slate
Roofing: MATERIALS ENCYcLOPEDIA
Applications for this system
Roof sheathing
Wall sheathing
Basic materials
Slate tiles
Ceramic or metal ridge and hip caps, in some cases
Roofing nails
Wooden strapping
Ratings Chart for Slate roofing
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- WASTE
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- DURABILITY
- CODE COMPLIANCE
- RAINWATER COLLECTION CAPABILITIES
- RESOURCES
- FUTURE DEVELOPMENT
Slate roof System
Slate roofing uses wide, flat pieces of quarried slate in a manner similar to all other shingle-style roofing systems. The slate is quarried to a predetermined thickness, length and width, with holes drilled at the top edge to accept nails. The slates are nailed to a solid roof deck or appropriately spaced strapping. The weight of slate often necessitates additional strength in the roof framing and decking.
As with most shingles, a double row is laid at the edge of the roof with the next course laid over top of the preceding one such that a set amount of the lower course is left exposed. The seams between shingles are offset between courses, and the “reveal” exposed on each course is such that a cross section of the roof would show three layers of slate at any point in the roof. The slates come in varying widths, making it easy to stagger the joints between courses.
Ridge and hip capping is often achieved by careful overlapping the slates to provide positive lapping and drainage. In some cases, ceramic or metal capping is mortared onto the slate at hips and ridges. Valley flashing is usually a metal flashing with the slate cut to leave the metal exposed in the valley.
Environmental Impact Rating
Harvesting — Low to Moderate
Slate is only quarried in a few regions and the quarries have been established for a very long time, so the immediate effects on local ecosystems took place many decades (sometimes centuries) ago.
Manufacturing — Low
The cutting of slate uses basic machinery and is not energy intensive. The final stages of cutting are still done by hand in most facilities.
Transportation — Low to High
Sample building uses 9,202 kg of slate:
13.8 MJ per km by 15 ton truck
8.65 MJ per km by 35 ton truck
2.3 MJ per km by rail
Slate only occurs naturally in certain regions in North America, largely in the northeast, and is a very heavy material. Shipping will accrue significant impacts the further the material is moved from the point of production.
Installation — Negligible to Low
Most of the work of laying slate is manual labor, with no or minimal use of power tools.
Waste: Low
Compostable — All slate offcuts can be left in the environment. Quantities will vary from negligible to moderate, depending on the complexity of the roof.
Recyclable — Metal fasteners and flashing. Quantities will be negligible to low.
Landfill — Underlayment (if required). Quantities will be negligible to low.
Chart of Embodied energy & carbon
Energy Efficiency: n/a
A slate roof will not have a large impact on energy efficiency. Dark-colored slate will create heat buildup in the summer that can add to air-conditioning loads.
Material costs: high
Slate tiles are a specialty roofing option produced in comparatively small quantities, putting them at a cost premium.
Labour Input: High
Slate tiles are fastened to the roof with only one or two nails per tile, and the layout of the roof is the same as with other shingle or tile systems. Slate is heavy, and additional labor time is accrued in the moving of the material from ground to roof. The necessity of avoiding walking on the roof once slate is in place also adds some preparation time not required with other roofing styles.
Shaping the slate decoratively will add significantly to the labor time.
health warning
Working at heights to install roofing has inherent dangers. Proper setup and safety precautions should always be taken when working on a roof.
Skill level required for homeowners
Decking — Easy
A homeowner with the ability to frame a roof will be able to lay the required decking for a slate roof.
Underlayment (if required) — Easy
Underlayment is lightweight and comes in rolls that are straightforward to apply.
Shingles — Easy to Moderate
Slate roofing skills can be learned and successfully applied by beginners. The premise is the same as any shingle or tile roof system, and the basic dos and don’ts can be found in accessible resources. There are workshops available to help acquire some experience.
Caps and Flashing — Moderate to Difficult
Beginners working from written manuals or instructions will be able to manage simple slate roofs. More complex ridge and hip caps and valley and penetration flashings require some experience, input from a professional or excellent instruction to complete successfully.
Sourcing & availability: moderate to Difficult
Access to slate materials and expertise will vary widely by region. The closer one is to slate quarries, the easier it will be to locate slate, installers and learning opportunities. The northeastern United States and southeastern Canada are the main roofing slate-producing regions in North America.
Durability: Very High
A properly installed slate roof can be exceptionally durable, with some roofs still in serviceable condition after a hundred and fifty years. Slate that is stressed by foot traffic or other point loads can crack and fall from the roof, but otherwise the material does not deteriorate, corrode, wear or break down from UV radiation.
Code compliance
Building codes in regions where slate roofs have been used traditionally are likely to still have provisions to accept slate roofs in a modern context. There is a very thorough ASTM standard written for slate roofing that will be referenced in codes or acceptable to code officials. Where slate roofs do not have historical precedent, it may be more difficult to persuade code officials, but the ASTM standard should help.
RAINWATER COLLECTION CAPABILITIES
Water from slate roofs is typically of high quality and would be suitable for use inside the building with regular filtration strategies.
Resources for further research
ASTM C406 / C406M — 10: Standard Specification for Roofing Slate
Jenkins, Joseph C. The Slate Roof Bible: Understanding, Installing and Restoring the World’s Finest Roof. White River Junction, VT: Chelsea Green, 2003. Print.
Stearns, Brian, Alan Stearns, and John Meyer. The Slate Book: How to Design, Specify, Install, and Repair a Slate Roof. Stowe, VT: Vermont Slate and Copper Services, 1998. Print.
Future development
The methods for quarrying or installing slate are not likely to change. The high cost of slate will likely see it remain a less popular option.
Tips for a successful slate roof
1. Be sure to size roof framing members and decking to be compatible with the weight of slate tiles.
2. The use of the proper non-corroding nails is essential. It is important to drive the nail deeply enough that the head sits fully in the countersunk hole or else it will rub on the underside of the slate above and potentially cause cracking; at the same time it is important not to drive the nail too deeply such that it exerts pressure on the slate that might cause it to crack. Careful installation is the key.
3. Roofers must be kept from stepping on slates. While slate is very durable, it is prone to cracking when point loaded with foot traffic. Roofing ladders, scaffolding or other means of keeping workers off of slates must be used.
4. Be sure the slate you are using is intended for roofing purposes. Many types of slate that are used for interior flooring applications are not suitable for roofing, as freeze/thaw cycles on a roof can destroy them.
