|Distance:||10.8 miles (17.4 km)|
|Meets:||A470, B4595, A4223, B4278, A4233, B4278, A4119, B4223, B4512, A4061|
|Old route now:||B4278, A4061, B4522|
|Route outline (key)|
The A4058 runs right into the heartlands of the Rhondda Valley.
It starts at a GSJ with the A470 to the east of Pontypridd and heads west across the River Taff into the town centre. The road then heads westwards and soon the mountains of the Rhondda come into view, though the road itself never seems to quite break free of terraced houses on either side. There are single-carriageway bypasses of Trehafod and Porth, after which the A4058 takes over a section of the erstwhile B4278 (the present B4278 on the far side of the river is the old A4058) before running on a relief road that was reclaimed from old railways after the coalmines shut down. There's a roundabout with the A4119 from Llantrisant, then after a mile we're back on the original road.
The remainder of the route is a fairly slow affair, passing through one long stream of terraced houses after another through Llynwypia, Ystrad and Pentre. It ends at a set of lights in Treorchy, the road ahead and left being the A4061.
The A4058 originally went further along the Rhondda Fawr valley as far as its head at Tynewydd, where the most upstream coal mines were. The extension into the mountains past this to the A465 was built as part of an unemployment relief project and opened in 1929 (with the rump becoming the B4522), and the A4061 was extended along this route in the 1935 Road numbering revision, truncating the A4058 back to its present point.