This structure was completely below ground with raised entrances level to the surface and was aligned north-east to south-west. Both entrances were 42cm above the chamber roof and capped with corrugated iron and concrete. The lip for the concrete cap on the north-east entrance was 10cm deep at the chamber end and 5cm deep at the sides and entrance. The lip on the south-west entrance was 5cm deep at the sides and 10cm deep at the entrance and chamber end. Both entrances were completely blocked by earth and rubble that could not be removed; therefore it was not possible to ascertain the number of steps. The north-east entrance was 5.75m long by 1m wide, the south-west entrance 6.45m by 1m wide. Both entrances led down into a corridor that terminated in an 87cm deep recess with centrally positioned 10cm-diameter exhaust vents, and both had poured concrete floors with five 60cm by 46cm concrete slabs set into them.
The central chamber was identical to shelters F1 and F2 in terms of measurements and fittings. <1>