This report has been prepared for Westok Ltd as an extension to the guidance given in RT983. The report contains design information to enable the required thickness of intumescent coating to be determined under Option 1 of AD 269, for the fire protection of cellular beams with d/t limits up to 100ε. In addition, further studies have shown that guidance in RT983 and the first issue of this report, RT1006 may also be applied to beams with elongated openings that may fail by Vierendeel bending. The guidance in this report (for d/t limits beyond 80ε), only applies to beams designed using the Cellbeam software. This software has been written specifically for Westok Ltd by SCI for the analysis of cellular beams manufactured by Westok Ltd using rolled sections. The Advisory Desk Note (AD269) was published in the November/December edition of New Steel Construction and was supported by the later publication of RT983 ‘Interim Guidance on the use of Intumescent Coatings for the Fire Protection of Beams with Web Openings’. RT983 provided background information on the engineering behind AD269 and also provided a more detailed approach for determining the thickness of the intumescent coating required for composite beams with web openings. RT983 and AD269 covered the design of beams with d/t ratios up to 80ε.