Since they are Debris overlays, any terrain under them is no longer a factor. They are now Debris hexes.
Per ITR6... All Debris (O1) rules apply normally.
O1.1 DEBRIS1 (EX: RBK3) is Inherent Terrain (B.6) [EXC: if in a Factory; 5.41] and also Concealment Terrain (A12.12). It has a +1 TEM and is a Half-level LOS Hindrance (both of which are cumulative with Factory TEM/Hindrance [EXC: see 5.41]). Debris may not be Kindled, nor may a Blaze spread to it; however, debris in a Roofless Factory Hex (EX: RBS13) can catch Fire along with the building Location(which may still be Kindled in the normal manner [EXC: Gutted; 5.51]). Debris may not be created during play.
O1.2 MF/MP: Infantry must expend 1 MF + COT to enter a debris Location (EX: 2 MF to enter ROO21, 3 MF to enter ROO22 from O21). The only vehicle type that may enter debris is a fully-tracked AFV, which does so by expending ¼ (FRU) of its MP allotment and taking a Bog Check with a +1 DRM (and an additional +1 Bog DRM for entry of a Factory Location if applicable) [EXC to all: if Bypassing a Factory debris Location 5.42; if entering from "outside" the Factory, normal building entry applies]. Dash (A4.63), road bonus (B3.4) and the ½-MP road rate are NA in a road hex covered by debris except via a TB (O.1).