This structure was completely below ground with raised entrances level to the surface and was aligned north-west to south-east. The north-west entrance was 42cm above the chamber roof and measured 6.65m long by 1m wide. It was capped by corrugated iron covered in concrete, which sat on a lip in the entrance wall that was 5cm deep all the way around the interior. Eleven steps led down into a corridor (70cm wide) ending in a recess (87cm deep) with a centrally-located 10cm-diameter exhaust vent. The south-east entrance was 45cm above the chamber roof and measured 6.35m long by 1m wide. This was capped with concrete covered corrugated iron that sat upon a 5cm deep lip at the sides and the chamber end of the entrance. The entrance was blocked by earth and rubble so it was not possible to ascertain the number of steps. However, there was an 87cm deep recess with a 10cm-diameter exhaust vent centrally located in the roof, present at the end of the corridor. Both entrance corridors had a poured concrete floor with five 60cm by 46cm concrete slabs set into them.
The central chamber was identical to shelters F1-F4 in terms of measurements and fittings. <1>