How long post SHTF before a city is safe to re enter?

[NorthernRaider]

It only takes one normal autumn for thousands of buildings, offices and factories to see their gutters clogged with fallen leaves, the same can be said of millions of drains and gullies. Add a couple of decent winters , bit of snow, bit of frost, bit of hail and tiles, bricks etc will be falling aplenty. As the clogged drain cause rain water and melting snow to find other routes the water will soon undermine foundations, roads, towers, plylons, flyovers etc this will cause concrete spalling and cracking more to fall on peoples bonces. Add vandalism, arson possibly post combat damage, flood damage, no maintenance etc you can be sure there will be glass panels, windows, fascia and shop fronts,guttering, telephone and power cables, bits of slate and tile etc all waiting sods laws opportunity to drop onto your head. The prepper approaching any city after TSHTF is going to approach very cautiously avoiding packs of dogs, unstable ground, damaged building, open spaces where they can be spotted. Given the chance the preppers will approach along old quiet abandoned railway lines, storm drains and tunnels, ALL will be inlit, all will have loose bits of masonary, cracked supports, hanging wires etc. In a world without hospitals and medicines but plenty of diseases a hard hat will protect your head from falling material, it will stop you scraping your noggin on that low roof and getting Weils disease etc into even small cuts n scrapes.
Imagine rummaging around dark unlit properties looking for safe routes of stuff to salvage and you scrape your head on a hanging ceiling tile, one thats been dampened by the weather blowing through open windows and doors, one that a rapid pigeon shit on before you scraped your nut on the tile. Its just as deadly as standing on a rusty nail, shard of glass, splinter of wood etc, kneeling, leaning against, falling onto, etc etc. Injuries to heads can be utterly lethal without a 4 pound lump of cornice smashing your head in.

The climbing hat, site hat, crash helmet will be as essential as gloves, wellies, high boots and dust mask if you decide to go to Harrods and Selfridges.

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Note also that the chartered institute of something or other reported than since 2008 maintenence of buildings is at an all time low because of the reccession, basically no one is taking care of their buildings any more.