The structural performance of timber beams in bending could be considerably improved if the behavior of the flexural tension face were enhanced. This paper reports on an experimental and analytical study on the structural behavior of full-scale beams made of glued laminated timber, reinforced with natural fiber sheets. Two 5m-span beams were strengthened with flax fiber sheets and tested to rupture, along with a non-reinforced reference specimen. Comparisons between analytical predictions and experimental results proved very satisfactory, although the latter were rather disappointing. The paper further discusses improvement options for the applied reinforcement concept, as well as other reinforcement applications for flax fiber sheets.