The win leaves Dale fourth, with 44 points from 13 games, while Alnwick drops to ninth in the National 3 North table.
Despite setting off to the North East at 7 am, the visitors were the faster out the traps with tighthead-prop Nick Flynn crashing over the whitewash following a five-metre scrum after eight minutes. Fly-half Lewis Allen then added the simple conversion to give Rossendale a seven-point lead.
A try from Alnwick's James Bird, converted by Callum Burn, then levelled the scores after 20 minutes.
But Druw Frey crashed over following intense pressure from the visitors. Allen couldn't add the conversion, but Dale still held a narrow 12-7 lead at half-time.
A penalty from Allen proved to be the only points of the second half, although it took heroic defence in the final 20 minutes to ensure Alnwick didn't set the game up for a grandstand finish.
Next weekend, Dale will try and turn two consecutive wins into three when they face eleventh placed Lymm at home next weekend. Their visitors, who sit eleventh, narrowly won the reverse fixture 17-15, setting it up to be a very close encounter.
Steven Smith