Vale of Glamorgan council gritting vehicles are laying crushed rock dust to protect road surfaces as they reach record temperatures.
Similar problems have been reported in Pembrokeshire, Carmarthenshire, and Gwynedd.
Ken Evans, acting routine maintenance manager at the Vale of Glamorgan council, said highways staff recorded road temperatures of 49C (120F) last week.
Pembrokeshire council is carrying out emergency repairs to one of the main roads in Tenby which has also been affected by the weather.
The surface of a section of the A4218 known as Broadwell Hayes – between Serpentine Road towards The Green – began breaking up in the heat.
Councillor Rob Lewis, cabinet member for highways and planning, told the BBC: “There is no doubt that the recent spell of extremely hot weather is responsible for the deterioration of the surface.”
In Carmarthenshire, motorists were being urged to watch out for uneven roads and drive carefully.
Highways maintenance crews in the county have been sent to repair potholes and seal road surfaces with stone dust or sand.
Carmarthenshire council executive board member Colin Evans said: “It is important that drivers are made aware that the high temperatures have caused minor damage to some of the roads and that they should take notice of any signs and drive accordingly.
“The areas affected are mainly in the rural parts of the county and are few and far between.
“It is not a major issue and we do not want to alarm people unnecessarily.”