Nomads, who started the day second, took ten points from a 52-run defeat at Brighton & Hove. Alasdair Wilson struck 106 not out and Lucas Barker hit 67, before Nomads were all out for 218 as Lachlan Sperling picked up 4-70.
Roffey, who play the Sussex Development XI today, dropped to second after collecting just two points in a six-wicket loss at home to Horsham.
Roffey were all out for 115 as Michael Munday took 5-21 and Travis Muller claimed 4-39.
Horsham reached their target in 27.3 overs as Will Beer top-scored with 47.
Third-placed East Grinstead went down by seven wickets away to bottom-of-the-table Hastings.
Darryl Rebbetts hit 60 as Grinstead were all out for 155, before Harry Finch struck 93 not out to guide Hastings to victory.
Steffan Piolet blasted 176 as Cuckfield defeated Eastbourne by 177 runs. Cuckfield scored 320-6, before Eastbourne were dismissed for 143 as Nicholas Beard took 7-58.
Three Bridges picked up a one-wicket win at home to Bexhill, after the visitors had scored 180-9.
In Division 2, leaders Middleton won by six wickets away to Lindfield. Lindfield were all out for 107, before Middleton got home in 17.4 overs.
Billingshurst remain second after a 111-run win over Findon, while third-placed Worthing made it six successive wins after a 15-run victory at St James’s.
Goring moved off the bottom with a six-wicket win over Chichester, who dropped to the bottom, while Ifield defeated Crowhurst Park by 81 runs.
Division 3 leaders Pagham recorded a seven-wicket victory at Stirlands.
Ansty remain second after a 134-run success at Pulborough, while third-placed Crawley Eagles won by seven wickets at home to Rottingdean.
Bognor Regis won by eight wickets at home to Burgess Hill and Haywards Heath defeated Glynde & Beddingham by three wickets.
Portslade clinched promotion from Division 4 with a eight-wicket win at Keymer & Hassocks, which was their 11th win from 12 games.
Owain Jones took 5-27 as leaders Steyning eased to a eight-wicket win away to Slinfold.
Hellingly beat Lewes by 147 runs, Rye overcame Henfield by three wickets and Wisborough Green won at Crawley by 139 runs.