Landscape Detailing Volume 3

Structures

Michael Littlewood

• Publisher: Routledge
• Year: 2013
• eBook ISBN: 9781136153976
• Edition: 3
• Topic: Architecture
• Subtopic: Urban Planning & Landscaping

Michael Littlewood's Landscape Detailing is now well established as a valuable source of reference for architects, landscape architects, other professionals and students designing external works. For this third edition it has been split into three volumes to give a greater
depth of coverage than ever before.Volume 3 covers pergolas, arbours, arches, gazebos, summer houses, sheds, shelters, decks, footbridges, furniture and roofs. Each section begins with technical guidance notes on design and construction. This is followed by a set of drawn-to-scale detail sheets. These details can be traced for direct incorporation into the set of contract drawings. A list of relevant references, bibliography and a list of association and institutions indicate where further guidance can be obtained. A ready reference for landscape designers and an indispensable time-saving tool, Landscape Detailing is an essential for the design office.

Roofs

Design Factors

1. Structural loadings
2. Drainage
3. Membrane
4. Planting medium
5. Irrigation
6. Materials
7. Features
8. Services
9. Safety

In designing a residential garden or a commercial landscape on a building above the ground there are special factors to be considered compared to ground level developments. These are:

Access – to the roof from the ground, especially during the construction phase and afterwards for maintenance.

Location – if the building and roof are very exposed on all sides considerable protection from wind and sun, as well as climatic extremes of hot and cold.

Roof – whether the existing roof is capable of having a garden or whether it can be amended to take the additional weight of a garden.

Drainage – there must be quick and positive surface and subsurface drainage to prevent flooding due to heavy rains. The weight of accumulated ice and snow must not exceed the weight limits of the roof’s structural system.

Planting Medium – this must be lightweight, long lasting, good water permeability, air retention capacity and root penetration, with a high water intake and storage followed by a slow and continuous supply of water to the plants.

Irrigation – this is essential to ensure the survival of plants in a hostile environment and must be allowed for in the design. Materials – it is essential that these are lightweight but robust to resist harsh climate conditions.

Features – the design of features must ensure the protection of the integrity of the roof and structure, particularly water.

Services – these are integral to the success of the roof garden/landscape and need to be considered at the outset.

Safety – the need for fire escape is a priority and the design of the roof landscape must ensure that there is not restriction of fire exits/ladders.

Protection of users going close to the edge is equally important.

1. Structural Loadings

Provision has to be made for the weight of soil, materials, plants and for people on a roof garden. A structural engineer should always be consulted before commencing any type of roof design.

Structural movement joints are one of the points likely to fail and should not be covered by any planters/plants. If it is unavoidable there are ways to design a solution to the problem.

2. Drainage

Adequate falls on the existing roof are necessary to ensure the removal of surplus water along with the appropriate number of drains. Roof drains, like all others used in the garden, should be designed to collect both surface and lateral subsurface drainage water.

This type of planting medium used on roofs ensures downward percolation of water direct to the sealed surface. Positive lateral drainage is then necessary through the subsurface of the soil to the drains.

There are a number of typical drainage details that can be used on roof landscapes and these are shown in illustrative form. Provision should be made for cleaning of the drains at periodic intervals.

3. Membrane

It is essential that the material chosen for the waterproof membrane is appropriate as there are for the project two types – continuous membranes and jointed membranes. For projects with a large area the only continuous membrane is asphalt. Smaller projects would use jointed membranes.

A typical section through a roof consists of the structural framing or reinforced concrete slab, sometimes a sloping layer of thin concrete to provide drainage to roof drains, a layer of waterproofing material, a layer of insulation, and a layer of lightweight concrete to protect the insulation and the waterproof membrane. This final layer of lightweight concrete is sloped to carry off surface drainage.

The most important consideration is to protect the roof and its waterproofing at all times and it is the designer’s responsibility to ensure that this is achieved. This starts with the bottom of the subdrainage layer to the finished roof.

4. Planting Medium

The critical factors in the formulation of a suitable planting medium for roof garden planting are lightweight, the ability to hold nutrients for plant growth, and the capability of developing a firm but easily drained soil structure.

There have been various mediums used in roof gardens. A base layer of lightweight clay granules topped with lightweight compost containing vermiculite, perlite or polystyrene with a 50/50 mix of loam and peat substitute is common. Grodan – a lightweight soil substitute produced from volcanic rock could be used; in addition to being a uniform growing substrate it has sound and temperature insulation qualities. It is available in slab form and placed over a reservoir of lightweight aggregates and, providing water is replenished regularly, capillary action ensures that the root zone is kept moist and the water/air balance constant.

Ready-mixed soils are available commercially, but a suitable soil mixture can be prepared for each project following the given formula per cubic metre:

½ m² fine sand

½ m² pine bark

500 gms calcium nitrate

1½ kg single superphosphate

500 gms sulphate of potash

3½ kg dolomite lime

When this mix is too heavy it may be lightened by substituting 3 parts expanded shale in No. 8 screen and 3 parts expanded shale No. 10 (texture of fine sand) for the sand elements. Expanded shale is commonly used in place of gravel in lightweight concrete and is generally available from suppliers of concrete materials.

Fine sand

The fine sand used in soil mixes is of critical importance in terms both of the soil’s drainage characteristics and of its ability to function without blocking the permeability of the filter blanket.

Soil Amendments

Organic materials such as fertilisers – particularly nitrogen, but also other minerals which go into solution – will gradually dissolve and be leached away by watering. Local public water supplies may also lack calcium, magnesium and sulphur. Thus, periodic replacement of these and other materials by surface application may be needed. At least once a year, soil tests should be made by a competent soils laboratory to determine deficiencies and to recommend additives.

Roof Weight Reduction

If optimum soil depths result in excessive weight on the structure, then lig...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright
• Contents
• Foreword
• Acknowledgements
• Introduction
• Pergolas, arbours, arches and lych gates
• Shelters, gazebos and sheds
• Decks and boardwalks
• Pedestrian bridges
• Outdoor furniture
• Signs and information
• Roofs
• Appendix A
• Appendix B
• Appendix C
• Appendix D