Plants in soft, loose mats, yellow-green to whitish green, seldom brownish, even in exposed sites, irregularly and sparingly branched or rather closely branched with the branches ascending to decumbent. Primary stems to 15 cm long with leaves markedly larger and proportionately broader than branch leaves. Leaves to 3.5 mm, 1.8–2.2: 1, symmetric and straight, strongly concave especially just below the base of the acumen,, not markedly wrinkled or plicate, broadly ovate, loosely ascending. Leaf apices abruptly acuminate to apiculate, strongly twisted and often reflexed. Margins plane to erect or with a small recurvature in the alar region, narrowly decurrent in most stem leaves but almost not decurrent in the branch leaves, distally dentate with the marginal cells which form the teeth markedly broader than the adjacent interior laminal cells. Median cells smooth, narrowly prosenchymatous, thin-walled, to 6 µm wide, mostly 10–15: 1, not pitted. Basal juxtacostal cells 2–3: 1, short rectangular with walls thin, not pitted, to 30 µm wide. Alar group poorly defined, mostly short rectangular gradually broader and shorter than the adjacent median cells. Alar cells to 25 µm wide, 2–3: 1, rather thin-walled and pellucid. Marginal cells of alar region rectangular, to 10 µm wide, to 4: 1. Costa extending 1/2 to 3/4, tapering to apex but seldom terminating in an apical spine. Juvenile leaves to 200 µm long, elliptic, about 2: 1, coarsely serrate across the rounded to truncate apex. Axillary hairs to 70 µm, 2–3 celled with a single basal brown cell. Rhizoids sparse, some short and unbranched, others long and planar much branched, light-brown, smooth, to 12 µm in diameter, clustered at the abaxial costa base. Stem cross-section with central strand, a stereome 2–3 cells thick, but without an hyaloderm.
Dioicous with male plants typically somewhat smaller than the females. Perigonia sparingly produced on prostrate stem and large branches. Perichaetial leaves 4–5 mm long, ecostate and entire or nearly so, sheathing at base but reflexed to squarrose distally, with narrow and elongate cells throughout and with no alar differentiation. Seta papillose but often finely so, 10–25 mm long, distally geniculate. Capsule dark brown to reddish brown, inclined to horizontal, not distorted or sulcate when dry. Urn to 2 mm, about 2: 1 but so strongly arcuate as to make difficult such a measurement. Exothecial cells irregular in size and shape, 15–20 µm wide, 1–5: 1, thick-walled with lumen: wall ratio about 3: 1. Stomata present on slightly raised pustules at base of capsule, short and vertical pored. Operculum conic, about 1/4 as long as urn. Exostome teeth yellow to yellow brown, narrowly lanceolate and obscurely shouldered, to 450 µm long, about 10: 1, horizontally closely striate, inflexed when dry, Endostome segments closely papillose, very narrow, about as long as exostome with basal membrane about as long as segments. Cilia 2–3, strongly appendiculate. Spores finely papillose, 12–18 µm.
Brachythecium hylotapetum is one of our more easily recognised members of the genus. At least some of its leaves have a rather deep concavity immediately below the rather abruptly defined acumen. That character combined with the large size and the loose foliation will allow easy field identification.