You are asking the wrong question. EMP and "magnetic stuff" is not your true concern.
First, "EMP/magnetic stuff" will not wipe your harddisks, unless something really unexpected happens, such as a nuclear bomb going off in the stratosphere (and that will likely destroy the circuits, but not likely wipe the platters), or you putting the harddisk onto an induction cooking plate, or similar things. In which case you have more serious stuff to worry about anyway.
Even placing a harddisk in immediate proximity of a strong permanent magnet for several hours (I've actually done that by accident!) doesn't seem to affect it in any significant way, although I wouldn't know what the effects might be if you put your harddisk between two layers of neodymium magnets for 5-10 years.
A "storage facility", such as a shoebox on a table placed in an otherwise empty, closed garage, provides enough protection against any "magnetic stuff" that might reasonably occur.
The actual problem: Harddisks are not made for long-term storage, and they are definitively not made with zero failure rates. You should reasonably not expect a harddisk to store your data reliable for longer than 5 years. Yes, there exist harddisks from the mid-1980s that still work today (I own one of these), but that's anecdotical and meaningless since they have practically nothing in common with present-day devices.
Typical rates of failure (numbers vary, depending on whom you ask) are around 5% per year within the first two years and around 10% per year thereafter.
Typical unrecoverable sector failure rates on consumer drives are specified as 1 in 1014 bits, server-grade disks having 1015 instead. Those are lower bounds as specified by the manufacturer (and no guarantee of failure), but still.
With a typical present-day disk having around 1013 bits, this means that if you fill such a drive to its full capacity, there is roughly a 50% chance that at least one sector may be unrecoverable.
There exist better media for long-term storage. M-Disc promises 1,000 years of lifetime, and while those 1,000 years are almost certainly a false promise, you probably won't live to tell a difference. If they live 50 years, that's mighty fine. You need an extra powerful writer to burn them, but every normal DVD player can read them.
Since earlier this year, these exist in blu-ray format, which should give enough capacity to hold your most important stuff "forever". Verify the disc after burning, and better burn two of them to be sure, put them into a wooden box inside a rockwool bed inside a steel box (to protect against the casual storage facility fire), and keep that box locked in your storage facility, door closed. Best put a shrink-wrapped DVD reader in there too, just in case they don't make them any more in 50 years.
Unless a nuclear bomb explodes or you fail to pay the rent for your storage room, your data will live longer than you.