Green Roof Design Considerations

Green Roof Design Considerations Overview

There are fundamental principles that apply to the design of a green roof, regardless of the proposed landscaping, location or climate. The specified solution must replicate nature within the build up and be able to support the vegetation.

The first step - What is the landscaping finish to look like?

The primary decision is the type of landscape required that best suits the rationale behind the development, whether it is an intensive, simple intensive, extensive or biodiversity roof. Whatever the landscape chosen, the plants will have some basic requirements to sustain them; nutrients, a balance between moisture and drainage to suit the vegetation, and aeration to the root systems.

The next step - Planning the green roof to meet the needs of the vegetation

Once the vegetation finish has been determined, the structural and design considerations and green roof components can be proposed.

Energy efficient roofs

Reducing energy usage through effective roof insulation. There are different options depending on whether you are constructing a warm or inverted roof above a heated building, the usage of the building and if it's domestic or commercial, and the specific project requirements and parameters such as if you need to create falls using the insulation or if there are limited installation heights, such as for terraces.

Building regulations across the UK, Scotland and the Republic of Ireland cover the conservation of fuel and power through energy efficiency, be that through retaining heat in the winter months or preventing excess heat from entering the building during the summer months.

Warm roof insulation

The rigid board insulation is encapsulated by the waterproofing and can be mechanically fixed or bonded, depending on the waterproofing system. There are different options depending on the requirement, each with its benefits.

If falls to drainage need to be incorporated a tapered insulation scheme is cost-effective and our technical department can design insulation solutions to comply with the latest regulations and guidance documents, along with providing U-value calculations and a detailed layout scheme to aid installation on-site.

Go to warm roof insulation

Inverted insulation

The principle thermal insulation is installed above both the roof structure and the waterproofing and is always ballasted with a variety of finishes ranging from a fully accessible intensive green roof to a biosolar PV system, or simple pebble ballast or paving. A Water Flow Reducing Layer (WFRL) is required to be included in the specification under BS6229. The WFRL is a separator sheet that reduces the water flow between and below the insulation to reduce the cooling effect of rainwater and therefore improve the thermal performance of the insulation.

Go to inverted roof insulation

Uplifting U-values on an existing flat roof

Increasing the thermal insulation within a roof makes perfect sense and has many benefits to the building, including its reduction in energy usage for heating or air conditioning as well as supporting the environment.

Many current flat roofs will require the addition of insulation to meet current Building Regulations, and this is added ideally as the waterproofing is renewed. We advise engaging with and seeking advice from the Local Authority Building Control to ensure that if any compartment walls exist beneath the flat roof their position is identified and the correct solution for fire performance is specified.

The design of the solution should ensure that thermal bridging at perimeters and details is reduced as much as possible.

Read more about improving the thermal properties of an existing roof in our technical centre.

Creating falls on a roof

It is a requirement of Building Regulation Part H that adequate provision is made for rainwater to be carried from the roof of the building. According to BS 6229 & BS 8217, flat roofs should be designed with minimum falls of 1:40 to ensure a finished fall of 1:80 can be achieved, allowing for any inaccuracies in the construction. This applies to general roof areas along with any internal gutters.

A cost-effective method of creating a fall on a flat roof is to incorporate it into the warm roof system by using tapered insulation which is quick and easy to install. It is available in different thicknesses to achieve the required fall and desired thermal requirements. Our technical department will design bespoke tapered insulation schemes and layout plans in accordance with the latest building standards for each roof area. We have three options for tapered insulation; BauderPIR FA, BauderGLAS and BauderROCK.

Falls on a roof can also be achieved by adjusting the height of the supporting beams or purlins, by using tapered supports, or by the addition of firring pieces before the deck is laid. In the case of a cast in-situ concrete slab, falls are normally provided by the use of a screed.

Read more about incorporating tapered insulation in a warm roof in our technical centre.

Insulation for acoustic solutions

The use and combination of insulation in a flat roof can reduce sound transmission into and out of a building and reverberation. Rain noise performance is generally the biggest issue faced by building designers as there are targets to be met in schools and hospitals and these targets vary depending on the room type.

BB93: Acoustic Design of Schools” states a 25dB max increase in sound pressure level (LAeq) during heavy rain.
Health Technical Memorandum 08-01: Acoustics” states 20dB max increase in sound pressure level (LAeq) during heavy rain

The solutions for reducing sound transmission vary according to the type of deck construction, the selected waterproofing system and whether the solution can include a green roof and/or solar PV.

An acoustic consultant can provide advice regarding mitigating the type of noise that needs to be negated and we can assist with the selection of products to deliver the solution.