick Hall, Durham, for a nature trail through carr. On this and other nature trails, one-way use is encouraged which makes traffic flows much easier to manage.
Short (i.e. under about 10m) , single-width, obligatory boardwalks may be successful, but the path will need surfacing at either end where people gather to wait their turn.
Several very effective single-width boardwalks have been built on the Dorset Coast Path between Swanage and St Aldhelm’s Head, to cross springs that seep through a clay topsoil. In summer when the path is heavily used the springs are almost dry and the boardwalks are not needed. In the winter when the ground is waterlogged and unpleasant to cross, the boardwalks become ‘obligatory’, and are sufficient to cope with the smaller numbers of walkers.
Single-width voluntary boardwalks are not usually worth constructing. If trampling pressure is such that a boardwalk is needed, a single-width boardwalk will not be enough to contain it.
Stobs or bearers
Stob construction is usually better than using bearers as less timber is required and levelling is easier. However, bearers will be required in very soft ground in which stobs sink. Test by knocking in a few trial stobs before deciding.
Bearers can be made cheaply of local timber or boulders, if available. Timber bearers are not suitable in ground where flooding occurs, as the boardwalk will be washed away. Boardwalks with stringers laid directly on the ground are not usually successful unless very heavy, as they are difficult to level and tend to move underfoot. This also applies to pallets, which are only useful for temporary boardwalks or those on sand dunes, where blown sand holds them in place.
Foundations
If a very high standard job is required, for example linking a carpark and an information centre, it may be necessary to dig foundations to make a sufficiently solid base. At Risley Moss, Warrington, foundations were dug at 1.6m centres in a double line to support a prefabricated boardwalk. When measuring centres, always measure each time from the beginning to avoid accumulating any error.
Trampling during the excavation phase may alter the levels, so all the foundations must be re-levelled before the boardwalk is put in position.
Decking
Most boardwalks have decking of sawn timber, laid transversely, with a gap of about 20mm. Boardwalks designed for use by the disabled or people in wheelchairs should have a maximum gap of 10mm on transverse decking, and no gap on longitudinal decking.
Transverse deckboards can either be attached flush with the sides of the stringers, or with an overlap.
A flush finish gives a neater appearance and the boards are less easy for vandals to remove, but the overlap is a stronger and structurally more efficient construction, as for a given finished width, the unsupported span is smaller.
The overlap method has the further advantage that the deckboards are less likely to split during construction , and the boards will have a longer useful life before rotting from the ends causes collapse. Overlaps of 50mm for narrow boardwalks and 100mm for wide boardwalks are suitable. Whilst attaching the deckboards, use a template the size of the overlap to ensure a symmetrical finish.
On boardwalks subject to continual wetting and drying, a problem may occur where the decking boards shrink around the nails and become loose. A footfall on the overlap can then spring the board out of place. At Malham Tarn, Yorkshire, the solution was to fit flush edge boards with a third central stringer to reduce springiness.
Pre-drilling
Pre-drilling of deckboards on site is not usually practical because it takes too long. However, as pre-drilling does reduce the possibility of the deckboards splitting, it may be worth doing this in a workshop before taking the materials on site.
Annular nails also reduce the chance of splitting, and being difficult to remove, are a deterrent to vandals.
Non-slip surfaces
One of the problems with boardwalks in tall or enclosed vegetation such as carr is that they become very slippery, as algae grow in the damp conditions. As these are the types of boardwalks that tend to be constructed for nature trails and teaching purposes, safety considerations are particularly important.
Non-slip surfaces can be made by stapling chicken wire to the decking. There may be damage to sensitive vegetation caused by leaching of zinc from the galvanising. Part non-slip surfaces can be made as shown with a strip of ‘Netlon’ netting, or thin battens cut from fruit and vegetable boxes.
Alternatively, the decking can be sprayed with tar and then dusted with grit, but this must be done with great care to avoid damage to the habitat. Protect the vegetation at the edges and under the boardwalk as necessary with plastic sheeting while spraying is being done. This method is suitable for railway sleepers, which are usually used close-boarded, and which already have a tarry coating.
None of these methods is very durable, and netting must be checked regularly to make sure it is firmly attached. Tar and grit coatings need renewing periodically. Boardwalks in open situations should present no problem, and are best left untreated.
Handrails and toeboards
Handrails may be fitted for safety or to keep walkers on the boardwalk. They must always be strong enough to bear the weight of people leaning against them. Timber sizes are given here.
Toeboards give some security for wheelchair users and for people who have a fear of walking on raised surfaces. They also discourage vandalism to the decking.
Prefabrication
Some prefabrication may be helpful, particularly if the weather is bad. It also means less duplication of tools. However, it does have its problems in that sections can become very heavy and awkward to carry, and difficult to level in ‘non standard’ situations. Its best use is for boardwalks without stobs, when the stringers and cross-beams can be assembled on a jig, and then the decking attached once the frame is in position.
Line and profile
Boardwalks in woodland, carr or reedswamp are usually hidden by vegetation, but those across open ground need careful siting and design.
For reasons of cost, the line taken is likely to be the shortest route possible. The appearance can be improved by one or two gentle angles or curves.
Construction of attractive boardwalks on sloping ground is less easy, but try to avoid a very raised profile, particularly if heavy timbers such as railway sleepers are used.
Steps
Board in the risers of steps to neaten the appearance and protect the stringer ends.
Flights of steps can be made easier to distinguish and so safer by alternating the decking between transverse and longitudinal. A slope can then be turned with the longitudinal decking, giving a very attractive result.
The advantages of constructing steps of longitudinal decking are that this allows off-cuts to be used, and the steps are more durable because the end-grain takes most of the wear.
Other methods of making steps are shown below.
