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A recent break-through in the domain of photonic integration is the use of sub-wavelength waveguide grating structures to engineer the refractive index of integrated components. These structures are an example of a case where a nanoscale simulation over the full device is not reliable and reliance must be placed on a simple design procedure. This paper reports a study of the homogenisation of sub-wavelength waveguide grating structures. It is shown how the properties these waveguides may be accurately modelled using simple design rules. A justification for these rules is found from the exact theory by a judicious choice of approximations. The temporal and spatial dispersive properties of the subwavelength grating waveguide are also evidenced.