The paper studies the asymptotic behavior of the solution to the Dirichlet problem for the Laplace operator in a domain obtained from an infinite horizontal strip by connecting a vertical infinite half-strip of small width. Using the methods of potential theory, the problem is reduced to an integral equation on the boundary of the domain. The Schwarz alternating method is applied to the obtained equation in a appropriate Banach space. The solution is expressed in terms of "reflection operators". The formula for one of such operators can be obtained only under additional restrictive conditions on the right-hand side of the equation, which consist in the finiteness of certain weight norms.