Boardwalks - Plastic or Wooden?
Draft notes by Scottish Natural Heritage based on LEADER+ work at Flanders Moss
Recycled plastics have been used in the construction industry for a comparatively short time - roughly since the mid-1990s. They have a number of benefits compared with the use of traditional treated softwoods - but they also have a number of drawbacks. The aim of this note is to compare these, to enable SNH staff to make informed and appropriate choices, when considering the construction of new boardwalks.
Types of Plastic
Recycled plastics come in two main types, with some further options discussed later.
- Low-Density Polyethylene (LDPE)
This is currently the most common material available, and less expensive - essentially recycled bale-wrap, it is softer and more flexible than HDPE.
- High-Density Polyethylene (HDPE)
Made from recycled drinks bottles, this is a more expensive material. It can be difficult to source (mainly European), but is stiffer and seems to have performed better in field studies.
- Composite Materials
It is possible to include other materials mixed into the plastic - the most common is sawdust. This changes the handling characteristics of the plastic - for example, a 50:50 plastic/sawdust composite will take paint or stains in a similar way to wood. However, the downside is that the composite material is more brittle than 100% plastic.
- Reinforced Materials
It is possible to introduce reinforcing into the plastic - either steel sections or glass fibres, both of which would give greater structural strength.
However, the composition of the materials is very heterogeneous - these are recycled materials, and batches can vary in colour and composition significantly, depending upon what plastics are available at the time.
Recycled plastic can be purchased in a number of forms - ready moulded for specific items, in standard profiles and lengths, or in custom-made moulds. For larger projects requiring a specific profile or shape, it may be economic to have a custom-made mould produced. Most applications will make use of standard items - the basic design and construction techniques are very similar regardless of plastic or timber materials.
All but the thinnest sections have a honeycombed cross-section, with a denser outer edge - this is due to the physical effects of moulding the sections. As a result, bolts are better used for structural joints - although most screws or nails will work, provided guide holes are drilled first.
Benefits Of Using Plastics
- Value for money - although prices are usually subject to negotiation, as a general rule, plastics cost about 3 times as much as treated softwoods. However, they have a predicted lifespan of at least 40 - 50 years, compared to 10-12 years for treated softwoods. Studies have shown that they do not deteriorate significantly, resist UV and chemicals and are not subject to insect/fungal attack.
- Maintenance free.
- Greater slip resistance - most recycled plastics have a textured surface that gives better slip resistance than treated softwood.
- Recycled plastic - less material going to landfill, and a practical example of recycling.
- Clean - as it is impervious to fungi and has minimal algal growth, plastic boardwalks appear "clean", and according to some sites, this is greatly preferred by children on educational visits.
- Inert - there are no potentially harmful chemicals to leach out into sensitive sites. There are issues if the material is burnt - but as the plastics can be re-recycled, this should only be an issue if you are expecting vandalism.
Problems of Using Plastics
- Variability - recycled plastic is inherently heterogeneous, particularly between batches. This can affect both physical properties and appearance.
- Expansion/contraction - plastic sections are far more affected by hot/cold temperatures than timber. Designs must allow for significant expansion/contraction - for example, by allowing beams to slide over supports, rather than being bolted down, or by using oval holes to allow movement.
- Bending/softening in hot weather - plastic posts do bend alarmingly in hot weather. One example was of a batch of plastic planks that were stacked against a lorry - after a lunch break, they had become distinctly banana-shaped!
- Ease of Working - although plastic can be worked using power tools in a similar way to timber, it does require sharp, clean cutting tools to avoid binding. Driving posts into the ground can also be difficult, as tools such as mels tend to bounce, and the plastic can splinter. Relatively few contractors will have much experience of working with it.
- Supply - there are only a small number of producers in the UK, some of which have been short-lived. HDPE in particular can be hard to source - and mainly comes from Holland or Germany. This can lead to delivery problems (delays of 1-2 months are not uncommon), particularly if additional batches are needed.
- Flammability - this is particularly an issue with LDPE, although fire-retardants can be added to any material at additional cost. For example, Leicester City Council lost a 30m section of LDPE boardwalk in an arson attack. Some plastics will allegedly only smoulder if an ignition source is present, and will not burn once this is removed - but a fire-retardant should be specified if this is likely to be an issue.
- Scarf & Scraps - cutting and drilling the plastic produces plenty plastic scarf, which can be difficult to collect and dispose of. Larger scraps can be recycled, assuming that they cannot used elsewhere. At Flanders Moss NNR, the cutting was done on the site of the new car park, and the scarf has been incorporated into the hard-standing material.
Slip Resistance
A non-slip surface is a common requirement on boardwalks - and a variety of solutions have been tried on wooden boardwalks, none of which have been entirely satisfactory. (One firm has now started to market resin strips set into treated timber, which may provide a better solution). Plastic sections have a textured surface, do not support algae, and as a result provide a greater degree of slip-resistance.
Chemical Leaching
Although opinions vary, treated timber does pose a risk if used on sensitive wetland
sites. The tanalising fluid used to treat the timber can leach out into the surrounding environment - although if left to dry thoroughly (c.3 months), this does seem to be avoided.
Being inert, this is not an issue with plastics - which may be a significant advantage at sensitive sites.
There is an issue if the plastics are burnt, when a variety of unpleasant/harmful chemicals are released - but the same is also true of treated timber.
Aesthetics
As a feature in the landscape, boardwalks can have a huge impact, and the layout of the route is important to minimise this. By using the topography of the site to hide the boardwalk, and avoiding long straight sections, they need not dominate the appearance of a site.
Recycled plastic can come in a variety of colours, of which the most common are black or brown. Examples that have been used for over 10 years are not showing significant fading in sunlight.
Experience on several sites so far is that black is more intrusive than brown - although this may just be that visitors expect the boardwalk to be wood, and therefore brown!
Green Issues
Although it is a recycled material, using recycled plastic may not necessarily be a green option.
The whole-life energy and carbon inputs (including the collection, sorting, moulding and transportation of the materials) are likely to be higher for plastics than treated softwoods. However, the reduction in waste going to landfill, and the educational message could also be taken into account.
There are some doubts about how the plastic is sourced - there have been some suggestions that virgin plastic is used if recycled plastic is not available. Unlike other parts of Europe, there is not yet a reliable recycled plastics stream in the UK - which is the reason for the supply problems mentioned earlier.
Undoubtedly, the green option is probably locally sourced, seasoned oak - but cost and actual availability are likely to make this impractical for most projects.
The use of recycled plastic should be considered as an option when developing path projects - it does offer a number of significant benefits over traditional materials (not least being the cost over its predicted lifespan). However, the choice is not a clear-cut one, and each case should be considered individually.
Case Studies
- Leicester City Council
They have used recycled plastics for some 13 years - partly because of the value for money (they plan for a 40 year lifespan, and regard it as maintenance free), but also importantly because of the educational value (practical demonstration of recycling).
They have used a variety of suppliers - and had a variety of problems. They now go for "best value", not "lowest price", and currently use a Dutch supplier ("Larkhorst") who have been more reliable.
They specify that the plastic must be 100% recycled material (with a preference for post-consumer waste), and use over-size sections. They use a 1.2 m wide walking surface (with passing places every 30m), made up of 60mm x 240mm planks.
The use of oversize sections reduces the "bounce" on boardwalks - using similar dimensions to timber results in a disconcertingly bouncy feel. Although sections do bend, they don't seem to break.
They are firmly committed to using this material, and feel that the benefits far outweigh any problems.
- Glasson Moss
Natural England used LDPE to build a boardwalk over Glasson Moss, Cumbria - but found the project fraught with problems, and would not necessarily use it again.
They experienced problems due to expansion/contraction, leading to buckling of some sections. They used screws between the legs and the longitudinal beams, and in retrospect would have used coach-bolts, as some have sheered.
They also found that the black LDPE had a greater visual impact than anticipated, and would have preferred a brown finish. A neighbouring RSPB reserve used composite wood/plastic for the decking, which is less obtrusive, and could be stained.
- Flanders Moss
A 500m boardwalk was built in 2006 across part of this NNR using HDPE ("Hanit" - produced in Germany) - this has less give, but expansion/contraction remained an issue. The longitudinal beams rest on bearers - sliding over them - this technique has less impact on the moss, and allows the boardwalk height to be adjusted simply by adding extra sections to the bearers. A bridge section uses steel beams for structural strength.
A significant issue at this site was the inert nature of the plastic - there is no impact on the natural history interest of the site.
Interestingly, the plastic surface does not seem to be noticed by visitors, although school parties seemed to like the "clean" surface to sit on.
Thanks to:
Phil Clarke (Paths for All Partnership)
Adrian Lane (Leicester City Council)
Colin Auld (Natural England)
David Pickett (Scottish Natural Heritage)
Andrew McBride (Scottish Natural Heritage)
Further Information:
Boardwalks - timber or plastic? (Conservation Land Management, Spring 2006)
Useful Websites:
California Integrated Waste Management Board - www.ciwmab.ca.gov/Plastic
American Plastics Council - www.PlasticsResource.com
Waste & Resources Action Programme (WRAP) - www.wrap.org.uk
Hahn Kunststoffe GMBH - http://www.hahnkunststoffe.de/index.en.php
Centriforce Products Ltd. - www.centriforce.com
Additional library resources:
Flanders Moss public consultation document
Flanders Moss consultation results