The Boars blasted 288-5 declared off 55.4 overs. Last year's champions initially floundered at 12-2 but two massive partnerships shot Roffey to a huge total.
Rohit Jagota (59 off 58 balls) and Jibran Khan (44) smashed a 106-run third-wicket partnership before Mike Norris (100 not out off 97 balls) and skipper Matt Davies (53) struck 105 at five.
The Horsham reply got off to a solid start as Sussex star Tom Haines (56) and captain Michael Thornely (76) forged 92 for the second-wicket, but the loss of the pair sparked a Lions collapse.
Superb bowling from Luke Barnard (5-50) and Ben Manenti (4-40) saw Roffey bowl the visitors out for 171 in 42.5 overs to seal a convincing victory and move Horsham to the division's basement.
Second-placed Brighton & Hove fell further behind the league leaders after a narrow eight-run home loss against Three Bridges.
Bridges were put in and were all out for 183 in 57.1 overs, despite a 68-run opening wicket partnership from Mahad Ahmed (18) and Sam Cooper (52). Matt Machan was the pick of the Brighton (4-18) as he expertly polished off the visitors tail.
But ferocious bowling from Adam Jones (6-37) helped Bridges tear through Brighton's batting line-up as the title challengers were bowled out for 175 in 49 overs.
Mayfield moved off the bottom of the table thanks to a 42-run home victory against East Grinstead.
The hosts were all out for 201 in 50.3 overs thanks to Kaushal Silva (80) and David Hooper (35) hitting 97 at four. EG's William Taylor performed valiantly with the ball, taking 5-50.
In reply, Grinstead were bowled for 159 in 28.2 overs. Ian Sturmer (64 off 43 balls) and skipper Darryl Rebbetts (41 off 37 balls) struck a quickfire 76 for the fourth-wicket but EG could not contend with a blistering display from Emmerson Wood (6-40) as Mayfield took a vital win.
Middelton moved up to third after an 88-run home win over Preston Nomads.
The home side were all out for 233 in 55.1 overs as captain Sean Heather topscored with 88. Nomads were then bowled out for 145 in 39.2 overs as eight-wickets were shared between Jack Carson (4-49) and Craig Fowle (4-42).
Eastbourne notched up a nine-wicket win at third-from-bottom Cuckfield.
Two hefty partnerships propelled Cuckfield to 241-8 off their allotted 58 overs. Dominic Sear (29) and Marcus Campopiano (109) wracked up 53 for the third-wicket before Campopiano and Abi Sakande (31) blasted 75 at eight.
Eastbourne saw James Hockley (eight) fall to Sakande (1-52) at 12-1 bringing Scott Lenham to the crease. Lenham (141 not out off 109 balls) and Malcolm Johnson (88 not out) then produced a scintillating display with the bat. The pair combined to hammer an almighty 232 runs to rocket Eastbourne to a clinical victory.
Division 2 leaders Hastings & St Leonards Priory played out an exciting draw with third-placed Haywards Heath.
Hastings posted 200-9 declared from 52.5 overs. Wicketkeeper Joe Billings (47) and skipper Tom Gillespie (56) produced a 78-run second-wicket partnership while Jonny Phelps (5-69) was the star man with the ball for Heath.
Phelps then proved just as adept with the bat as he topscored with 53 for the visitors. But dazzling bowling from Elliott Hooper (6-82) stifled Heath. The away side scored 199-9 off 47 overs to see the spoils shared.
Second-placed Bognor Regis wrapped up an emphatic eight-wicket win at bottom-of-the-table Ifield.
The host were skittled for 52 in 32.4 overs as Bognor's extraordinary trio of Ben Woolnough (4-4), Joe Ashmore (3-10) and Josh Sargent (3-18) blew through Ifield.
Despite losing two wickets in the reply, both to Matthew Graves (2-15), Bognor cantered to victory, hitting 56-2 in just 11.4 overs.
St James's Montefiore overcame Billingshurst at home by four-wickets to move up to third.
Openers Louis Storey (33) and Jordan Willoughby (31) struck 60, and Andy Barr (74) and Paul Osborne (42 off 39 balls) crunched 75 at eight to help Hurst to 232-9 declared off 51.5 overs.
St James's then produced two magnificent partnerships of their own to take the win. Captain Hector Laughton (23) and Toby Pullan (a remarkable 60 off 35 balls) hit a lightning 62-run partnership at two before wicketkeeper Joe Gilligan (51 not out) and Jordan Shaw (50 not out off 47 balls) smashed 92 at seven to take the hosts to 233-6 in 43.5 overs to claim victory.
Goring By Sea moved out of the relegation places thanks to a four-wicket home win over Chichester Priory Park.
Chi were all out for 165 in 44.4 overs as Joe De La Fuente (60) and Abhishek Raut (33) made 64 for the third-wicket. Despite De La Fuente (3-26) and Raut (3-61) then sharing six-wickets in the reply, great batting down the order saw Goring score 166-6 in 47.1 overs to ease their relegation fears.
Burgess Hill slipped into the drop zone after a three-wicket defeat at Lindfield.
An 82-run second-wicket partnership from Kevin Ramsay (68) and Ben Hopkins (45) got Hill off to a solid start but irresistible bowling from Lindfield's Tom Hinley (6-53) curtailed their innings as they struggled to 183-8 declared off 53 overs.
79 from Apoorv Wankhade (73 off 69 balls) and Harry Moorat (28) at two helped Lindfield to 186-7 in 39.2 overs to plunge Burgess Hill into relegation danger.
Division 3 West: Findon 209 all out lost to Ansty 202-5; Littlehampton, Clapham & Patching 224-7 beat Broadwater 221-9; Pagham 235-8 lost to Roffey 2nd XI 236-5; Stirlands 220-8 lost to Steyning 221-5; Worthing 137 all out lost to West Chiltington & Thakeham 138-3.
Division 3 East: Bexhill 96-1 beat Brighton & Hove 2nd XI 94 all out; Eastbourne 2nd XI 285-6 beat Hastings & St Leonards Priory 2nd XI 167 all out; Preston Nomads 2nd XI 169-4 beat Crawley Eagles 164 all out; Rottingdean 133 all out lost to Rye 193-9; Seaford 105-6 beat Portslade 102 all out.
Division 3 highlights: Morne Louw 6-33 for West Chiltington & Thakeham; Martin Blincow 5-36 for Rye.
Other highlights: Glenn Whitaker 5-12 for East Preston; Charlie Hobden 119 not out for Glynde & Beddingham; Craig Eaves 6-31 for Glynde & Beddingham; Wazir Khan 6-10 for St Peters; Tom Crathern 5-17 for Little Common Ramblers.