Renovating the Green Roof (2023)

NOTE: THIS IS A FOLLOW-UP POST TO
THE GREEN ROOF: LESSONS FROM THE LAST SEVEN YEARS »

(you might like to read it first)

My Green Roof was created in 2010 and extensively replanted in 2016. The new planting worked well for a few years, reaching its peak in 2020 (see photo below), at which point it had evolved into an exquisite tapestry of interwoven, semi-wild garden plants — a fascinating little ecosystem: beautiful, ever-changing, but ultimately, unsustainable. By 2023 the more thuggish plants were taking over, while others were dying out. The future trajectory was towards a less ornamental mix of plants with increasing weed problems.

It was clearly becoming unmanageable, leading me to rethink the planting once again (See my recent article: “The Green Roof: Lessons from the Last Seven Years“)

In 2023, along with the planting issues identified in that article, I also realised that some of the timber structures of the roof-garden were deteriorating (e.g. the white border around the roof in the photo above) and were in urgent need of replacement. All of this lead me to undertake some significant renovation with the aim of creating a long-term and low(er) maintenance roof garden.

Looking for a Long-Term, Low-Maintenance Solution

TLDR

The diagrams below summarise the main changes I made during the renovation. In the remainder of this article I go into the individual problems and solutions, followed by the practical steps involved in the renovation.

Before

Main improvement made in 2016 (orange)

After

Changes made in 2023

Problems and Solutions

1. Deterioration of edge capping

The original edging to the roof was capped with 4 x 1in (100 x 2.5cm) treated timber, painted with a white OSMO coating to protect it. It needed re-coating every five years or so. Unfortunately, the horizontal orientation of these boards allowed water to pool on top, inevitably leading to deterioration as you can see in the photo below. Furthermore, where lengths of capping were butt-jointed I had not used any sealant, so water had inevitably penetrated to the timbers below. At these weak-points, the lower timbers were beginning to rot. Solution: Replace the capping with white PVC fascia board with sealed joints to make the perimeter watertight and properly protect the timbers beneath.

White coated timber capping board (top) showing deterioration. With the adjacent capping board removed, damage is evident in the boards beneath where water had penetrated the butt joint. Importantly, however, the butyl liner and protection mat were still in good condition.

2. Front fascia board Weathering

The front fascia boards around the perimeter of the roof consisted of 6 x 1in (150 x 25mm) treated timbers, which were coated with black OSMO stain. They were generally sound but showed more signs of weathering than the main cladding on the walls of the building, especially at the corners of the roof where the end grain received more weathering. The perimeter timbers take the brunt of the rain and act as a drip rail, so are more vulnerable to decay than other external timbers. Solution: Cover the fascia with black PVC fascia boards. This will also reduce maintenance (i.e. no more painting).

3. Soil and Weeds in Pebble Gutter

The pebble gutter had accumulated dirt and debris over the years and weeds and adjacent ground cover plants had taken hold in several sections. Solution: Remove all the pebbles, remove the plants and clean the gutter.

LEFT — Plants invading pebble gutter; RIGHT — timber frame around soil rotting

4. Rotting frame around soil

The timber frame that surrounds the growing medium, despite being treated wood, had rotted badly several years ago, causing soil to begin to slump into the pebble gutter. This is not surprising as the timber was in constant contact with soil bacteria and moisture. Solution: Replace with rot-proof Accoya.

5. Gravel mulch has become a second growing medium

In 2016 when I installed the weed mat, I mulched it with fine gravel to protect the mat from UV and provide an attractive finish. Unfortunately, this tuned out to be a perfect growing medium for certain plants which spread by runners through the soil. This combined with atmospheric dirt and detritus from the plants (old leaves etc) turned the original gravel into a perfect secondary growing medium (see photo below) encouraging weeds. It is interesting just how quickly new soil can form. Solution: Remove all of the gravel mulch (along with the infiltrating plants and roots). Do not apply any mulch to the new weed mat. Do not replant with any plant that spreads by roots, stolons, or sub-surface shoots.

After lifting an area of gravel mulch, it was clear that it had turned into an ideal growing medium for plants and weeds, due to the infiltration of stems, roots and dirt.

6. Weed mat fraying

In 2016 when I installed the weed mat, I cut holes using scissors. This left a lot of frayed edges which only got worse as the years went by. Plastic strands were an eyesore and made it difficult to work close to these edges. Solution: use a butane gas torch to cut new weed mat to melt edges and prevent fraying.

7. Weed mat Overlaps

Another mistake I made with the 2016 weed mat was using a 1m wide roll, which I laid on overlapped strips. Not only did this create more frayed edges, but the overlaps ended up getting wrinkled and were impossible to get flat again as the gravel topping got underneath them so easily. To keep them flat I had to use lots of pieces of stone slab. Solution: Use a single sheet of weed fabric that covers the entire area in one piece.

8. Weeds colonise Bare soil around planting holes

In 2016 when I installed the weed mat, I retained dozens of plants from the previous planting. To accommodate these, I had to cut holes in the weed fabric. These holes were often larger than the base of the plant, leaving patches of bare soil where weeds could establish. This was especially bad around plants that have upright growth, or clumps of bulbs, or perennials like the large sedums. These, inevitably, ended up with colonies of weeds deep in their crowns that were nigh on impossible to remove. Shrubs like the helianthemum (rock rose) were far less trouble as it grows from a central stem, requiring a much smaller hole in the weed fabric, and their semi-evergreen crown. Solution: Retain as few plants as possible (so few holes have to be cut) and replant with only plants that grow from a central stem and form a weed-proof mound. e.g. small shrubs like heather.

9. Ground cover plants and Weeds control

By creating a continual carpet of plants across the roof it was impossible to use weedkillers. Weeding had to be done by hand, which took two or three days every year. This is not sustainable going forwards. Solution: the new planting as per solution 8. No ground-cover plants.

RENOVATION
(OCTOBER 2023)

Step 1: Remove existing plants

Digging off the plant layer was laborious but not complicated. The mulch mat that was laid in 2016 provided a clear surface to guide the spade under the gravel. The planting area of the roof is about 20m², which resulted in several dozen buckets of soil to be removed via ladder. A similar volume of plant material went to compost. Although most of my green roof plants were dispensed with, a few were carefully dug up and planted in pots.

The largest plant to be removed was a six-year old Eleagnus which had self seeded on the roof. In a few years, it had grown into a sizeable shrub which I pruned annually. The photos below show the shrub growing on the roof in 2020, and the extraordinary root mass that I uncovered and removed in 2023. Remember this was all growing in just 3 inches (8cm) of soil!

I had no idea how extensive the root system was until I started removing it. It just shows how some shrubs can thrive in the shallowest of soil, and how important it is to weed them out of a green-roof system before they get too well established!

Step 2: Remove gutter stones and soil Edging timbers

In the original design, the growing medium was retained in a treated softwood frame, separating the soil from the gutters which were filled with large pebbles. Inevitably, treated softwood in ground contact only has a lifetime of 10 years or so, so mine was badly decomposed and needed removing. r had rotted, so needed removing. Meanwhile, the gutters, which were filled with large pebbles, needed cleaning, so had to be removed in bucket and carried down a ladder: a long and tiring process!

After emptying the gutters, I used a vacuum cleaner to clear out the fine dust and grit. The underlying structures (butyl liner and pond-protection mats) was intact and in good working order.

Step 3: Replacing the capping boards with PVC fascia

Once all of the perimeter white capping boards were removed, I covered the existing six inch vertical fascia boards with black ash PVC fascia cladding, around the entire perimeter of the building. These are 9mm PVC L-profile boards that coverer the front and bottom edge of timber fascias on houses, providing a weatherproof, no-maintenance finish. They are simply nailed in place with self-coloured plastic-headed nails. Cover fillets are then cut and glued to cover the butt-joints between boards and at the corners.

After these were in place, 4inch (100mm) white PVC fascia cladding could be fitted in place of the top capping I had removed, ensuring the top edge of the perimeter boards were fully protected. The lip of this white fascia neatly fitted over the top edge of the vertical black cladding, creating a smart edge line. I bedded the white top capping on a bead of mastic running along the top of the butyl liner to create a weather-seal, before nailing it down. I took particular care with the cover strips at the corners and where the capping boards abutted to create a water-tight seal.

As you can see in the photos above, these fascia panels provide a slick, zero-maintenance, waterproof seal for the edge of the roof. I would recommend anyone building a green roof to use PVC fascias from the start.

Step 4: Replacing the Soil Edging Boards

After removing the rotten edging boards around the soil on the green roof, I replaced them with Accoya – an effectively rot-proof wood with a 25 year guarantee, even in permanent wet ground contact. In practise, it will outlive the building and me!

I used 4×1″ sawn Accoya timber, which comes in at a generous 110 x 30mm – solid and sturdy. I fixed these boards around the edge of the soil, using shims and little posts to hold the boards clear of the deck so water can easily drain from under the growing medium and get into the gutters. The roof has a slight fall to bring water to the two drains at the front, so the gap under the accoya boards is greater at the front than at the back.

I also added two cross boards, dividing the roof into three beds (not shown). These cross boards provide rigidity to the long sides, preventing them bowing or tipping outwards under pressure from the soil behind them.

Step 5: Replacing the Weed Membrane

The penultimate step was to cover the soil with heavy-duty woven polypropylene weed membrane. This time I used a single sheet, and only cut holes in it for the two plants I have carried over from the old planting: a large clump of Carex ‘oversold’ and one clump of Basque flower. I used a small butane torch to cut the holes (rather than scissors) as the flame seals the edges and prevents fraying. A few small stone slabs have been used to hold the fabric down, while the edges were tucked down between the soil and the Accoya boards.

Step 6: Replace Gutter Stones and Replant

The final step was to fill the gutters with the previously removed pebbles.

Once the weather improves I will be adding just a few plants. Primarily, these will be plants that sprout from a single central stem and spread out into neat, dense mounds on the surface. What I have in mind are: Helianthemum (rock rose), winter heathers (Erica carnea) plus a couple more golden sedges (Carex ‘Evergold’) as these ‘grasses’ do not seed everywhere.

The plan is for these to make distinct, large clumps with clear space between facilitating simple weeding (or even herbicide spraying). By growing the plants from small plugs, there will be almost no exposed soil where seeds can get established.

Well that’s the plan at least!

RElated posts

FURTHER READING


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