It would probably be ok but I would add an extension pole to put it up higher than the chimney to catch the best wind and off-set to avoid any issues with hot flue gasses. The rest of the ISS should not be put up there because of radiant heat from the roof distorting readings and other issues. I agree with mmoris...to make it easier, rather than run the cable all the way to the ISS, I would buy the anemometer transmitter. That way the ISS (rain bucket, temp\humidity sensor etc) could be located at ground level for better servicing which could be too far or difficult to run the anemometer cable to.