Straw bale walls need ample roof overhangs to protect them from humidity, much more so than more common construction techniques.
A hip roof provides ample overhang on all sides of the construction. While the roof itself is more complex to build, it rests directly on the bond beam on top of the straw bale walls. This considerably simplfies and speeds up construction. No triangular areas have to be filled with straw bales.
An excellent place to help you calculate build your hip roof yourself is http://www.blocklayer.com/Roof/default.aspx .
If you want to build a green or a similarly heavy roof on top of a load bearing straw bale constructions, make sure the walls can carry it. Have it calculated!
If your reason for choosing to build with straw bales includes an element of environmental concern then some options quickly become more attractive than others. In this context the main concerns are the embodied energy of the roofing system and the potential for reuse of the materials at a later stage. For example the production of new roofing tiles uses very large amounts of energy, which contributes to our burden on the environment. Clay/ terracotta tiles require large amounts of energy to bake. Concrete tiles must take the burden of the energy used in the extraction and heating of lime to make cement. On the other hand some types of roofing tiles can easily be removed and used again and again for several hundred years.
In many cases and depending on where you live, collecting rain water or minimising roof runoff can be important. If you are not collecting your roof water then a green or living roof can be an option. Who wouldn't like a roof garden?
Another direction for roofs in straw bale buildings is to make arches and vaults of straw bales so that they all press on each other giving a stable compressive structure just like the stone arches of ancient roman times. There is more about arches in the section on Straw Bale Construction/Techniques/Pushing_the_Limit.
The most typical solution is a conventional roof structure attached to a load-distributing plate or beam running all the way along the top of the bale walls.
The Green RoofEdit
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One advantage of green roofs is the stability they add to the temperature of the roof. Because of their size and mass they are slow to warm up and cool down. In places where there is a large temperature change from day to night this can be a great advantage.
The actual insulation value of a green roof (and here we're talking about not more than 30cm thick) is unclear. The presence or absence of water and roots makes such a large difference that it is hard to generalise. Naturally if the layer of soil is thick enough it will provide more insulation and a considerable amount of stability to the temperature.
Roof and Ceiling InsulationEdit
One of the easiest and most effective places to add extra insulation is in the space between your ceiling and the roof. So don't overlook this important part of the overall design.
Conventional roof structures may be insulated with straw bales, taking advantage of their high insulation values and good acoustic properties. Other alternative insulation includes rice-hulls, cotton or wool batts, soy-based foam and recycled cellulose. According to comments on an unrecorded test by Tim Owen-Kennedy of Vital Systems  rice hulls perform just as flame retardant as borate treated cellulose or better, without being treated. According to The Rice Hull House (Paul A. Olivier) from around 2004;
- ...rice hulls are unique within nature. They contain approximately 20% opaline silica in combination with a large amount of the phenyl propanoid structural polymer called lignin. This abundant agricultural waste has all of the properties one could ever expect of some of the best insulating materials. Recent ASTM testing conducted R&D Services of Cookville, Tennessee, reveals that rice hulls do not flame or smolder very easily, they are highly resistant to moisture penetration and fungal decomposition, they do not transfer heat very well, they do not smell or emit gases, and they are not corrosive with respect to aluminum, copper or steel.
(The following quote from a reference in the same article needs to be followed up)
- "Rice hull has a thermal conductivity of about 0.0359 W/(m.°C); the values compare well with the thermal conductivity of excellent insulating materials (Houston, 1972).” Juliano (1985), p, 696. The thermal conductivity of rice hull ash is reported to be 0.062 W.m-1.K-1. See UNIDO, p. 21. A more recent test done by R&D services of Cookville, Tennessee, indicates a 3.024 R-per-inch."
Many of these natural products have a very low impact on the environment and perform excellently, sometimes better than synthetic insulation like rock wool or fibre glass insulation.
If straw bales are used in the roof, their weight needs to be considered. Moisture is another consideration, and there is a fire risk if any loose straw is left exposed. Weight considerations are overcome by the fact that web-beams built to the height of the bales can easily bear their weight. To avoid moisture problems, it is important that the bales be treated just as walls are. They need to have good ventilation on the outer surface (a ventilation space) and should be coated with some plaster (typically clay or lime plaster) that can absorb, redistribute and release to the air any moisture. It cannot be overemphasised that no straw should be left exposed, plastering should be done in such a way that it acts as a suitable fire retardant.
- "Pitched roofing materials compared" is part of the National Green Spec website for encouraging sustainable building practises.
- "In 1999, Green Roofs for Healthy Cities, a small network consisting of public and private organizations, was founded as a direct result of a research project on the benefits of green roofs and barriers to industry development entitled "Greenbacks from Green Roofs" prepared by Steven Peck, Monica Kuhn, Dr. Brad Bass and Chris Callaghan. Green Roofs for Healthy Cities - North America Inc. is now a rapidly growing not-for-profit industry association working to promote the industry throughout North America."
- "Sustainable Roofing" is part of the website of the The Green Building Press
See the bibliography.