Fire-retardant powder comprising at least 30% by weight of mono ammonium dihydrogen phosphate and/or di-ammonium monohydrogen phosphate, at least 5% by weight of alkaline bicarbonate, at least 3% by weight of silica, and at least 5% by weight of a compound selected from the group consisting of: sodium chloride, potassium chloride, potassium bromide, potassium sulfate, magnesium carbonate hydroxide pentahydrate, magnesium chloride hexahydrate, iron(II) sulfate heptahydrate, zinc (II) chloride, and combinations thereof. The invention also relates to building materials preferably comprising natural fibers and comprising at least 5% by weight, and at most 30% of a powder according to the invention.