Avoid soggy roast beef sandwiches by selecting thick slices of bread or crusty rolls like baguettes that hold up better and retain structure. Toasting the bread slices or baguette halves can also help against moisture.  Placing the ingredients strategically on the sandwich is another way to prevent soggy sandwiches. For instance, the mayonnaise spread on this cold roast beef sandwich provides a barrier, preventing moisture from the other ingredients from being absorbed by the bread.  Lastly, any sandwich ingredients that are washed should be pat dry before being added to the sandwich to remove any excess moisture.