Click here to read and download the 1939 Directions for the Erection and Sinking of the Galvanised Corrugated Steel Shelter'.
The 14 gauge steel was both relatively thick and very strong. (14 gauge is 2mm.)
One weakness was the doorway. Householders were expected to provide their own wooden doors or else build something more substantial as below. Note the lucky horseshoe!
Every shelter kit was held together with bolts, as shown below, and provided with a spanner-tommy bar, also shown below. After construction, the spanner was kept in the shelter as it might be needed to loosen the clip bolts at the back of the shelter, and so create an emergency exit if the main door was blocked by bomb debris.
The face-to-face width of the bolt heads was around 2.25cm - just under an inch. (For scale, the length of the yellow label is 1.5cm.) The photo of the bolts can be expanded by clicking on it.
But some shelters had a rather better spanner. Al Manton kindly sent me these photos of a swivelling box spanner that he found in an Anderson shelter on his allotment. He commented that "It is not a scaffolder's spanner. The box section is stamped 1/2" W & 7/16" W & both sizes correspond with the original 1 1/2" long Whitworth nuts & bolts supplied to fit the shelter together. It has the podging end for aligning sheets, and the advantage when fitted on the bolt hexagon inside the shelter that it can be easily rotated without snagging on the corrugations." Also "These double ended spanners were issued during the 'phoney war' but were soon superseded as the sheer cost & quantity of spanners required later led to the introduction of the cheaper-to-manufacture open-ended rat tail spanner".