May 2011

design

see also Vapour barriers

Green roof design can affect several aspects of the building:

• Having generous roof overhangs can significantly reduce the amount of maintenance to woodwork such as doors, windows and cladding
• Careful sourcing roofing materials is important as a way of reducing the embodied energy and minimising manufacturing pollution
• The roof can be designed to harvest rainwater
• It can reduce flash flooding
• It may be able to recycle grey water
• As a living roof it might provide a habitat for wild life or a garden

 

Roof overhang

Simply because most rain in the UK falls within 5º of vertical, a generous overhang will prevent most rain reaching the walls. While this costs slightly more initially for the extra roof area, it does save money in the long run and usually adds to the style of the house, in turn making it more valuable.

Sourcing materials Roofing materials should be checked for embodied energy and for how polluting they are in manufacture. For instance most of the natural stone slates coming into the UK at present are coming from India and China. Lead for flashings is often manufactured in a very polluting way. This subject is dealt with well in the Green Building Handbook.

There is a potential pollution problem from zinc roofs.

rainwater Harvesting

With only the minimum of filtering, collected rainwater can be used for watering the garden, washing the car, the cold feed to the washing machine and flushing toilets. Particularly in parts of the country where water is becoming increasingly scarce, the low investment required for the first two of these makes a lot of sense.

Bear in mind that a living roof, see below, is not an ideal candidate for collecting rain as it tends to absorb quite a lot itself, particularly during dry periods when you need it most.

SUDS

The increasing areas of hard surfaces in towns such as paths, driveways, roads, hard standing, etc. put a very high strain on the sewerage system in most towns. This is because when it rains there is sudden runoff into the drains rather that gradual seepage into the earth. SUDS set out guidelines on how to prevent this on the CIRIA web site. One of the ways is to have a living roof (see below).

rooftop grey water recycling

Research has been going on at Cranfield University into the possibilities of rooftop cleaning up of grey water for use in flushing toilets. The result is GROW by Waterworks UK Ltd.

Timbrel vault roofs have made a rare showing with the house featured on Grand Designs on 20th February 2009. This house, which is to Passivhaus standards, has a living roof formed from a large arch constructed of thousands of plain hand made local clay tiles bonded together with plaster of paris and laid in three layers. There is a description of a timbrel vault roof at Pines Calyx.

timber roof structures

see also timber frame and SIPs | Walter Segal | Solid timber

From a green standpoint the main thing to remember is that timber should be woodmarked and sourced as locally as possible, preferably in the UK.

Structural timber has the advantage over most other structural materials of being renewable and also a relatively good thermal insulation material.

Structural timber is usually of the following types

• normal softwood sections such as floor and ceiling joists as specified in the building regulations.
• larger sections of softwood or hardwood forming beams, trusses etc. and usually calculated by a structural engineer.
• laminated structural timber
• engineered timber such as I beams.

The normal softwood sections are usually graded as C16 or C24 (respectively replacing SC3 and SC4) and this will be stated in the approved building regulation drawings. It is often available from UK plantations

Larger sections of softwood are also available from the UK and it is often possible to get them from local sawmills.

The larger hardwood sections may be more of a problem to source sustainably and in the case of tropical hardwoods, only the woodmark can be relied upon. (However it is seldom that tropical hardwoods are used structurally in housing). There is a considerable amount of timber produced sustainably in the UK which is not woodmarked mainly because it is produced in such small quantities that the certification procedure would not be warranted. E.g. there is a constant supply of hedgerow ash and to a lesser extent oak and other species which is not woodmarked but which gets replaced. There are also organisations such as Woodlots which may be of help in sourcing local timber.

Laminated timber (sometimes known as Glulam) is generally well sourced environmentally. However, due to the poor understanding of timber building culture in the UK it has been marketed mainly towards large structures such as offices, swimming pools, theaters etc rather than the housebuilding market, so it can be difficult to find merchants who are supplying off-the-peg structural members. The Glued Laminated Timber Association has a list of member companies. Also try Panel Agency Limited and Lamisell

Engineered timber such as Masonite beams represents a huge step forward in timber technology. Compared with traditional beams and joists Masonite sections are, for the same structural strength, much lighter, much more dimensionally regular and use considerably less timber. The dimensional stability with traditional beams and joists can be a major problem if they are not supplied at a moisture content of 12%, as shrinkage can cause considerable movement. It is not unusual to hear of 20mm movement over a two storey timber structure in the first year . They also help with the insulation because the webs, being much thinner, cause minimal thermal bridging. They do however run at about twice the price of solid timber sections with merchants such as Arnold Laver quoting around £4.80/m for 220 x 38 I beams. (supplied in 12m lengths).

James Jones do a set of roof joist details and span tables for their I beams

European Standards

On 1st April 2010 the new CEN Eurocode standards for structural timber came into force in place of the old BS standard. These are -

• BS EN 1995-1-1 Eurocode 5: Design of timber structures. Part 1-1 General – Common rules and rules for buildings
• BS EN 1995-1-2 Eurocode 5: Design of timber structures Part 1.2 General – Structural fire design

TRADA have published span tables in a new softback book called Eurocode 5 Span Tables: For Solid Timber Members in Floors, Ceilings and Roofs for Dwellings and various companies do online calculation software. However for practical purposes the self builder will still find the span tables in the old (archived) Approved Documents from 1992 to be useful in determining sizes for floor, ceiling and roof joists, binders, rafters and purlins. There is extremely little difference between the old span tables and the new ones. The slight discrepancy is mainly in spans shorter that 2.4m. See more on the Eurocode News site.

rainwater goods

Rainwater goods, mainly gutters, fall pipes and their related connectors, are made of various types of material. These are:

• metal
  • cast iron
  • galvanised steel
  • aluminium
  • copper
  • stainless steel
  • zinc
  • galvanised steel
• wood

• plastic
  • PVC
  • GRP

Cast iron, the traditional material for much of the rainwater goods in the UK, (although timber gutters were largely used in the north) has several drawbacks. It is very high in embodied energy and needs regular maintenance and painting to prevent it rusting.

Coated galvanised steel has traditionally been widely used in northern Europe and is environmentally somewhere in the middle; easily recyclable and not particularly high in embodied energy. However its endurance is not that great if the coating gets damaged. It then needs regular painting inside and out.

Aluminium, copper and stainless steel all make high quality rainwater goods though they are all very high in embodied energy and tend to be polluting during manufacture. They all have very good recycling rates.

Zinc rainwater goods are quite low in embodied energy, reasonably lightweight and and totally recyclable. Unfortunately they corrode fairly quickly and the run off is polluting to land and water. There appear to be no UK manufacturers for domestic systems although German and French suppliers have agents here.

Interesting bit of chit chat about repairing zinc goods here

Wooden gutters have been in common use, particularly in the North of England for the last few hundred years. They are extremely low in embodied energy and are easily disposed of at the end of their life. They are also very sturdy and do not suffer from ladders being leaned against them. However, the quality of timber used has declined from being very high up till the middle of the 20th century (often high grade Baltic redwood) down to almost any old rubbish now. Even when treated, water gets into knot holes and cracks and rots them relatively quickly. The practice of painting them internally with bituminous compound does little to extend their life as the bitumen flakes off in sunlight.

Lining them with EPDM works well because it keeps them dry and contributes very little to embodied energy. It also obviates the need for the quite difficult lead joints at corners and running joints. The only complication is having to fix top hats at outlets. This is not difficult once you have got your head round it. Timber gutters are very resistant to mechanical damage, eg. from ladders. They work very well with living roofs because they are strong enough to be able to support the growing medium along the edge of the roof, especially if the gutter has a 65mm plastic land drain laid in it with a covering of pea gravel. This obviates any problems of leaves blocking the gutter. The Roof membrane can be run from the roof down over and into the gutter so that the gutter is fully protected.

Although so much of the housebuilding market uses PVC guttering this is probably one of the less environmentally acceptable materials because of its embodied energy and the fact that it is so easily damaged (by ladders etc). It suffers from high thermal movement which can cause problems at joints if not installed very carefully. PVC is constantly having to be replaced because of damage and this further increases its effective embodied energy. PVC fall pipes tend to fair much better than gutters, partly because they are less susceptible to mechanical damage and they form much better joints than gutters. They can be reused if the joints are replaced (whereas this is much more difficult with gutter sections).

Corrosion of various types of materials is discussed here

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trade associations

The National Federation of Roofing Contractors

The Single Ply Roofing Association produces a design guide for single ply roofing with lots of useful information.