Flooding closed a Perranporth pub and the Co-Operative store in Truro.
Staff at the Co-Op were left mopping up the mess after torrential rain hit the centre of Truro.
The landlord of the Tywarnhale in Perranporth said he could be closed for weeks as a result of the flooding.
Cornwall Fire and Rescue Service said it had dealt with incidents across Cornwall from 8am.
Firefighters were called to flooding in Truro, Camborne, Redruth, Perranporth and Porthleven.
Mike Moyle from the Tywarnhale pub said: "At about 8.30 this morning there was water lapping at the door.I phoned the fire brigade and they were here within five minutes.
"They started pumping but it was too little too late."
Mr Moyle has had repeated problems with flooding since November last year and said he is still trying to clarify who is responsible.
He said: "It gets too far in then whoosh - it comes up through the drains. There was about eight inches right through the ground floor of the pub.
"I think the trouble is that all the storm water comes down the road and eventually goes into my manhole."
Mr Moyle hopes to re-open as soon as possible but said the clean-up operation may take weeks.
"There are thousands of gallons of water in there," he said.
A spokesman for the Co-Op said: "Like much of the local community we have been affected by today’s flooding in Truro, and in early hours of this morning the store on Boscawen Street became inundated with water. The store was not able to open as usual this morning as the clean-up operation got underway, and currently remains closed while this is ongoing.
“We are aware that our store is central to the local community, and we would like to assure our customers that we will do everything we can to reopen the store as soon as possible."
A spokesman for the Met Office said: “Further rain will affect parts of western England, Wales, northern England and southeast Scotland through today before easing away eastwards.
“Accumulations of 20-40 mm are likely in many parts with the risk of more than 60mm over parts of southwest England.
“Rain could turn thundery at times with some intense downpours within the longer spells of rain.
“The public are advised to be aware of the risk of surface water flooding.”
The Environment Agency has issued a flood alert for rivers in west Cornwall, including Lands End, Helston, Crowlas, St Ives, Hayle, Camborne and Perranporth.