In the search for superconductivity with a high critical temperature $T_c$, new materials with light elements are expected to favor electron-phonon coupling at higher frequencies, thus achieving a higher $T_c$. Due to their typically stiff bonds and the small atomic masses, carbon intercalated compounds are among the prototypes of light-element superconductors. Here, we report on the electronic and superconducting properties of ${\mathrm{Sc}}_{20}{\mathrm{BC}}_{27}$, a member of the carbon-based layered compounds. By using mostly muon-spin spectroscopy $\left(\mu \mathrm{SR}\right)$, we investigate the ${\mathrm{Sc}}_{20}{\mathrm{BC}}_{27}$ superconductivity as a function of temperature and hydrostatic pressure. The experimental data are compatible with a single $s$-wave-gap superconductor in the clean limit. In particular, we find that the magnetic penetration depth at ambient pressure, $\lambda =512\left(3\right)\mathrm{nm}$, decreases only slightly with increasing pressure (up to 16.7 kbar). The superconducting gap $\mathrm{\Delta }\left(0\right)=1.73\left(3\right)\mathrm{meV}$ is fully isotropic at ambient pressure, but it becomes anisotropic under hydrostatic-pressure conditions. At the same time, similar to other carbon-based superconductors, $T_c$ increases with applied pressure.

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• Spin-singlet pairing
• Superconducting gap
• Superconducting order parameter
• Superfluid density
• Muon spin relaxation & rotation