Editing Roof (section)

===Outer layer===
{{Main|List of commercially available roofing material}}
This part of the roof shows great variation dependent upon availability of material. In [[vernacular architecture]], roofing material is often vegetation, such as [[Thatching|thatches]], the most durable being [[sea grass]] with a life of perhaps 40 years.  In many Asian countries bamboo is used both for the supporting structure and the outer layer where split bamboo stems are laid turned alternately and overlapped. In areas with an abundance of timber, wooden [[Roof shingle|shingles]], shakes and [[Board roof|boards]] are used, while in some countries the [[Birch-bark roof|bark]] of certain trees can be peeled off in thick, heavy sheets and used for roofing.

The 20th century saw the manufacture of composition [[asphalt shingle]]s which can last from a thin 20-year shingle to the thickest which are limited lifetime shingles, the cost depending on the thickness and durability of the shingle. When a layer of shingles wears out, they are usually stripped, along with the underlay and roofing nails, allowing a new layer to be installed.  An alternative method is to install another layer directly over the worn layer. While this method is faster, it does not allow the roof sheathing to be inspected and water damage, often associated with worn shingles, to be repaired. Having multiple layers of old shingles under a new layer causes roofing nails to be located further from the sheathing, weakening their hold. The greatest concern with this method is that the weight of the extra material could exceed the dead load capacity of the roof structure and cause collapse. Because of this, jurisdictions which use the [[International Building Code]] prohibit the installation of new roofing on top of an existing roof that has two or more applications of any type of roof covering; the existing roofing material must be removed before installing a new roof.<ref>{{cite web|title = Chapter 9 - Roof Assemblies|url = http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_9_par135.htm|website = publicecodes.cyberregs.com|access-date = 2015-08-31|url-status = dead|archive-url = https://web.archive.org/web/20150903034500/http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_9_par135.htm|archive-date = 2015-09-03}}</ref>

[[Slate]] is an ideal, and durable material, while in the [[Swiss Alps]] roofs are made from huge slabs of stone, several inches thick. The slate roof is often considered the best type of roofing. A slate roof may last 75 to 150 years, and even longer. However, slate roofs are often expensive to install – in the US, for example, a slate roof may have the same cost as the rest of the house. Often, the first part of a slate roof to fail is the fixing nails; they corrode, allowing the slates to slip. In the UK, this condition is known as "nail sickness". Because of this problem, fixing nails made of [[stainless steel]] or [[copper]] are recommended, and even these must be protected from the weather.<ref>{{cite web|title=Six Steps to Building a 150 Year Roof|url=https://kocaeliwebtasarim.medium.com/six-steps-to-building-a-150-year-roof-4624749b33c2|access-date=2021-08-18|website=kocaeliwebtasarim.medium.com|archive-date=2021-08-18|archive-url=https://web.archive.org/web/20210818184240/https://kocaeliwebtasarim.medium.com/six-steps-to-building-a-150-year-roof-4624749b33c2|url-status=dead}}</ref>

[[Asbestos]], usually in bonded corrugated panels,  has been used widely in the 20th century as an inexpensive, non-flammable roofing material with excellent insulating properties. [[Asbestosis|Health]] and [[Asbestos and the law|legal issues]] involved in the mining and handling of asbestos products means that it is no longer used as a new roofing material. However, many asbestos roofs continue to exist, particularly in South America and Asia.

Roofs made of cut turf (modern ones known as [[green roof]]s, traditional ones as [[sod roof]]s) have good insulating properties and are increasingly encouraged as a way of "greening" the Earth. The soil and vegetation function as living insulation, moderating building temperatures.<ref>{{cite web|date=2020-02-06|title=Green and Cool Roofs|url=https://drawdown.org/solutions/green-and-cool-roofs|access-date=2020-12-05|website=Project Drawdown|language=en}}</ref> Adobe roofs are roofs of clay, mixed with binding material such as straw or animal hair, and plastered on lathes to form a flat or gently sloped roof, usually in areas of low rainfall.

In areas where clay is plentiful, roofs of baked [[Roof tiles|tiles]] have been the major form of roofing. The casting and firing of roof tiles is an industry that is often associated with [[brickworks]]. While the shape and colour of tiles was once regionally distinctive, now tiles of many shapes and colours are produced commercially, to suit the taste and pocketbook of the purchaser. Concrete roof tiles are also a common choice, being available in many different styles and shapes.

[[Architectural metals|Sheet metal]] in the form of [[copper]] and [[lead]] has also been used for many hundreds of years. Both are expensive but durable, the vast copper roof of [[Chartres Cathedral]], oxidised to a pale green colour, having been in place for hundreds of years. Lead, which is sometimes used for church roofs, was most commonly used as [[Flashing (weatherproofing)|flashing]] in valleys and around chimneys on domestic roofs, particularly those of slate. Copper was used for the same purpose.

In the 19th century, iron, electroplated with [[zinc]] to improve its resistance to rust, became a light-weight, easily transported, waterproofing material. Its low cost and easy application made it the most accessible commercial roofing, worldwide. Since then, many types of [[metal roof]]ing have been developed. Steel shingle or standing-seam roofs last about 50 years or more depending on both the method of installation and the moisture barrier (underlayment) used and are between the cost of shingle roofs and slate roofs.
In the 20th century, a large number of roofing materials were developed, including roofs based on bitumen (already used in previous centuries), on rubber and on a range of synthetics such as [[thermoplastic]] and on [[fibreglass]].
<gallery mode="packed" class="center" caption="Outer layer">
File:The Elms Jersey.jpg|Slate, [[Jersey]]
File:Miskolc omassa peasant house.jpg|Terracotta tiles, [[Hungary]]
File:Niigata NCM Peasant Rice Farmers House.jpg|Thatch, using rice straw, [[Japan]]
File:Njem house in Cameroon.jpg|Banana leaves, [[Cameroon]]
File:Neot smadar.jpg|[[Date palm]] branches, [[Neot Semadar]], [[Israel]]
File:Minenverwalter Kolmannskuppe.jpg|Metal sheeting, [[Namibia]]
File:Gassho-zukuri farmhouse-02.jpg|Repairing thatch, Gassho-zukuri farmhouse, [[Japan]]
File:Stone Roofing House - Palchan - Kullu 2014-05-10 2507.JPG|Stone used as roofing material in [[Himachal Pradesh]], [[India]]
File:Green Roof in Norway.png|Turf roof in [[Norway]]
</gallery>