Granite Cladding Wind Load: Who Calculates It, and What the Stone Yard Still Owes You
Wind load on granite cladding is calculated and signed by the design side, not by the stone yard, yet it decides how the panel is made. A 30 mm panel weighs 81 kg/m², and 1 kN/m² of wind load pulls about 1.26 times that weight, acting on the inside of the slot shoulders. The four factors, the load path, and six lines to put on an enquiry.
How is wind load calculated for granite cladding?
Short answer: not by the stone supplier. Wind load is set, calculated and signed off by the design side, using the load code in force where the project is built. The yard does not produce that number and should not sign it. What the yard does control is the first link of the chain the force travels down: panel, then bracket, then steel frame, then anchor, then the main structure. Our part is making the panel able to carry its share — 30 mm thickness, the module size on the layout drawing, the slot position and dimensions, the thickness tolerance. Get any link wrong and the most careful calculation upstream is wasted. So when you talk about wind load on granite cladding, separate two questions early: who calculates, and who makes the panel hold.
Why can't you just multiply the wind pressure on the drawing by the panel area?
Because the number on the drawing is usually not the wind load. Characteristic wind load = basic wind pressure × shape coefficient × height variation coefficient × gust factor. All four are multiplied: the basic wind pressure for the site at the required return period, a shape coefficient (a field zone, a corner zone, and a parapet or eaves zone do not share one value), a height variation coefficient that grows with height, and a gust factor for the short-term fluctuation. Multiplying by the area is not the mistake; the multiplier is. Multiply all four and you have a number you can use; drop one and you do not. The commonest purchasing error is taking the basic wind pressure as the wind load and multiplying it by the area. The basic value describes local climate only — not the shape of this building, not its height, not the zone. Two panels on the same site, one mid-field and one at a corner, can end up far apart. Standard names are public (the load code for building structures); use the edition in force where the project sits. For a sanity check on site you can run 0.5 times air density of 1.25 kg/m³ times velocity squared: 30 m/s gives about 562 Pa, or 0.56 kN/m². That figure is the basic wind pressure only — three more factors go on top of it before it becomes a wind load. It is physics, not a code value, and it only tells you whether the figure on the table is off by an order of magnitude.
Wind pulls outward — why is suction harder to deal with than pressure?
On a dry-fixed granite facade the governing case is normally negative pressure: the wind does not push the panel onto the wall, it pulls it off. Under pressure the panel is pushed inward and the slot and bracket take it in compression, which is the forgiving direction. Under suction the panel is drawn outward and everything rests on the two shoulders of the slot gripping the bracket tongue from the inside. The directions differ too: suction pulls the panel out, self-weight presses it down onto the upper face of the slot. Different faces, different directions — they do not simply add or cancel. That is why the check is run on the worst combination rather than on a sum. How much shoulder you leave, and how clean the slot is, decides how much grip this link has; we went through that in our slot depth article and will not repeat it here.
How much force does one panel actually take, and why is the module fixed before the wind load?
Two steps. Characteristic wind load in kN/m² times the area of one module in m² gives the force on one panel in kN; 1 kN is about 102 kg. A 30 mm panel at 2.7 t/m³ weighs 81 kg/m². Both quantities scale with area, so the ratio of suction to self-weight does not depend on module size: all four rows below give 1.26, and the idea that a bigger panel takes a bigger multiple is wrong. What the module does change is the absolute force on a panel and the number of brackets it needs. And the module is usually fixed before the wind load is: it comes off the architect's layout, and the yard cuts to it.
| Module (mm) | Area per panel (m²) | Self-weight (kg) | Suction per 1 kN/m² of wind load (kg) |
|---|---|---|---|
| 600×300 | 0.18 | 14.6 | 18.4 |
| 800×600 | 0.48 | 38.9 | 49.0 |
| 900×600 | 0.54 | 43.7 | 55.1 |
| 1200×600 | 0.72 | 58.3 | 73.4 |
What can the stone yard give you, and what can it not?
We can give: self-weight from thickness and 2.7 t/m³; module sizes taken from your layout; slot dimensions including the shoulder left on each side; thickness tolerance; packing and loading (finished panels shipped on edge, container load quoted from the actual standing pack), 100 m² minimum order, 5 days for standard production. We cannot give: the design value of flexural strength, or the load capacity of brackets and anchors. Those come from laboratory reports and product literature, and have to be accepted where the project is built. Keep two situations apart — one is that we do not have that number; the other is that the number is not the stone yard's to sign. Mixing them is how a project ends up with nobody accountable.
What should be written on the enquiry and on the drawings?
Six lines worth copying: the height band and the zone the panel sits in (field, corner, parapet or eaves); the characteristic wind load from the design side, and whether it is the four-factor product or only the basic pressure; the module size and layout, including internal and external corners and closing panels; the bracket model and tongue length, with slot dimensions cut to the bracket actually supplied; the thickness and tolerance set by the design side — we keep 30 mm for cladding; packing and loading, panels standing, container load quoted on the standing pack. Send those six and a quotation becomes arithmetic instead of guesswork.