Thatched magic

[Lightspeed]

To shield against EMP, a faraday cage needs to completely surround an object,

Interesting that Devonian has experience of having to install external GSM antennas and repeaters in a steel roofed building. That just shows that the roof is shielding against radio waves sufficiently to reduce reception within the building.

EMP is effectively high power radio frequency energy. A steel roof, or chicken wire clad straw roof would reduce EMP but not shield from it completely.