The disclosure relates to difficulty flammable wooden articles, especially chip boards comprising solid and pipe boards, prepared by extrusion from wooden chips and/or woody fibrous materials. The articles contain as a flame-retarding addend a product of(a) 50 to 99.5 weight % of a flame-retarding agent physically and chemically stable under extrusion conditions, preferably 75 to 99.5 weight % of a pulverulent, free-flowing ammonium polyphosphate of the general formulaH.sub.(n-m)+2 (NH.sub.4).sub.m P.sub.n O.sub.3n+1in which n stands for a whole number with an average value of about 20 to 800 and the ratio of m/n is about 1, and(b) about 0.5 to 50 weight % of a cured water-insoluble artificial resin encapsulating the individual particles of the flame-retarding agent or ammonium polyphosphate.The disclosure also relates to difficulty flammable boards veneered with wood or a laminate.