Whin Sill Grassland Roof

The most ambitious part of the building is the Whin Sill Grassland Roof, it has never been tried before.

Built to mimic the shapes and geology of The Great Whin Sill, the grass roof is planted in a substrate inspired by the surrounding landscape. Whin makes thin, poor soil, so plants have to be able to cope with tough conditions; this means that the area gives a home to a unique community of plants and our grass roof reflects this.

We tested different soil mixtures and planting techniques for the roof in a number of trial plots. All the seed we’ve used has come from Northumberland and Cumbria, so we know the plants growing on the roof are local.

The roof has been carefully designed so it is wheelchair accessible, giving all visitors the opportunity to enjoy the Whin Sill roof and take advantage of the elevated views across the landscape.

The roof was designed to help the building fit into the landscape as a key aspect of the architectural design but it also serves as a viewing point accessible to all, insulation and a place where locally native plants can grow. The building is constructed to reflect the local geology so we wanted the roof to reflect the surroundings too. The shape mirrors the shape of the Whin Sill.

The Whin Sill

The Whin Sill is one of the special geological features of the North East and forms some of the area’s most dramatic landscapes. The hard, dark rock of the Whin Sill was once molten, but now stands out as a spectacular ridge in the Hadrian’s Wall area. The Whin Sill formed 295 million years ago when stretching of the Earth’s crust caused molten rock, or magma, at over 1000°C to rise up from deep within the Earth. It didn’t reach the surface but was injected between layers of limestone, shale and sandstone. The magma cooled and solidified underground to form the Whin Sill – a vast sheet of rock up to 80m thick, which lies beneath much of north-east England. After millions of years of erosion the Whin Sill is now partly exposed at the surface. This is what we can see from the roof and in this area it is what Hadrian chose to build his wall on.

The Whin Sill is made of a hard, dark, crystalline rock (known locally as whinstone). As it cooled, the Whin Sill contracted, producing vertical cracks along which the rock breaks into rough columns. This is a distinctive feature of most Whin Sill outcrops and provides the geometric shapes on which the overall design of the building and the internal design features of The Sill have been taken.

The name ‘Whin Sill’ was first used by local quarrymen. To them ‘whin’ was a hard dark rock, and ‘sill’ was a term for a flat-lying layer of rock. Geologists who studied the Whin Sill in the 19th century recognised its molten origins and the term ‘sill’ was adopted for all similar bodies of igneous rock. The Whin Sill is thus the original sill of geological science and is well known to geologists worldwide.

The soil we've used

The soil is made from local whin chippings, soil from a local disused whin quarry and compost – bark and bracken. Slow release granules with nutrients were also added. Plants that grow on the whin sill need to be tough as it can be dry, it's exposed and windy, and cold for many months of the year.

visitor looks at the green roof of The Sill

Seeds

A seed mix with grasses such as sheep’s fescue and sweet vernal grass and plants such as selfheal, yarrow and birdsfoot trefoil was added to the soil in April 2017. A coir matting was laid over the top to help keep moisture in and provide little areas of shelter for the growing seedlings.

Plants

In May 2017 6,000 young plants of 40 different species such as rockrose, stonecrop, maiden pink, thyme, salad burnet, crow garlic, ox-eye daisy, plantain, cowslip, red campion, lesser trefoil and harebell were added, which grow on whin sill soils along Hadrian’s Wall or at the Northumberland coast.

photo of a grassland rooftop with a layer of coir

Changes over time

The soil needs to develop and get organic material (from roots, leaves etc) bacteria, fungi, worms and other soil creatures. This will happen naturally over years as they move in the air, drop from the plants and migrate from the surroundings. Some of the soil that came with the plants had micro-fungi added as well. We have found slugs, snails, worms, voles and even rabbits on the roof so far and an owl has been spotted perching on the fence!

photo of a slug on a sign which reads 'landscape of the whin'

We are expecting some plants may die, the ones that are best suited to certain areas to thrive, and also some to make their own way here – we have seen a few new species including an orchid that we didn’t plant. Since planting there have been some very dry spells of weather and the tough plants and grass suited to these conditions have survived as they do in the surrounding landscape. Every year though it looks different and changes.

Here’s what you might see throughout the year…

Spring (March - May)

The grass starts to grow again and looks a bit greener. Early flowering species such as cowslip, violets, red campion, mousear and lesser trefoil are first to be seen and then, thyme and rockrose. From March you can see and hear curlews calling in the surrounding fields.

photo of cowslip plant in the foreground, and a building 'the sill' in the background

Summer (June - September)

This is best time to see most of the plants in flower. Bright maiden pinks are stunning and ox-eye daisy, thyme, stonecrop, birds foot trefoil, black medick, and knapweeds give a mixture of white, yellow and purple.

photo of planta maiden pink and birds foot trefoil on grassland roof

Autumn (September - mid November)

Species such as harebell, devil’s bit scabious, red campion, maiden pink, plantain and lesser trefoil are still flowering and the fluffy seed heads of hare’s foot clover are really obvious.

photo of the plant 'hare's foot clover' amongst grass

Winter (November - mid March)

The grass turns browner and not much flowers on the roof, but it helps the building merge into the landscape as all the grassland around changes colour too. A few hardy yarrow, clovers or trefoils may keep their flowers. Dead heads left on plants are homes for insects and other creatures.

photo of the grassland rooftop with the sill building on the right

Looking after the roof

The aim was to have a low maintenance roof which did not need watering and the plants could look after themselves. We have cut it twice as initially the grass grew longer than we expected while the nutrients settled down, but we are hoping that this will not be frequent from now on.

We have a great team of knowledgeable volunteers who help keep the paths clear, cut back some strong plants that may take over, and we are now creating some new bare areas for thyme and other species that like shorter turf. We have discussed getting a few sheep to graze it, but they are not needed at the moment.

A volunteer tends to plants on roof of The Sill

The future

We will keep looking at which plants survive and the composition of the soil. We hope students will continue to be involved monitoring the plants and the soil conditions. We have added a bit more locally collected seed and also some soil plugs to see if we can get waxcaps – bright grassland fungi that grows on the whin sill locally – to establish. There has been a massive change since 2017 and we are excited to see how it changes in the future.