The spectral energy distribution of 500K brown dwarfs peaks in the in the $J$ band in the near infrared (even colder objects should emit more energy in the mid-infrared), and they are therefore most easily detected in that wavelength range. Their NIR $JHK$ colours are, however, not distinctive at a modest signal-to-noise ratio [see @Metchev.2008 for instance], and brown dwarfs are more easily recognised by including at least one photometric band blueward of 1 $\mu$m. At those wavelengths the steep slope of their spectra stands out, and they have very distinctively red $i'-z'$ and $z'-J$ colours. The CFBD Survey [@Delorme.2008b] took advantage of their distinctive $i'-z'$ colours to identify hundreds of L and T-dwarfs. However, the reddest and coolest brown dwarfs have extremely red $z'-J$ colours and are much easier to detect in the NIR than on original CFBDS far-red images. A WIRCam $J$-band coverage of CFBDS fields is a straightforward and efficient way to cumulate both the very effective selection criteria using far-red colour information and the NIR detection sensitivity to ultracool brown dwarfs. Since the overlap of CFBDS with existing NIR surveys with a relevant depth [such as UKIDSS, @Lawrence.2007] is marginal, it was impossible to rely on 