Convent of Mercy, Thornhill, Culmore Road, Londonderry, BT48 8JF

This is a large stone-built house of two storeys with numerous stepped gables of varying size, natural slated pitched roofs, moulded stone chimneys, and attics and basement. The architectural style is severe with Scottish Baronial overtones.

The west front and main entrance is some eight or nine bays wide, featuring a central square projecting entrance bay one storey high with a recessed arched opening containing an almost diminutive square-headed door. The entrance sits at the end of a higher ridged section which thrusts forward, with another projecting single-storey bay at the other end likewise with a flat roof. Behind these bays, two narrow stepped gables break the roofline; that behind the entrance rises another storey. Windows throughout are generally narrow and tall, arranged in groups of two and three with some singles. Multiple windows are divided by stone transoms and mullions. Above the entrance bay are coupled windows capped with angular pediments, while above the other bay sits a segmented pediment. This principal block, three rooms deep, contains the main spaces.

To the north of the entrance and set back from it extends a lower two-storey wing terminating in the kitchen, which has separate entrances. The kitchen wing thrusts forward with a crow-stepped gable and its round-headed entrance is sheltered by a lean-to roof. Windows here comprise a mixture of singles, doubles and triplets, with two round-headed singles at first floor abutting the higher block. Above these sits a pleasing oculus, one of the few refinements in the overall composition.

Chimneys are tall, rising well above the ridge line, built in ashlarwork with rounded narrow sides and moulded bands and caps. Roofs are steeply pitched with two two-light dormers with square heads on the higher roof. The walls are built of snecked sandstone obtained from Dungiven. Windows are timber, painted white. Kitchen windows are wider and taller, divided in five panes, similar to those in the stable block. There are no roof overhangs, with a simple plain string course beneath cast iron gutters and matching rectangular downpipes, with trunkheads where necessary.

The north elevation of the kitchen wing is three bays long with a double-pitched projecting porch at the north-east end. The porch has a crow-stepped gable with a pair of round-headed windows and an oculus placed centrally above. Windows on this side are irregularly spaced and square-headed with sliding sashes at first floor. Steps lead down to a basement doorway beside the projecting porch. The steep pitched roof has three two-light dormers.

The east elevation shows some symmetry in the composition of the high block, which thrusts boldly forward from the service wing. It is five bays long with stepped gables at each end projecting forward and containing between them a veranda with curved metal roof supported on slender cast iron columns. A central polygonal bay pushes through the veranda roof and forms a steep sloped pyramid roof. Each end bay has large square windows at ground and first floor, subdivided with sandstone mullions and transoms. Intermediate windows are two-light, similarly divided at first floor, whilst at ground floor these become French windows. One bay of the veranda is open, the other two have metal decorated balustrading. The service wing has a mixture of square and round-headed windows. A group of three windows to the service staircase express its semi-circular flights. The gable of the main block has irregularly spaced two-light windows, and the return walls of the bays are punctuated with single-light square-headed fenestration. Usually each stepped gable sports a slender round-headed window, though this gable unfortunately has a recent square window fitted.

The south elevation has changed since a convent chapel was added in the 1960s. It otherwise repeats the characteristics of other elevations with square and round-headed windows displaced here and there, and a rectangular bay at ground floor on the west gable. A contemporary narrow conservatory abuts the connecting porch to the chapel. Originally there was a curved glass roof conservatory, visible in photographs from around 1920.

Thornhill enjoys a magnificent site on the west bank of the River Foyle where it flows between Rosses Bay and Culmore. Set on sloping ground well elevated above the river, it commands panoramic views of the surrounding landscape. The grounds are well laid out with mature trees dotted across lawns. From the veranda, steps lead down to a formal path which dips to the river. Further down and to the side is a cemetery for deceased members of the Sisters of Mercy community. A winding avenue sweeps from Culmore Road past the former walled garden, now a commercial nursery.