Posted by dave on May 16, 2010 at 3:14 PM

Roofing Slates

Roofing Slates Slate is an excellent roofing material, for centuries it has attractively cover roofs and prevent water ingress into a roof structure and living space.

•Slate Characteristics

•About Roofing Slates

•How Was Slate Formed

•Slate Characteristics

•How Roofing Slates Are Made

•Dressing of Slates

•Welsh Slate Sizes

•How Long Do Slates Last

•Slate Quality

•Roofing Slate Tests

•How To Choose Slates For You Roof

•Roof Pitch

•Roof Loading

•Local Weather Conditions

•Slate Roof Maintenance

DIY Roofing:

DIY How To TipsSlate is strong and durable, chemically inert, non-combustible, non-porous and doesn't warp, shrink or rot.

About Roofing Slates

Roof slates are a natural, roof covering whose function is prevent ingress of water into a roof. Laying or hanging slates is by fixing then so onto secured, regular rows of battons so they overlap each other and the framework of a roof.

People choose slates because they meet both the aesthetic requirements for housing and they withstand environmental forces such as weathering and wind.

Because slates are an attractive and high quality roofing the material natural variation, allows us to exploit their different characteristics for specific roofing uses. they re are alternatives on the market. These man made substitutes include such as reconstituted slate and fibre cement 'slates'.

Because each roofing slate is unique, the natural variation allows for different characteristics of slate to be exploited for specific uses.

For example, experienced slaters will identify and order slated according to thickness, natural curvature, surface texture and appearance this allows the strongest slates to be laid at the areas of the roof that experience most stress.

At a UK scale, we select the most economical slate for a specific roof requirement. We select slate based on its ability to withstand the extremes of weather found at the particular geographical location and height of a building.

General Characteristics of UK Roofing Slate

•Thickness: 4mm to 12mm

•Colour - The chemical and mineralogical composition of slate determines its colour. These factors differ between different slate quarries so it is possible to find slates in a variety of colours and shades and matching existing slate colours and sizes requires an experienced eye. The available colours include: Heather Blue, Blue Grey, Westmorland Green, Blue Black, Grey Green, Black Black, Green, Grey, Blue, Weathering Grey, Green Green

•Size - table below from: Large, Medium, Small - 10 x 5 to 26 x 16 inches

•Finish - Fine rubbed or sawn; Very Smooth, Smooth, Normal, Textured, Rough

•Weathering Rate by wind, rain and snow: Slow, Medium, Fast

•Sources/Origin of Slate: Wales, Cumbria, Spain, China, Argentina, Brazil

How Was Slate Formed

Slate is natural stone, which has been formed from fine-grained particles deposited in horizontal strata after which through the metamorphic processes such as time, pressure and heat. It can thus be split along these fine strata into slabs or thin slates.

Slate Characteristics

Being a natural material, even after roofing slates have been created and individually dressed they can retain natural variations in their characteristics.

We select according to specific characteristics such as quality, thickness, colour, size and finish.

How Roofing Slates Are Made

Slate dressing uses natural, mined slate that has been extracted from a quarry or mine. The first process is the extraction of large slate blocks or slabs once they are quarried.

These are then made uniform in shape when they are cut to the required dimensions using, for example, hunter or diamond saws. The faces of the blocks are then planed (in a planer) to create flat surfaces.

Dressing of Slates

The dressing processed requires splitting of the block along the cleavage into thin sheets. The thin sheets are then cut or trimmed to a regular size using a trimmer machine.

Slates may then have holes drilled in them so there are holes in them for the roofing nails used to secure the slate to the roofing battons by a slater.

Welsh Slate Sizes

Traditional name for British slate sizes


All sizes are in inches - length x width



26 x 16


Princess or Wide Duchess

24 x 14



24 x 12


Small Duchess

22 x 12



22 x 11


Broad/Wide Countess

20 x 12



20 x 10


Small Countess

18 x 10



18 x 9


Wide Lady

16 x 10


Broad Lady

16 x 9



16 x 8


Wide Header

14 x 12



14 x 10


Small Lady

14 x 8


Narrow Lady

14 x 7


Small Header

13 x 10



12 x 6



10 x 5


How Long Do Slates Last

Natural states have a life expectancy of up to 200 years and need to be used on a roof structure and on other roofing materials that posses a similar life expectancy.

The life span of a slate roof will also depend on the level of maintenance it receives, for example, on a slate roof that has dislodged or lost slates then water may be able to permeate and then moisten and rot the battons , or roof rafters.

Always choose an experienced roofing contractor such as K-Roofing that has knowledge and experience of slate roofing techniques to ensure slates are fitted to professional standards and so maximise the lifespan of a roof.

Slate Quality

There is currently no UK or EU standard that is generally accepted or can be relied upon to always provide a guarantee against poor quality. Three Standards that are commonly referred to are:

•UK - BS 680

•French Testing Standard P32/302

•American ASTM

Many European countries select the French Testing Standard P32/302 as the most stringent testing standards available to grade slates.

Roofing Slate Tests BS 680 Part2 1971 French P32/302 Class A

Water Absorption




Density (g/cm³)

No requirement



Calcium Carbonate

No requirement



Thickness (mm)




Resistance To Flexation


No requirement

>70 mpa



No requirement

>40 mpa


Frost Test/Loss Of Mass

No requirement





No requirement

Not permitted


Non Leaching

No requirement

Permitted if not through


Sulphuric Acid Immersion Test




Wet & Dry Test




Grain Longitudinal


No requirement


How to Choose Slates for Your Roof

The Characteristics of Slate

Characteristics of slate that go into the materials decision are roof pitch, gauge and slate size.

These effect the appearance, mechanical resistance, mass/weight, durability, cost, thermal resistance, air tightness and vapour permeability of the roof.

Roof Pitch

Roof pitch is a key feature in choice of slate. The greater the pitch of a roof (more steep) the more suitable the roof is for smaller slate sizes. Steeper slate roofs are generally more weather tight and attractive.

Buildings regulations allow up to 70 degrees.

Lower roof pitches - down to 15 degrees - will shed water less quickly and have greater vulnerability to lifting and permeability during extreme storms.

Roof Loading

Roof loading is defined as the total weight of roof at any one time.

The loading factors include: the 'dead load' which is the weight of the roof materials themselves; imposed loads such as snow or rain; and wind load (positive and negative) which is a characteristic of location in the country and that effects caused by wind swirling and very local conditions such as height and orientation of the building, and by more localised pressure effects caused by chimney stacks and eaves.

Local Weather Conditions

Slate roofs are formed from individual slates, applied in courses (layers) and designed to shed precipitation and prevent water penetration either by wind or capillary action.

As a result, the weather performance is directly related to the quality of laying of the individual slates needs to be such that the head and side laps and pitch are as designed. This will ensure that nail holes are not weak points in the roof. Surface texture of slates will also effect capillary action.

D&C-Roofing are experienced roofing contractors and we professionally assess each roof to determine the best size of slate, lap and extension eaves for the local conditions.

Slate Roof Maintenance

Slate roofs require little maintenance other than replacement of individual slates that become damaged. Plastic clips, pop riveted to the bed of the replacement slate may be used to secure invisibly the replacement slate.

Man Made Slate

Many tile manufacturers now make artificial slates formed from reconstituted slate or concrete alternatives. They have certain advantages over the natural product:

•Attractive and natural appearance

•Excellent properties to keep the roof watertight

•Long life expectancy

•Lightweight to reduce roof loading

•Weather and frost resistant

•Easy to cut and fix

•Vermin and rot-proof

•Asbestos free


•High strength and does not delaminate

•Resistant to freeze-thaw and standing snow

•Cost-effective compared to natural slate

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