How Many Composite Slates Do I Need Per Square Metre?

The count per square metre comes from one sum: slate width multiplied by the gauge. Here is how to do it, with worked examples and the extras people forget.

For the common 600 x 300mm composite slate laid with a 100mm headlap, you need roughly 13 to 14 slates per square metre. Smaller 500 x 250mm slates at a 90mm headlap need nearer 20. The exact figure always comes from the manufacturer's fixing table for your roof pitch and exposure, because that table sets the headlap you must use.

Everything else on this page explains where those numbers come from, so you can check a quote or order sensibly for a shed, porch or garage yourself.

The sum behind the number

Slates are double lapped, so each visible course only shows part of each slate. The distance between the batten lines is called the gauge, and it equals the slate length minus the headlap, divided by two. A 600mm slate with a 100mm headlap therefore has a gauge of 250mm.

Each slate then covers its width multiplied by the gauge. For a 300mm wide slate that is 0.3 x 0.25, which is 0.075 square metres. Divide one square metre by that area and you get 13.3 slates. Swap in your own slate size and headlap and the same two steps give your figure.

Why the headlap changes with pitch

Shallower roofs drain more slowly and wind can drive rain further up between the slates, so manufacturers ask for a bigger headlap as the pitch drops. More headlap means a smaller gauge, more courses and more slates per square metre. A steep, sheltered roof can use the minimum headlap; an exposed, low pitched one cannot.

Most composite slates also have a minimum pitch below which they are not approved at all. If your roof is close to that limit, it is worth asking whether a different covering suits the slope better before buying anything.

The square metre count is for the open field of the roof only. You will also need extra slates at the eaves and top course, slate and a half or half slates at the verges depending on the product, and allowance for cuts around valleys, hips, chimneys and rooflights. Adding a waste margin for breakages and offcuts is normal practice, and a roof with many cuts needs more than a plain rectangle.

Remember the fixings too. Composite slates are usually nailed, sometimes with a disc rivet or clip at the tail, and the manufacturer specifies both. Ridge and hip pieces, underlay, battens and any ventilation products are separate items.

Measuring the roof properly

Measure each slope along the rafter, from eaves to ridge, not the flat footprint seen from above. A plan measurement understates the area, and the lower the pitch, the smaller the error; on steep roofs it can be large. Multiply rafter length by the width of each slope and add the slopes together.

If you cannot measure safely, do not climb. Drawings, the pitch from a loft measurement or a roofer's survey will give a reliable area without anyone going up a ladder they should not.

Composite or natural slate?

The counting method is identical for natural slate, but natural slates vary in thickness and need sorting, which affects laying speed rather than quantity. If you are still deciding between the two, our natural versus composite slate comparison covers weight, appearance, lifespan and conservation area suitability.

How many 600 x 300 composite slates cover one square metre?

At a 100mm headlap the gauge is 250mm, so each slate covers 0.075 square metres and you need about 13.3 per square metre before cuts and waste.

Does a lower pitch mean I need more slates?

Usually yes. Low pitches call for a larger headlap, which shortens the gauge and adds courses, so the count per square metre goes up.

Should I measure the roof from the ground plan?

No. Measure along the slope from eaves to ridge. A plan area leaves out the extra length created by the pitch and leads to short orders.

Can I use one manufacturer's figures for another brand?

Not safely. Slate sizes, minimum pitches and headlaps differ between products, so always use the fixing table for the slate you are buying.

Will you work out quantities as part of a quote?

Yes. When we survey for a new roof or re-roof, the slate count, fixings and extras are worked out from the actual slopes and the chosen product.

Want the numbers checked?

Send us the roof size and the slate you have in mind, or book a visit, and we will work out quantities from the real slopes.

Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.