Mineral Wool on Green Roofs – Chance or Challenge?

By Jörg Breuning, The Green Roof Patroller

Graphic by Jörg Breuning.

Mineral wool (also known as stone wool, rock wool, and glass wool), mineral fibers, or man-made mineral fibers are products made from natural or synthetic minerals.  The term "man-made mineral fibers" is generally used to refer solely to synthetic materials including fiberglass, ceramic fibers and stone wool.  Since their first commercial production in 1871 (Georgmarienhütte, Osnabrück, Germany), mineral wool found its way in many applications such as thermal insulation (as both structural insulation and pipe insulation), and soundproofing.  Patents in this regard date back to the 1930's.  The hydrophobic properties of mineral wool make it ideal for building products.
Much later hydrophilic mineral wool was developed for controlled horticultural purposes like germination of seedlings or simple hydroponic systems for vegetables.  Hydrophilic mineral wool has very high water retention so it was no surprise that this specially prepared mineral wool came to the attention of the German Green Roof industry in 1985.  International patents from 1986 and later in 1990 by Gruenzweig + Hartman (one of the largest mineral wool manufacturers in the world) document their involvement with the Green Roof industry.

Unlike the US, where I believe new components and materials flood the market with little or no research, G+H made extensive tests over more than five years before their scheduled introduction to the Green Roof market.  The research and development was led by Prof. Dr.-Ing. Stephan Roth-Kleyer at the University of Geisenheim, Germany.  Prof. Dr.-Ing Stephan Roth-Kleyer is one of Germany's leading researchers for modern Green Roof technology, having published more than 300 publications, coaching theses and dissertations in the last 30 years.

Research fields in Germany in 2000.

The first publications about mineral wool in lightweight Green Roof systems were finally available in May 2001 (Dach + Grün 2/2001).  The tests (almost a decade long) revealed results that where shattering the big expectations like a piece of glass that falls on the ground.

Without doubt, the mineral wool has advantages in germinating vegetation but in the medium-run or long-run any traditional Green Roof (according FLL) will outperform a system containing mineral wool.  The location of mineral wool in the green roof buildup doesn't matter.  Back then even the world's largest Green Roof component suppliers (like Optima/Optigreen) stepped away from the once so promising mineral wool products for Green Roofs.

The discovered issues with mineral wool components on Green Roofs cannot guarantee medium-term or long-term success for the vegetation on Green Roofs.

This might be different in North America where most building owners are not spoiled with high quality at decent costs and focusing on longevity at the same time.  At the same time many Green Roof component suppliers emphasize short-term profits, dependencies with long-term warranties, and additional unnecessary components like irrigation or other services that have no impact on healthy driven vegetation.

My company, Green Roof Service LLC /Green Roof Technology (GRS/GRT), cares about the future of modern Green Roof technology as an active and integrated system for environmental protection.  Based on our research on existing projects with mineral wool and interviews with outstanding Green Roof experts, we feel there are sufficient reasons why GRS/GRT hesitates to design, specify, or allow mineral wool products on our Green Roof projects.

These are as follows:

1) Dry mineral wool is lightweight and requires ballast on a roof;

2) When saturated it is as heavy as a specifically engineered drainage board that has increased water retention or as heavy as saturated LWA (lightweight aggregate);

3) The relative air content is very low when saturated comparing to LWA– this saturation can cause problems over the long run, especially with succulents like Sedum (they don’t mature, like peat moss in pre-vegetated Sedum mats);

4) The ballast (mentioned under 1) can compress mineral wool overtime and this can reduce water retention, but more importantly it can further reduce air content;

5) It is proven that the high water content (when saturated) of mineral wool is hardly available for the plants;

6) Mineral wool has very little or no ion exchange capacity (dead material);

7) Mineral wool can contain a large amount of chemicals that make the material hydrophilic (how long will this last?);

8) The insignificant air content of mineral wool – open space – can be compressed or filled with fine particles;

9) No equivalent or compressive strength compared to LWA or specifically engineered Green Roof drainage boards;

10) Drainage capacity of mineral wool is poor (below the requirements of FLL);

11) High costs;

12) Difficult to recycle once in place over decades;

13) High energy consumption in production (over 1600°C/2900°F);

14) Potential health issues of fibers (during install and in runoff water);

15) Attracts birds and rodents for nesting material;

16) No sufficient tests in North America (one year or less);

17) Mineral wool material is not used anymore in the German Green Roof industry by professionals over the last decade – most existing Green Roofs have been replaced by other systems.

Mineral wool is getting more and more popular in the US despite the fact that it can cause many issues.


Jörg Breuning


DACH+GRÜN 2001/2
Personal Interview 2014: Prof. Dr.-Ing Stephan Roth-Kleyer
G+H Webpage
ZinCo: Press Release December 2012
Pictures: DACH+GRÜN, Open Source, Jörg Breuning

You can also see our case study page under "Mineral Wool" on Green Roof Service LLC/Green Roof Technology.


Jörg Breuning, principal of Green Roof Service LLC/Green Roof Technology

Jörg was born in the green roof capital of world. He began working with green roofs in Stuttgart, Germany in 1980. Since then, he has installed and maintained tens of millions of square feet of green roofs throughout the world.  Remember, everyone, that English is not his mother tongue!

During his career Jörg has earned multiple degrees and certifications in landscaping, horticultural, and green roofing. In January 1983, Jörg Breuning was one of the first people in the world to earn a green roof professional accreditation from Optima, now Optigrün AG, who were the first green roof manufacturer to offer an in depth educational system for the installation and maintenance of their products. In order to be eligible for the accreditation, a candidate had to have completed 3 years of apprenticeship with a certified landscaping company. In 1985 he earned his national certification and horticultural technician degree – a program of work and study that required 5 years of practice in an approved landscaping company and 4 semesters of University study.

In 2003 Jörg transferred his German Company (founded in 1985) to the United States after being the key consultant for Chicago’s City Hall green roof. As an experienced horticultural expert, he has been able to educate many of North America’s green roof pioneers, providing them with fundamental green roof knowledge to start and grow their own businesses.

His company Green Roof Service LLC (Green Roof Technology) has introduced numerous innovations to the North American green roof industry. Many of these innovations have been adopted by companies such as Roofmeadow, Emory Knoll Farm, GAF, CETCO, Rooflite, Green Roof Solutions, Celebrity Cruises Miami and Optigreen.

Jörg’s articles will strongly focus on topics that ensure common sense is always practiced, ensuring that the horse stays in front of the cart, and provide valuable insights into making green roofs as popular and efficient as they are in Europe.

Contact Jörg at jorg (at)

Past Green Roof Patroller Articles

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