People are always asking why certain types of natural stone cannot be used outside. In many cases it is simply the thickness of the stone has been specified for interior use. It is generally considered that any material below 25mm is inadequate for exterior use. On the same note the thickness of interior stone tiles can range from anything between 10 & 20mm. Materials like sandstone, granite and travertine are suitable for both interior and exterior use. It is generally accepted that limestone and marble are too soft for use outside. Acid rain is prevalent in the UK and can cause damage to softer types of limestone and marble. Indian limestone from the Kota region are acceptable for exterior use as are types of Jura & Moleanos limestone.