3mm natural slate spacing, why so large?

[epsilonGreedy]

I will be fixing some (350) natural slates this week to cover a 30 degree pitch hipped roof.

 

While creating a gauging measure stick to see how many slate and a halves I would need where the slates meet the hip I was surprised how excessively far apart the slates appear with a 2.65mm gap (which is less than the regulation 3mm gap). I dropped two 30mm x 2.65mm copper nails between each slate to space them out closer to the regulation gap. 

 

It feels counter intuitive to lay slates with such a large spacing. What factor drives the 3mm spacing standard, or put another way what downsides might there be to laying the slates a bit closer?

 

My personal intuition is that a 1mm to 1.5mm space looks about right and should cope with thermal expansion.

 

Mr FixMyRoof calls it the "slate abutment gap".

 

 

https://www.fixmyroof.co.uk/videos-and-guides/pitched-roof/slate-a-roof/

[makie]

Capillary attraction.

You are also looking into this too much mate, just lay them with a slight gap instead of using a nail on every one.

You will be there until next Christmas.

[joe90]

My slates were automatically spaced because we used clips (about 2-3 mm), I guess the gap is for expansion (they can get quite hot being black!).

[Temp]

American article?..

https://slateassociation.org/wp-content/uploads/2017/12/Tech-Bull-2-Vertical-Joint-Spacing-D.pdf

 

Vertical joint spacing refers to the amount of space between two adjacent slates within the same course1.  As roofing slates are hung by the shank of a nail, or a slate hook, a slight amount of movement may be expected in any given slate shingle due to wind loads and thermal expansion and contraction.  To compensate for this, a small space is typically allowed between side-byside slates within the same course.  The basic recommendation is that adjacent slates be placed tightly together, but not so tight as to overlap, as overlapping of the vertical edges of adjacent slates can subject the slate to premature breakage2.  A  slight gap of 1/16” to 1/8” between adjacent slates (roughly equivalent to the point of a slate hammer) is recommended.  This gap can, however, vary from 0” (slates just touching) to 3/8”, or more, based on the width and thickness of the slate, roof slope, seasonal rainfall, the severity of local climate conditions (e.g., sea-coast vs. arid plains), and regional practices. A gap at the wider end of the range can enhance the shadow lines and textural appearance of the slate, especially when thicker shingles are used (Figure 1). 

 

As you are slating at a cold time of year I wouldn't lay them touching but it says 1/16" is OK. 

 

[Temp]

Hold my beer. Time for some maths... Google found Slate's coefficient of expansion is 10 * 10^-6 meters per meter per degree C

 

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

 

Slates vary from 250mm to 300mm wide.

 

Let's suppose you install them when they are 0C and they heat up to say 100C. That's a 100C variation. 

 

So a 300mm slate would expand by ...

 

100 * 10 * 10^-6 * 0.3 = 0.0003m or 0.3mm

 

I think 1 or 2mm gaps should be fine.

[ToughButterCup]

When they touch   the temperature variation , especially in winter causes them sometimes to break.

 

Ask me how I know ☹

[markc]

I was told the gaps are to stop wind blown water from rolling across the roof, the gaps force the water to stop rolling and head down hill towards the gutters instead of off the edge

[epsilonGreedy]

Ok lots of food for thought in this thread, thanks chaps. In reverse thread order:

 

1 hour ago, markc said:

I was told the gaps are to stop wind blown water from rolling across the roof, the gaps force the water to stop rolling and head down hill towards the gutters instead of off the edge

 

 

I can see the sense in this, i.e. breaking up wind driven horizontal laminar flow. I don't want a mini tsunami building up against the hip rafter capping and sneaking under. The riven edge to each slate creates a small gully even when the slates were tightly abutted but I can see that an extra 2mm forms a better gully.

 

1 hour ago, ToughButterCup said:

When they touch   the temperature variation , especially in winter causes them sometimes to break.

 

 

Yup can see that, also if my nailed slates wiggle a bit in a strong wind a 2 or 3mm gap stops the adjoining slate edges clatting each other.

 

1 hour ago, ToughButterCup said:

Ask me how I know ☹

 

 

Nope, I require only positive self builder vibes this week. ?

 

1 hour ago, Temp said:

So a 300mm slate would expand by ...

 

100 * 10 * 10^-6 * 0.3 = 0.0003m or 0.3mm

 

I think 1 or 2mm gaps should be fine.

 

Good to get the science behind my original thermal concern. My first intuitive trial layout of 16 slates indoors yesterday featured 1mm gaps and given the other concerns I am erring towards a 2mm gap so well beyond the thermal expansion risk.

 

Your US link also mentions that a larger gap allows a poorly cut oversized slate to be fitted without throwing out the overall bond pattern.

 

2 hours ago, joe90 said:

My slates were automatically spaced because we used clips (about 2-3 mm), I guess the gap is for expansion (they can get quite hot being black!).

 

The US article linked to earlier mentions that a larger gap facilitates future repairs with slate hooks. Another vote for a 2mm minimum gap.

 

3 hours ago, makie said:

Capillary attraction.

You are also looking into this too much mate, just lay them with a slight gap instead of using a nail on every one.

You will be there until next Christmas.

 

Capillary action never crossed my mind. My reading of this is that any water sneaking upwards under the lap will be constrained to a single tile because the capillary action will be broken at the 2mm to 3mm side gap.

 

Re sizing the gap with nails, this was not done while laying slates on the roof. After dark yesterday I arranged my builder trestles indoors with two 4.5m roofing battens running cross them and then did a trial positioning of 16 slates to get an idea of the eave course. My first intuitive positioning attempt resulted in 1mm gaps. I then tried again with roofing nails as spacing pegs to see what 2.6mm looked like. At this point I thought 3mm gaps they must be joking hence this thread.

 

My confidence in published industry standard practice was knocked last week after a visit to a specialist roofing merchant, that deserves a separate thread.

 

 

 

[makie]

1 hour ago, epsilonGreedy said:

My confidence in published industry standard practice was knocked last week after a visit to a specialist roofing merchant, that deserves a separate thread.

 

Just because they are a specialist merchant doesn't necessarily mean they know what they are on about.

 

What advice did they give?

[epsilonGreedy]

4 minutes ago, makie said:

 

Just because they are a specialist merchant doesn't necessarily mean they know what they are on about.

 

What advice did they give?

 

See my new thread on nail size for fixing roofing battens. Would be interested in your opinion.