Intumescent coating was studied relative to the fire performance of nail-laminated timber. Three NLT specimens were coated with three different intumescent coating thicknesses (1, 2, and 3 mm) in even-numbered laminae and compared to uncoated NLT specimens. As a result of the coating, the internal temperature of the coated specimen increased more slowly than that of the uncoated specimen. The average charring rate of the intumescent coating specimen was reduced by 12.8% (1-mm thickness), 14.1% (2-mm thickness), and 15.4% (3-mm thickness) compared with the uncoated specimen. However, statistical analysis showed there was no significance between 1-, 2-, and 3-mm coating thicknesses. The combustion of wide surfaces of timber laminae between the plywood was delayed due to the coated plywood, and the timber laminae became a one-dimensional charring rate problem. Therefore, if even laminae are coated with an intumescent, then the NLT can be designed with a one-dimensional charring rate condition.