Contract Altro Stronghold 30 Midnight K30421 Speckled Effect Non Slip Commercial Vinyl Flooring. Altro Stronghold 30 achieves Altros highest rating for slip resistance and is designed to minimise risk in wet and greasy conditions for the lifetime of the flooring. As well as helping to keep staff safe from slips, it also helps reduce their fatigue, thanks to its 3mm thickness which provides noise reduction and comfort underfoot.