Skipper Williams touched down for the first try on 13th minutes when he was on Nick Robinson's shoulder after a great break by the fly-half and with the tries from Thomas and Roberts following soon after, Ulster looked to be in for a long evening.
However, an improved Ulster performance in the second half, whilst not enough to erase all of Cardiff's lead, did see two tries, one each for Ryan Caldwell and Tommy Bowe.
Caldwell drove under the posts on 53 minutes though was sin-binned on behlaf of the team in general according to the referee, just a few minutes later.
The Blues were unable to capitalise on the one-man advantage and as conditons deteriorated they were punished for their poor handling when Ulster kicked through from their own half and Bowe was awarded the touchdown for Niall O'Connor to convert.
Neither side was able to secure a bonus point, the fourth try eluded Cardiff, while Ulster didn't get close enough to pick up the losing point. Ulster will now finish out the season with three home matches, the re-arranged fixture against Munster on Wednesday 30th April, a clash against Glasgow on Saturday 3rd May before facing Cardiff again in the final tie of the season the following week.
