Metal vs. GRP – Which Material Should I Specify?

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Metal vs. GRP – Which Material Should I Specify?

A discussion of the relative merits of Metal vs. GRP as a material for planters.

IOTA is a fully-integrated, sheet metal fabrication business – so, naturally, we will put a strong and informed case for Metal.

But, first, the case for GRP – as the material does indeed have merit in certain situations. 


GRP [Glass-Reinforced Plastic, or Fibreglass] can do some very clever things. And with modern fibres, and modern resins, and modern manufacturing technologies and production design, GRP is often a far better choice than Metal to achieve super high-tech design objectives [wind turbine blades, aerospace, high-performance sports etc.] 

However 99% of landscape products do not require the application of space-age, composite material science. At the level of cost and complexity relevant to planters, GRP is a better choice than Metal in two distinct circumstances:

1. GRP wins over Metal in large production runs of the same design

GRP has benefits for large production runs, of relatively small planters, to a standard design.

As a moulded product, there are volume scale economies in GRP, where the initial GRP mould set-up cost is amortised over a sufficiently large production run, which can then be batch manufactured.  Volume scale economies do not apply to anything like the same degree with Metal.

So for [say] 100nr. window boxes of dims. L 1000 x W 300 x H 300mm, in a single design, then GRP will be a highly cost-effective solution.

2. GRP wins over Metal for complex, 3D, sculptural forms

As a moulded product [and, again, assuming a reasonable production run], then GRP also beats Metal for complex, 3D, sculptural planter forms. Most of these complex planter forms can be achieved in Metal, but at a very high cost. For example:

  • ‘Bellied’ shapes can be achieved in Metal through the process of spinning, but the tooling set-up costs are extremely high.
  • Compound [double curvature] curves can be achieved from flat sheets of Metal via the English Wheel, and IOTA does have these technologies and skills in-house – but, again, the labour is highly skilled, and the costs are extremely high.

So, for complex curved or rounded planter shapes [bowls, vases, orbs etc.], and given a reasonable production run, then GRP will be a far cheaper option than Metal.


So GRP is preferred for large production runs, and/or for complex, sculptural forms. However, in reality, most planters specified do not fit this profile.

Most planters specified are some combination of:

  • Essentially geometric in plan
  • Relatively large in scale
  • Required in short production runs [i.e. a large number of different designs, rather than a large number of a single design]
  • Require customisation, and/or design additions, within the overall scheme [such as drainage, irrigation, inclusion of seating and lighting etc.] 

Thus, in the vast majority of cases, the benefits of GRP do not apply, and Metal is the preferred material.

And there are indeed a great many benefits to Metal.

The Weight Myth

It is a widely-held belief that GRP is “the ideal lightweight material for planters”. This view is both erroneous, and irrelevant in practical terms.

Yes, GRP can be light – but so can Aluminium. However if GRP is light, then it is weak – so there is the risk of comparing apples and pears. If a planter is made to the same strength and stiffness, on a genuinely like-for-like basis, in either GRP or Metal – then the weight difference will be negligible.

And even if GRP might be a few kilos lighter, on a like-for-like basis, this weight difference is irrelevant in practical terms. In the vast majority of conventional planting schemes, 75-85% of the weight is down to the planting [drainage materials, soil/compost, water, plants/trees etc.], and only 15-25% of the weight is down to the planter. So, if weight is an issue, then the way to address it is via the design of the planting, not the specification of the planter.


The Cost Argument

Modern sheet metal fabrication is highly automated, whereas even modern manufacturing processes in GRP still rely heavily on manual labour. Therefore, for larger schemes, Metal can often work out cheaper than GRP, on a like-for-like quality basis.

Also, as previously discussed, Metal will often be cheaper than GRP where there is not a large production run, of a standard design, of modestly-scaled planters [i.e. in the majority of cases].

Specification Clarity and Contractual Risk

Specification in Metal is much simpler and more transparent, leading to greater contractual clarity and lower contractual risk. For example: 3.0mm thick 316-grade Stainless Steel is a wholly complete and incontestable specification; whereas 10.0mm GRP could describe a range of different materials, depending on the exact resins and fibres used, the lay-up schedules, and the quality and method of manufacture. 

The relative clarity of specification in Metal, is also a benefit where structural liabilities are contained within the contract. For example, gaining Structural Engineer sign-off in Metal is a relatively simple and low-cost undertaking, whereas with GRP it can be complex, time-consuming and expensive.

Specification Clarity and Design Life Performance

The fact that Metal can be accurately specified, also bears on Design Life Performance.

The longevity, the ageing characteristics in different environments, the maintenance needs [or absence thereof] – with Metal, all of these things are scientifically established, and are inherent to the material. 316-grade Stainless Steel will last 25+ years – it’s as simple as that. 

So where Design Life Performance commitments, and/or specific warranties, are required, then these can be extended for Metal with a very high degree of certainty.

With GRP, these commitments are infinitely more problematic, as there is no provable performance measure inherent to the material. For example: a GRP planter using excellent materials, but badly made – this will most likely fail before one that is expertly made, but with poor materials.

Resistance to Damage or Destruction

In public realm, the client may be less concerned about how long the planter will last; and more concerned about what happens if it is damaged, or even destroyed – particularly as it relates to public liability issues, and operational site disruption.

Imagine [say] 20 planters around a market square, with a twice-weekly market, and the space otherwise allocated to parking. In this case, it is less a question of ‘if’ a planter will be damaged, but rather ‘when’. And the central concern is what then happens, after the damage has occurred.

With GRP, anything other than a minor ‘knock’ is highly likely to irreparably damage the planter; and a heavy vehicular impact will almost certainly split the planter, with soil, plants/trees etc. being released onto the road or pavement. The site is thus disrupted, and immediate action must be taken to mitigate public liability risks.

With Metal – most particularly Steel – even severe vehicular impacts are almost always absorbed. The planter may be ‘dented’, but it remains functionally intact; and recovery/remedial action can be deferred until a convenient time. Metal is also much more resistant to minor impacts than is GRP; and with some Metal – most notably Corten Steel – no surface repairs are required, and minor dents are often unnoticeable.  

Design Precision

In most cases, GRP moulds for planters will still be made by hand; whereas modern sheet metal fabrication technologies are infinitely flexible, and manufacturing tolerances are extremely precise [less than 1.0mm].

Therefore, creating complex geometric forms, to a high degree of precision, is much more assured with Metal than GRP. The larger the scheme, and the more complex the geometry – the more that Metal is to be preferred.

Adding Structural Complexity to a Planter

With Metal, it is possible to easily achieve a desired alignment of form and function; whereas, with GRP, the aesthetic [form] and the functional elements of the construction tend to be separate.

For example, if there are structural additions to a planter [such as balustrades, seating, trellises or fencing], then these can be designed-in to a Metal planter with minimum fuss, and great strength. Whereas, with GRP, a material change to functionality tends to require a major revision to design; and, ironically, within a GRP planter with additional seating etc. the internal load-bearing structures will most likely be Metal anyway, glassed-in to the exterior GRP form.

So where the brief calls for ‘more than just a planter’, then Metal will be greatly preferred. This is most particularly the case where public liability issues exist, and/or where Structural Engineer sign-off is required – as the Metal solution will be stronger and easier to certify.

Surface Finishes

Surface finishes in GRP are limited to what can be painted on, and/or tinted into the gel coat – and whatever finishes can be achieved in GRP can also be achieved in Metal.

In addition, there is a huge range of surface finishes achievable in Metal, that cannot be achieved in GRP. For example:

  • Polyester Powder Coating [PPC] technologies have advanced dramatically in the last 20-30 years, and there are now a huge variety of coatings available. These include coatings – for example – that replicate Anodised Aluminium and Corten; hardwearing, textured coatings ideal for high-traffic areas; plus myriad specialist ‘designer’ ranges developed by powder coat manufacturers like Akzo Nobel, IGP and Syntha Pulvin.
  • Because PPC is also used in industrial applications, there are also industrial coatings available [which can equally be used on planters] such as anti-graffiti coatings; and coatings suitable for extreme environments, like coastal.  
  • Stainless Steel can be mirror-polished, brushed, or shot-peened – each with a very different aesthetic, and technical performance. There are also a wide range of specialist Stainless Steel finishes available, from suppliers such as Rimex – which can introduce texture, pattern and specialist colour finishes into planter design.
  • Corten Steel and Lead are simply beautiful – just the way that they are, with no maintenance.
  • The natural patination on Zinc, Brass / Bronze and Copper does benefit from being maintained; however this is not strictly necessary, and the evolving patinas for each material are both beautiful and completely distinctive.
  • Hot-Dipped Galvanisation [HDG] is a ‘Marmite’ finish, with a hard/industrial edge. However those that like the look, really like the look – and HDG also has the benefit of extreme longevity.

So whatever is the desired aesthetic in terms of surface finishing, the choice in Metal is quite simply vast.

And, finally, the delivered aesthetic with Metal is often more visually pleasing than GRP, and considered higher value. For example, radiused transitions tend to be much tighter in Metal than GRP, and flat surfaces smoother, and edges crisper and straighter; and thus the general level of finish in Metal can often be much higher.

Fire Certification

In recent years, there has been a significant increase in planter-based soft landscaping schemes specified within new build developments. Not only are such schemes specified more frequently, but the extent and ambition of schemes has also grown - often at the plannning stage, and linked to BREEAM sustainability assessments.   

This planting may be internal to the building, or external but at a potential fire assembly and/or evacuation point [such as a roof terrace]. In either case, it has become much more common for the planters to have to be fire certified - and this has become a major disadvantage of GRP. GRP, in whatever spec., is made from flammable resins, and it will burn - Metal meanwhile will not burn. It is that simple - and in situations where fire certification is required, then it is a clear choice, and GRP simply cannot be specified.  

The Environment

Finally, it is increasingly the case that the environmental impact of materials will be considered as one factor within an overall specification.

Both Metal and GRP have good longevity characteristics, and that is a positive environmental argument. However, frankly, it is hard to claim that either Metal or GRP are 'eco' materials; as manufacturing the former is energy-intensive, and all materials for the latter are ultimately derived from the petrochemical industry. However in one - very important - respect, Metal is most definitely the preferred option. 

GRP recycling exists, but the industry is in its infancy; and recycling is simply not a realistic option today for planters, and/or other landscape product. Whereas Metal recycling is today absolutely standard practice, and is backed-up by environmental legislation, in all sectors [whether the planters are on a commercial building site, or in a private residential garden]; and recycling is as prevalent for planters, as for any other object. 

Thus 95% of GRP planters today will end up in landfill. Whereas 95% of Metal planters will end up being recycled - to live another day, as whatever they may become. 



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