I would suggest anything that is of a non slippery type. And high enough for the dogs to get under to chase off the vermin that will take refuge beneath.

i would use scaffold boards with chicken wire on top as the stuff you pay threw the nose for is rubbish . good pine comes from the much colder climates wear the summers are short and is double the age of the stuff grown here making it very soft in the wide summer growth rings . it should also be stacked properly outside for a year which it isnt . they just sell it green for a fortune .

personaly my 6 lovley dogs have there own dog run which is mostly cement . its on one or two of my blogs . it wont rot , quit cheap ,easy to wash down plus its not slippery with the added bonus of keeping there nails warn down . you can use old council slabs if not actual cement .

whatsort of dogs just out of interest ? ow the other bonus you wont have rats or mice running about under cement either .