Why lock half of a double door?

June 29, 2006 9:36 PM

When a building entrance has a set of double doors, why is one of the two door locked? I've seen this in many buildings. Is it for security? It's one of my pet peeves, especially when there is not a "use other door" sign on the locked door. I think it's a safety issue. I really want to understand why people do this. Any building managers, or business owners care to comment?
posted by hockeyman to grab bag (20 answers total)

A lot of double doors latch one door into the floor and top rail of the door frame, and the door that opens just latches to that fixed door. It's an easy set up to install, and doesn't come out of alignment in use frequently.

Double doors in multi-story buildings are often there for emergency access issues, with the expectation being that both doors will only be open in fire situations, or perhaps times when large objects are being moved in or out of the building through those entrances. The rest of the time, only a single door is allowed to be opened, to reduce air flow into the building, and to control ground floor lobby wind due to stack effect. In tall buildings with significant stack effect, there can be a dangerous periodicity to the stack effect, that will allow both doors to be opened, only to start a massive inrush of air that accelerates up the elevator shafts, and eventually slams the doors shut with such force that they injure people and break glass. In very tall buildings, revolving doors, or double doors arranged in sequential pairs are required to form functional airlocks to control this problem.
posted by paulsc at 9:51 PM on June 29, 2006 [4 favorites]

If one door isn't locked any wind will blow them open all the time.
posted by tellurian at 10:17 PM on June 29, 2006

Great question, hockeyman. My peeve, too. And paulsc's answer is valid, but doesn't apply in most cases where I live. It often occurs in small stores, perhaps out of laziness or a fear that the doors will not lock properly at closing if someone forgets to throw the slides in. And frequently there is no "use other door" sign.
posted by weapons-grade pandemonium at 10:19 PM on June 29, 2006

I used to have to walk through a hotel to get from the metro to my apartment, and they had two sets of double doors one after the other. In each set, the opposite door was kept locked. There were always lots of tourists coming through, know one knew which door to use, and there was often a bit of a line just to get through the doors.

One day I asked the hotel manager why the doors were set up like this. "It's for anti-terrorism," he replied. Um, okay.....
posted by hazyjane at 11:45 PM on June 29, 2006

I used to be a building manager and we did this because the condensers on the door (that keep the door from slamming shut) would go out all the damn time. Like, every couple months.

If it was broken, we locked that door. If it wasn't broken, we would still often lock the right hand door (if you are inside the building facing out), since people would automatically go to the right on their way in anyway. This would save the condenser from going for a few extra weeks.
posted by SassHat at 12:35 AM on June 30, 2006

I assumed it was just a custodial tactic to get out of the building with less strain. After you've heaved this and loaded that, lifted this and carried that all over the building, the last thing you'd want to do is reach up high to latch the top notch, and bend way down to latch the bottom notch while so sore. Getting the straining/bending done with before the soreness had set in made things easier on the way out. Just my guess, though.
posted by vanoakenfold at 1:11 AM on June 30, 2006

I have a double-door out onto my balcony. The one door has two pins that lock up into the doorframe, one on the top, and one on the bottom, to keep it from moving. If I don't keep these pins locked in, so that the door can't open, a small wind will force both doors open. The door do not exert pressure on each other, and there's only a small catch in the middle to keep them together. The typical doorknob catch is not deep enough to keep them stationary; given a small amount of force, the doors swing slightly and the catch comes out of its recess in the opposite door, allowing them both to open. So that's -my- reason for doing it, anyway.
posted by po at 1:32 AM on June 30, 2006

Could this have to do with keys? Like perhaps a special key is required to open the second door, which only the maintenance staff has, whereas any teacher (thinking school here, just as an example) with a standard key can unlock the main half.

I've also seen this where the second door has no outside handle, so having it unlocked would only confuse people. I guess in that case the second half is only there to be opened when taking something really large through the door, and to provide something for the main half to latch against.

I've seen some double-doors where one half had to close before the other, else it would get all tangled up. In that case if both halves were left unlocked and visitors were not diligent to close them in the right order (which they never are) then the doors wouldn't latch and that would be a HVAC issue.
posted by Rhomboid at 2:00 AM on June 30, 2006

The purpose of double doors is easy access. Not so much for people, but for things. Whether moving furniture, fixtures or stock into and out of buildings.

When not being used for that pupose the doors will not remain closed securely (in a wind, etc.). The pins for locking are used for that purpose. Period.

Seriously.
posted by FlamingBore at 2:55 AM on June 30, 2006

Sometimes people just forget to unlock the second door.
posted by drezdn at 3:17 AM on June 30, 2006

A quick tip to people peeved by this (from my dad, who has been in the package and truck industry for years) -- always push open the door with the deadbolt lock on it. It will always be the unlocked on. This may be obvious, but it wasn't to me.
posted by trey at 4:21 AM on June 30, 2006

After working retail for a bunch of years, I agree with drezdn. Sometime, you just forget to unlatch the second door. The first one unlocks with the dead bolt, then you have to reach up and down to unlatch the second. If you've been letting the morning staff in by just unlocking the dead bolt, it's easy to forget to unlatch the other door at opening time. Or sometimes I would give my keys to an employee (if I was busy) to unlock, and they didn't know about the latches.
posted by kimdog at 6:14 AM on June 30, 2006

I frequently unlock the second door when I find myself in this situation. No one has ever said anything when I 'unlock' the two pins (one in the top, one in the bottom) on my way through. As far as I can tell, there is no reason good enough to build two doors and only use one.
posted by maxpower at 6:45 AM on June 30, 2006

I have a policy that if one door is locked, and the pen on the counter is out of ink, I change banks. Done it twice now.
posted by StickyCarpet at 7:25 AM on June 30, 2006

paulsc: "stack effect" -- thanks for that! I wondered why there was an enormous rush of wind when the elevator doors opened at the top of a tall building.

I see the point of two double doors. (And if you've ever moved a couch, you'll agree that ALL doors should be doubled, everywhere.) And I see the point of keeping one door locked most of the time. But I wish there was a reliable convention more obvious than the deadbolt location. I automatically default to using the right-side door, and occasionally smack into one that's locked. Doh.
posted by Tubes at 7:40 AM on June 30, 2006

If you leave the second door unlatched, you can't close the first one. They'll both just swing free at the slightest push/wind, etc.
That's it.
posted by signal at 8:03 AM on June 30, 2006

Agreed with those talking about the stack effect.

I used to run a gas-station convenience store, with the double doors at the front. If both doors were unlocked, one of them would always always blow open everytime the heating or a/c came on. We regularly called out the HVAC compnay to adjust the ventilation to eliminate the positive pressure, but for some unknown reason this would only solve the problem for a few days at a time.

In a 24 convenience store in the wrong area of town, having one door open all the time is a major safety concern. We had magnetic locks that come on with the flick of a switch (to keep approaching bad guys out, or keep people with their pockets full of unpaid merchandise in) but it only engages if the doors are shut. So we'd frequently lock one of the doors and put up a big "Use Other Door --->" sign. No one reads the sign. No one. They will push and push and pull and pull and bang on the door, turn to the clerk and say "why are you locking me in?" and swear and kick the door, but they don't see the sign. Even if it's on hot pink paper right at eye level on a glass door, they don't read the sign.

The door problem was one of those tiny stupid things that made that job suck.
posted by raedyn at 8:39 AM on June 30, 2006

Unfortunately, building code sometimes requires that the second door be operable to provide an adequate width for the egress of building occupants, based on the occupant load. So, it's entirely possible that some of these places that leave one door locked, for whatever reason, would get in serious trouble with the fire marshall. This is certainly not always the case--stores in a strip mall with double doors hardly need 72" of exit width. Sure, you can unlock the second door by pulling the pins in the floor and ceiling if you need a sudden escape, but by code, that makes the exit too hard to use, especially in a panic situation.
posted by LionIndex at 9:16 AM on June 30, 2006

Here was an earlier thread on this question. In that thread I posted links to two Washington Post articles that might be of interest.
posted by llamateur at 10:46 AM on June 30, 2006

llamateur - My search on this subject in AskMefi returned no results. Thanks for the Post articles.

I think for small establishments, it the broken door or "lazy" reasons for the locked.

Thanks for all the great responses.
posted by hockeyman at 2:40 PM on June 30, 2006