This work describes how a new, extremely porous, hairy layer of carbon nano-fibers (CNFs) can be prepared on the surface of porous inorganic bodies, e.g. wash-coated monoliths. CNFs were prepared catalytically by methane and ethene decomposition over a Ni catalyst. The influence of the Ni particle size and growth conditions on the properties of the resulting material is reported. It turns out that the thickness of the CNF layer at the outermost surface (ca. 1 mm) as well as the diameter of the fibers increases with mean Ni particle size. The structure of this layer resembles the inverse structure of a traditional inorganic support material, combining high surface area, high porosity and low tortousity. Growing CNFs using methane leads to immediate fragmentation and doubling of the thickness of the washcoat which is independent of the amount of CNFs, forming a macro-porous composite layer of entangled alumina particles and CNFs with a typical diameter of 10¿30 nm. Immediate fragmentation is due to the fact that some of the fibers are too thick for the pores in the washcoat. The total porosity decreases with the amount of CNFs whereas the surface area per gram of monolith increases. Large Ni particles are able to grow CNFs for longer times, resulting in detachment of the washcoat from the cordierite, which is caused by extensive growth of CNFs out of the washcoat. Furthermore, extended growth of CNFs inside the cordierite body causes disintegration of the monolith body when macro-pores are locally overfilled with CNFs. Methane is preferred over ethene for growing CNFs because ethene grows CNFs rapidly even on relatively large Ni particles, resulting in thick fibers up to 70 nm in the macro-porous cordierite, destroying the monolith. Controlling both the Ni particle size and Ni distribution as well as choosing the right activity of the hydrocarbon are essential to grow CNF washcoats without damaging the monolith structure.