The key difference outdoors is coping simultaneously with sun, heavy rain, slipperiness and temperature swings. Hong Kong summers are hot and humid with heavy typhoon rain, so selection must prioritise low absorption, slip resistance and weather resistance ahead of appearance. In practice granite is the dependable choice outdoors.
Granite is 6 to 7 on the Mohs scale with absorption below 0.5 percent and high compressive strength, so it will not warp or fade in sun and rain, and flamed or bush-hammered finishes add grip. Marble is vulnerable to acid rain, becomes slippery when wet and wears faster, so it is generally not recommended outdoors.
Outdoor paving needs a drainage fall, generally 1 to 2 percent, with adequate outlets, and a free-draining substrate so standing water does not cause efflorescence. Rooftop paving must also consider waterproofing and heat insulation; raised pedestal systems make later inspection and repair far easier.
With high slip risk outdoors, choose flamed or bush-hammered finishes targeting R11 to R12. Rough textures trap dirt, so schedule periodic pressure washing; treat moss with dedicated cleaners rather than prolonged bleaching.
Travertine and sandstone have a natural, warm character suited to garden paths and feature walls, but pores collect water and dirt, so specify filled or closed-face finishes and avoid permanently wet locations. Driveways need sufficient thickness, 30 mm or more, and granite with high compressive strength.
Generally not recommended: it is vulnerable to acid rain, slippery when wet and less wear resistant. Use granite outdoors. If marble is essential, limit it to covered semi-outdoor areas with a slip-resistant finish.
Waterproofing, drainage falls and heat insulation. Raised pedestal systems are recommended so the waterproof membrane can be inspected and repaired later without standing water problems.
Footpaths generally 30 mm or more; driveways and vehicular areas need greater thickness and high-strength granite, determined by the design load.