The Workshop Construction: The Green Roof

PART 5: Construction of the green roof: base and liner

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OSB Deck Boards

The deck was laid with 16mm Smartply (OSB) decking boards. This was a nice job as these boards are matched (T&G) on all four sides and slot together tightly. They were then screwed down to the rafters, quickly making a firm roof deck that could be walked on whilst it was worked on.

Arris rail was then attached all the way round by screwing through from the underside (see below, right) and 8″x1″ fascia boards screwed to this to form an up-stand.

Drainage Details

I fitted flat-roof rainwater spigots into each end of the front of the roof. To prevent pooling behind them they needed to be recessed flush – a router came in handy here.

Butyl Roof Liner

The one-piece, 1 mm butyl liner was supplied from a DIY flat roof company – you simply give them the roof dimensions and it arrives in one (extremely heavy) piece. Getting it up on the roof was one of the hardest parts of this whole project! However, once up it was relatively easy to install. First, glue was applied over the timber deck, then the liner was rolled out and pressed into place with a broom.

Green roof construction drawing

In the corners a fold was necessary. The only cuts made were for the rainwater spigots, and here the butyl was bedded onto a ring of mastic to seal it (below, left). The edges of the liner were folded over the top of the 8″ fascia board then pinned in place by a 4″x1″ timber that was screwed flush with the top of the fascia all the way around. Any excess butyl was cut off, leaving a neat finish.

Pond underlay was then added as an overlay (see above, right). This protects the butyl from the gravel that is used to create the drainage layer that comes next. (Although it is called pond underlay it is just carpet underlay as far as I can tell!)

A treated timber retaining frame was constructed to eventually hold the soil. It is raised 1cm on short posts and it sits under its own weight on the underlay. Within the frame pea shingle was placed in a 20mm layer to create a drainage layer. Around the outside 20-40mm angular shingle creates a plant-free border that ensures free drainage to the spigots. Note that the spigots have leaf-guards, primarily to keep the shingle out, and these need to remain accessible so they can be easily checked and cleaned if necessary.

Before the growing medium, the inside of the frame was lined with woven landscape (weedproof) fabric. This prevents soil washing down and blocking the drainage layer. The growing medium was then lifted, bucket by bucket, up onto the roof. This was the most exhausting, repetitive job and took much longer than I expected! The growing medium I used was supplied by Aldingborne Nurseries and consists of 50% sand and 50% recyled green compost. Good green-roof growing medium should be low fertility and free draining (hence the sand) and water retentive, (hence the compost).

Finishing touches included a 6″x1″ faschia board screwed to the 4″x1″ around the edges, and finally, a 4″x1″ capping rail that frames the whole roof – we painted it a contrasting white.

Here it is, finished with some of the early planting installed:

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Find out more about the green roof here»

7 thoughts on “The Workshop Construction: The Green Roof”

  1. Hi,
    Many thanks for this explanation of how the living roof was constructed. 7 years on how is it doing. Is there anything you would have differently. I am just about to embark on a similar project.

    • Hi Mark. Would I change anything? No, I don’t think so, not in terms of construction that has been fine.

      What I have learned is that you get a much better effect if you control the weeds and water in dry weather. My mistake was to assume that letting it take its own course would select the toughest, most drought tolerant plants and reduce maintenance. Problem is that many annual weeds cope well with annual drought on a neglected roof as they go to seed before the summer. Also, grass will get established and push out everything else – even ornamantal grasses like pony grass and festuca glauca – both become weeds after a few years.

      I have written a post about my experience of the first five years here. I am now on stage 2: keeping the best clumps of plants, spraying off the weeds, laying weed matting over the soil, and planting spreading alpines through the matting. Some slabs and gravel between plants is keeping the soil moist for longer and the results are much better. I’m planning to post an update soon to show how well it’s going!

  2. Hi Keir,

    We are about to embark on green roof, in a south west facing garden. The building is on the north side. I had hoped for a wildflower meadow, but it’s looking like it won’t get enough light.

    I like your planting plan a lot, so may see if any of those elements would work.

  3. hi, is it possible to install green roof without pebble gutter and the frame for growing medium? can the whole area between the four arris rails are filled up with the growing medium?

    • If you do that you are likely to end up with drainage issues (waterlogging). The pebble gutter allows excess water to easily find its way to the downpipes. Part of my maintenance is to make sure the gutters remain clear, so I remove seedlings that grow in the pebbles.


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