Roofing Slate - Users Guide
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(1) Lifespan of Slate
The quality, performance and durability of roofing slate can vary enormously. The lifespan of slate in Australia may be as short as 10 years or longer than 150 years depending on the origin of the slate that is used and the quality of building design and workmanship.
The only slate that is known to perform 150 years or more under Australian conditions is Welsh Penrhyn – a slate that came to Australia as ballast in trading vessels the early 1800s.
The introduction of Spanish slate such as Cupa & Samaca has only occurred in the 15 – 20 years so less is known about how they perform on roofs under Australian conditions. Test results reveal they will last up to 75 years – only time will tell.
Condensation in roofs is a major cause of slate and batten failure. Irrespective of slate quality, the performance of a slate roof is at the mercy of architects and builders – ventilation of the roof cavity is a critical determinant of longevity. Quite often moisture build-up from condensation will rot out battens long before the slate is affected. Ultimately, slates too will perish from excessive condensation – it will just take longer because of their density and water resistance.
(2) Measuring Slate Performance Objectively
The top slate manufacturers in the world routinely test slate as part of their quality assurance programs – the ones that don’t usually have something to hide.
There are a number of relevant tests for roofing slate that collectively indicate how well they will perform. There are many types of roofing slate available – the best slates have test results that validate their performance.
Each of the following tests has been designed to replicate the most common causes of failure.
Water Absorption Test – a slate should not increase in weight after repeated cycles of wetting (distilled water), boiling and drying. Levels of weight increase under 0.25% are optimal and under 0.3% acceptable. Slate is effectively impermeable to water but water trapped under the lapped zone can create failures on the under-surface of the slate.
Wetting & Drying Test – slates should not split, flake or delaminate after 15 cycles of soaking, cooling and drying in a ventilated oven. The best slates dry quickly. Water that adheres to the slate surface will dry slower and trap dirt and airborne biological matter which then holds moisture on the slate for longer predisposing the slate to premature degradation.
Acid Immersion Test – slates immersed in sulphuric acid baths for 240 hours should so no signs of swelling, softening, flaking or delamination along the edges and no gas formation or release.
Modulus of Rupture Test – this attempts to measure the flexural strength of slate to simulate foot traffic and hail on slate roofs. The test originated in the US and is based on the ability to withstand a 9000lb load on a 3/16” slate.
Flexure Test - These test methods cover determination of the breaking load, modulus of rupture and modulus of elasticity of slate by means of flexure tests
Weather Resistance Test – includes a depth of softening test and a chemical weathering test that measures slate disintegration that correlates with the durability of slate in actual weathering.
(3) Standards Protect Consumers
1. ASTM C406-06 – Standard Specification for Roofing Slate
The American Society for Testing & Materials (ASTM) was formed in 1898 but these days ASTM standards are truly international. ASTM C-406 is an extremely rigorous standard for roofing slate that has 3 grades – S1 (the top grade), S2 & S3. Slates that meet the S1 standard have a durability of 75 years or more. Penrhyn, Cupa and Glendyne slates are all S1 compliant.
The grades are based on physical tests:-
- flexure testing (ASTM C120- 06e1)
- water absorption (ASTM C121-06)
- weather resistance (ASTM C217-94)
- compressive strength (ASTM C170-90)
They also incorporate physical requirements:-
- minimum thickness 3/16” (4.76mm)
- free from broken corners
- curvature less than 1/8” (3.18mm)
- no “knots” or “knurls” that affects the smoothness of split
- freedom from “ribbons” i.e. rippled surfaces
- less than 1% broken or cracked slates
- maximum allowable deviation from face dimensions is 3mm from specification
- slates to be square & corners cut to be right angles.
2. NF P32.301 & P32.302 - French Norm or Standard for Roofing Slate
These are French standards for roofing slate widely regarded as being the toughest most stringent standards for roofing slate in the world. This is because they only test slates randomly sampled by independent representatives of the testing house rather than having company selected slates sent for testing. In addition, sampling may take place at any time in any place. This means the test results are genuinely statistically significant. The standards incorporate all the tests outlined above.
It would be highly unlikely to achieve continual compliance with these standards unless a company had one hellishly good quality assurance program backing its products. Cupa slates are NF: P32.301& P32.302 compliant.
3. BS680 Part 2 1971 – British Standards for Roofing Slate
These are superseded British standards that were applied to samples submitted (rather than randomly sampled) for the following tests:-
- water absorption test
- wetting & drying test
- acid immersion test
This standard was criticized because it did not address iron pyrite content - some types of which are reactive and cause slate to rust out causing roof failures. For what it is worth, all slates distributed by Bellstone were BS680 compliant.
4. BS EN 12326-1 2004 – European Standards for Roofing Slate
This European standard came into force in May, 2006 and replaces BS680 Part 2 1971. These standards were set by member countries of the EU and are a compromise and because of this they are not particularly useful to the consumer, because the bar has allegedly been set “too low”.
Its contents include:
- Symbols and abbreviations
- Origin and petrography
- Thickness
- Bending strength
- Grain Water absorption
- Freeze–thaw resistance (only for code A2 from Table 2)
- Thermal cycle test
- Carbonate content
- Sulphur dioxide exposure test
- Non-carbonate carbon content
- Defects
- Dimensions
- Release of dangerous substances
- Number of slates required for type tests and factory production control
- Evaluation of conformity
- Testing frequency
- Type of tests
- Factory production control
- Procedures in case of non-conformity and/or complaint
- Marking, labeling and packaging
(4) Quality Assurance – ISO 90001:2000
Quality assurance (QA) is a management process that addresses the critical control points in delivering slate for mountain to market. Quality management is documented and compliance with all the steps along the way to deliver product to specification is audited to ensure 100% compliance. Independent auditors are used to ensure company integrity.
Slate is not just about good raw material it is also about how diligently a company processes the slate and the care it takes in checking quality.
ISO 90001:2000 is generally regarded as the most rigorous QA standard in the world. Welsh Slate, Cupa, Samaca & Glendyne have ISO accreditation.
Buying slate from companies without independently audited quality assurance programs is a risk not worth taking.
(5) Slater / Fixing Skills
Slater is a term of respect for a roofer or roofing contractor that bestows reverence for the enormous skill levels that are required to “fix” or lay a slate roof. Achieving a uniform roof surface with every slate lying flat with no “kickers” is an enormous challenge especially on roofs with steep pitches.
Many slaters working in Australia are of British or European descent – some 2 nd and 3 rd generation.
Choosing an experienced slater is vital. If in doubt ask to see their last job or better still get independent, professional advice.
(6) Grading Slates - Flatter Roofs
The best slaters will carefully grade every slate before it goes on the roof. They grade slate thickness into three groups using the thinner slates at the top of the roof and thicker slates near the eaves. Usually only a small percentage of slates are bowed or twisted; these are set aside for “cuts” on hips, valleys and verges.
A professionally fixed roof using graded slates will have a wonderful harmony from one slate to the next and will contain no “kickers” – bowed slates that sit up and ruin the uniformity of the roof line.
(7) Headlaps – Hidden Hurdles
Headlap is a technical term that relates to how slates are lapped and fixed on a roof. In Australia, the usual headlap for a roof with a pitch of 40 degrees is 75mm. This means that 15.4 slates per covered metre will be required if 500x300mm slates are used and 18.5 slates per covered metre if 500x250mm slates are used.
Flatter roofs e.g. 30 degrees, require bigger slates and headlaps. The reverse is true for roof pitches greater than 45 degrees.
If lower headlaps are used, fewer slates are required to do the job which either increases roofing margins and/or lowers cost but this may not be good practice. Headlaps are specified by architects for all new buildings. If you are re-roofing an old building make sure that the quotes for work clearly state the headlap to be used.
(8) Copper Nails are for Keeps!
Hundreds of nails are used to fix slate to roof battens. Sometimes the longevity of a roof can be determined by the quality of nails used. If nails corrode in salt or polluted air slates can slide off a roof long before their lifetime performance has been met. Falling slates are dangerous and both nails and slates are costly to replace.
The best nails are copper, they cost a little more especially when copper prices are so high but if their initial cost is written off against their longevity, they are a better investment than cheaper alternatives. Ask to have the type of nails to be used specified when seeking quotes for work.
(9) Oregon Battens Last Longer
Oregon is the timber of choice for use as battens under slate roofs. While pine may be popular for many short lived roofs it is essential that the timber used under slate has the ability to match its lifespan.
(10) Patination Oil Prevents Leaching
New lead is commonly used on slate roofs for ridge capping and flashing around chimneys. Rain reacts with the lead to form an unattractive, lead carbonate stain that is white in colour. Over time, white leach marks gravitate down the slates and persist for many years.
These stains are unsightly and can be easily prevented by coating lead with patination oil that forms a semi permeable membrane on the lead.
If lead is used on a new roof, it is essential to have it treated with patination oil prior to, or immediately after fixing.
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