Living Roof on Dutch Barge, London Docklands


Steel boats are ideally suited to having green roofs. The deck is designed for the point-loads imposed by people and equipment, and so is structurally well capable of supporting the distributed load of a wet green roof. The main design constraint in this case was that a deeper intensive green roof would, aesthetically, have spoilt the lines of the barge, a 1924 Dutch Motorschipp built in Amsterdam.

The clients (my partner and myself) wanted a wildflower meadow, but this requires a substrate depth of 150mm or more. As a compromise, the suppliers, Wallbarn, suggested a sedum roof on 40mm substrate, which was seeded with a specialist London Wildflower mix. It will be ‘hit-or-miss’ as to which of these species survive in competition with the sedum, but we see this as an on-going experiment.

The roof materials arrived on a pallet : a 20mm drainage layer, geotextiles, bagged substrate, and rolls of sedum matting grown on a coir-fibre mesh. It was important to unroll the sedum as soon as possible.

The stern cabin roof is approx. 2m wide x 2.2m long with a slope of around 5 degrees towards the back of the wheelhouse and a slight curve across the beam.

There was an existing 50mm angle-iron upstand which defined the position of the additional edging required to contain the roof layers.

All paintwork was initially treated with rust-stabiliser (Vactan), 2 coats of  Jotul marine-grade (one-pack) primer/undercoat and several topcoats.

The roof was edged with bamboo, supplied in 150 x 1000mm rolls, cut to 75mm and cable-tied to holes in the angle-iron upstand. The masking tape kept the individual pieces in place until pushed against the angle-iron by the layers of the green roof.

The paintwork was protected with a felted geotextile layer. The white cross is steel bars protecting a small fixed porthole which it was decided to cover.

The drainage layer was cut to the shape of the roof and weighted down in position, butting against the bamboo edging.

A second layer of geotextile was then cut to turn up inside the bamboo edging. The purpose of this layer is to retain the substrate, and to prevent it from filling the indentations in the drainage layer, which are designed to hold water.

This layer also prevents the substrate from being washed into the drainage channel, which is formed from cobblestones. Both felt layers are turned up, containing the substrate but allowing passage of excess water.

The substrate/growing medium is a crushed brick material with low organic content, as per FLL guidelines.

The substrate was spread evenly, to a depth of 40mm, and lightly tamped with a block of 3x2. This pushed the bamboo edging neatly into the steel upstand.

The sedum matting was laid out, hanging out over the edge. This was then trimmed to follow the inner edge of the bamboo by sawing down in-situ with a sharp knife.

The bamboo edging was pre-cut such that the sedum would be flush with its top edge. This gave a clean line which doesn’t interfere with the lines of the barge.

The result is an attractive mixture of natural forms and materials.


Wildflower Species :

Agrimonia eupatoria, Agrimony

Anthyllis vulneraria, Kidney Vetch.

Centaurea nigra, Common Knapweed

Clinopodium vulgare, Wild basil

Echium vulgare, Viper’s Bugloss

Galium verum, Lady’s Bedstraw

Hypericum perforatum, Perforate St John’s Wort

Knautia arvensis, Field Scabious

Leontodon hispidus, Rough Hawkbit

Leucanthemum vulgare, Oxeye Daisy

Linaria vulgaris, Common Toadflax

Lotus corniculatus, Birdsfoot Trefoil

Malva moschata, Musk Mallow

Origanum vulgare, Wild Marjoram

Plantago media, Hoary Plantain

Primula veris, Cowslip

Prunella vulgaris, Selfheal

Ranunculus acris, Meadow Buttercup

Ranunculus bulbosus, Bulbous Buttercup

Reseda lutea, Wild mignonette

Sanguisorba minor ssp. Minor, Salad Burnet

Silene Vulgaris, Bladder Campion

Benefits of Green Roofs :

Apart from the obvious aesthetic qualities, green roofs have several major benefits, whether they're on a barge or a building. The most important factor, especially in the highly developed urban area in which we live, is the ability of vegetation (and soil) to remove air pollution. In addition, they're a major component of SUDS (Sustainable Urban Drainage Systems), slowly releasing rainfall to stormwater drains and so avoiding Combined Sewer Overflows (CSO's). (This isn't a major concern on a boat, surrounded by water...)

They're also beneficial in the thermal performance of buildings, providing an additional insulation layer which prevents both heat losses in winter and also heat gains in summer. Moreover, the evapotranspirative and albedo effects of growing vegetation both serve to cool the surrounding air, reducing the Urban Heat Island Effect (UHIE). 

The important biodiversity potential of this roof will depend mostly on how many of the wildflower species take root in the sedum matting.

Other quantified benefits of green infrastructure components include health and well-being, and even reductions in crime.

My MSc Dissertation went into a lot more depth on these issues, and I'd be happy to email a copy of a conference paper that I presented on this subject if anyone out there reads this and wants to know more.