One or more sheet material layers in a multi-layer laminated structure is reinforced by adhesive bridges that extend through openings in the layer and tie together the layers on either side of the reinforced layer. A plurality of openings are provided in the layer or layers to be reinforced. The openings are spaced apart and distributed over the surface of the layer. Accordingly, when the layer is incorporated into a multi-layer structure, the adhesive applied to the layer, or to the layers on either side thereof, is forced through the openings so that a continuous adhesive connection is formed between the layers on opposite sides of the reinforced layer via the adhesive bridges that extend through the openings.