PTI Contours for Mortgage Affordability Risk

Mortgage affordability risk has increased as mortgage rates normalized while U.S. home prices remained elevated. This paper proposes a transparent, reproducible dashboard centered on payment-to-income (PTI), which appears as a contour map over the interest-rate and home-price space with decision thresholds at 25%, 30%, 35%, and 40%. Using official public data from FRED (30-year fixed mortgage rate, median house price) and a widely cited compilation of U.S. median household income consistent with Census reporting. This paper constructs a PTI surface, and quantify local sensitivities via a tornado chart, and compare a low-rate 2021 anchor with a representative 2024 anchor. With a 20% down payment and a 30-year fixed mortgage, the representative 2024 setting (price $419300; income $80610; rate 6.8%) implies a PTI of about 33%, i.e., above the commonly used 30% affordability threshold. Sensitivities show rate and price shocks dominate PTI changes at todays levels, while income shifts mitigate but more gradually. The approach is data-light and fully replicable, providing meaningful insights for integration into underwriting and macro-monitoring dashboards.