In a construction material comprising a flexible carrier substrate impregnated or coated with a water-hardening resin, the resin contains as an additive polyether polysiloxane polyurethanes of the formula ##STR1## in which R.sup.1 is lower alkyl,m is the average number of siloxane groups in the range from 5 to 25,R is ##STR2## in which X is lower alkylene,Y is an aliphatic, cycloaliphatic, aromatic or araliphatic moiety, which is unsubstituted or substituted by further ##STR3## Z is a polyether moiety based on ethylene oxide units propylene oxide units, or both with the average number of ethylene oxide and propylene oxide units being in the range fro 10 to 100 andR.sup.2 is lower alkyl.