Incorrect gauging results in taller, pig, courses and small head laps which will leak.

Correct gauging with drop courses means the margins reduce evenly and there are adequate head laps.

The characteristics of random slating are:

  1. Slate lengths which reduce up the roof slope.  The consignment of slates is sorted into length sets by the slater and laid from the largest at the eaves to the smallest at the ridge.

  2. Evenly diminishing margins.  The margin of a slate is the height of the exposed part.  (Margin is also used to mean the area of the exposed part).  This means that every margin must be the same size or smaller than the margins below.  This is achieved by reducing the lath (or batten) gauge at the first course of shorter slates.

  3. Gauge is the spacing of the laths and is calculated by subtracting the specified head lap from the slate length (measured below the peg hole) and dividing the result by two.  For example on a roof which starts with 20 inch slates at the eaves, the 20 inch slate minus three inch head lap gives an eight and a half inch lath gauge.  If the next slate length is 18 inches the gauge is seven and a half inches which will apply to all the 18 inch courses except the first one - the drop course.  The gauge for this must be reduced by half the difference in the slate lengths, that is one inch.  Failure to do this produces a taller margin over a shorter one - known as a pig course or a gaper - and a head lap which is too small and which will leak.

  4. Because the slate sizes reduce up the slope there are more slates at the ridge than at the eaves. These extra slates are introduced by the use of backers, narrow slates laid roughly centrally on a wide slate (known as wide butts) in the course below.  Each true backer adds one extra slate to its course.  There are also false backers which are narrow slates overlain by a wide butt.  That’s not a technical problem, it just looses the benefit of the narrow slate.  When introducing backers care is needed to achieve adequate side laps.

  5. Side lap. This is the lateral overlap of each slate over the slate immediately below.  Ideally slates should be chosen and laid so that their perpendicular joint is central over the slate below.  But this isn’t essential and may be impossible which is OK provided they do give the minimum sidelap.  The minimum side lap depends on the building’s exposure to driving rain and the roof’s pitch.  For backers the width must be must be at least twice the minimum sidelap and the width of the wide butt at least four times.  And if those values are the minimum allowable then the backer must be central on the wide butt.

Pig courses marked.  (Carboniferous Pennant sandstone)

Small side laps which will leak

Small head lap on pig course and resulting wet batten.

Pig - tall margin over shorter one

The characteristics of random roofs

Random roofs in Britain and Ireland

Head and side laps

It is critically important to ensure the slating has adequate head and side laps.  These are what keep out the rain.  The minimum head lap will normally be specified - typically three or four inch.  It will depend on the type of slate or stone, the building’s driving rain exposure and the roof pitch.  The traditional head laps used for a slate type and region will have been proved by trial and error and should be regarded as an absolute minimum.  They are based on the region’s traditional roof pitches and building roofs with lower pitches risks leaks.

At change courses - where the slates change from a longer to a shorter set - the lath gauge must be reduced to ensure evenly diminishing margins and adequate head laps.  If there is a tall margin over a shorter one (a pig course) the roof has almost certainly not been laid correctly

A minimum side lap may be specified but is more often left to the slater’s judgment.  This requires skill and care because each slate must be chosen and laid so that the side lap is satisfactory.  It is especially important for heavily shouldered limestones.  If the head or side laps are too small the roof will leak.  This may be hidden for many years if an underlay has been used but the laths will inevitably be wet and will eventually rot allowing the slates to slip.