Ventilating the Roundhouse was a major issue in the days of steam. Smoke and steam continually filled the air and a primitive form of extraction was provided by smoke hoods positioned originally above every road. The locomotives were backed onto the road with their chimneys under the smoke hoods. The majority of the smoke and steam was vented in this way. There were no fans to assist the process and the atmosphere was always slightly hazy when locomotives were in steam.
The original vents were about three times the size of the present replicas now in position. They were originally made from timber with metal straps. Why wood? Cost and availability of raw materials were major factors but also mild steel, no matter how well it was protected, was subject on a daily basis to heat and moisture, perfect conditions for rust. Timber, although it eventually rotted, had a much longer working life and was easily and cheaply replaced by the shed carpenters.
If we look carefully we can also see that the roof has ventilation at the top of the eaves to release any smoke that missed the main hoods. In later days this feature proved essential to provide part of the through ventilation needed to deal with the exhaust from diesels.
Once located above every road, smoke hoods were essential in a working Roundhouse in the days of steam locomotives.