This structure was completely below ground with raised entrances level to the surface and was aligned north-east to south-west. The north-east entrance was 42cm above the chamber roof and measured 6.4m 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), above which was a 10cm- diameter centrally located exhaust vent. The south-west entrance was 37cm above the chamber roof and also measured 6.4m long by 1m wide. This entrance was blocked by earth and rubble so it was not possible to ascertain the number of steps. The south-west corridor was 70cm wide and terminated in an 87cm deep recess with a centrally located 10cm-diameter exhaust vent. Both entrance corridors had poured concrete floors with five 60cm by 46cm concrete slabs set into them.
The central chamber was identical to shelters F1-F3 in terms of measurements and fittings. <1>