The B39 starts at a very sharp south facing fork on the A27 in the scattered settlement of Ronague and heads north, curving across the hillside while the A27 climbs above it. The two routes stay almost side by side for half a mile before finally turning away from each other. The B39 then drops steeply down to cross a burn, before climbing again along the southern edge of the Cringle Plantation. A small car park lies near the summit, and then the road drops steeply down once more, passing below the grassy slopes of the Dam for the Cringle Reservoir. A much larger car park sits on the roadside above the burn, and this time the ascent is just as steep as the descent. After winding around another block of forestry, the road resumes its north easterly course across fields.
The first house since those at the start of the route, is passed just after the trees, and then next to the next house, the B41 turns right, dropping down the hillside to Grenaby. The B39, however, continues to climb, past the 200m contour and on to a summit of around 230m above Corlea Farm. This section of the route is narrow, with some passing places, but reasonably straight with occasional, slightly sinuous bends. Beyond Corlea, the road runs along the southern edge of the South Barrule Plantation, and soon comes to an end at a T junction on the A3 south of Foxdale.
