What are fibre washers used for? Red fibre washers are generally used within the plumbing trade where a seal to prevent leakage is required. Vulcanised fibre washers purpose is to absorb the fluid they come in contact with and swell slightly to fill the joint or gap within the pipe of flange. Do fibre washers expand when wet? The unique property of a fibre tap washer - sometimes referred to as vulcanised fibre washers - is that the material naturally expands when it becomes wet.  Widely used in many areas of plumbing, fibre washers tend to be somewhat easier to compress than rubber or nylon versions. Red vulcanised fibre washers are generally used within the plumbing trade where a seal to prevent leakage is required. Vulcanised fibre washers purpose is to absorb the fluid they come in contact with and swell slightly to fill the joint or gap within the pipe of flange.