# How can screwing a massively heavy object to a wall with four screws (like a water tank) be safe? I feel its always going to fall, taking a piece of the wall with it.

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Or screwing workout equipment which you constantly pull with your own weight.

EDIT: Forgot to add, I’m not in the US, I’m talking about brick or concrete walls, not drywalls. Although probably the basic principle applies when it comes to explaining how force works.

In: Physics

You don’t screw it into the plaster. You screw it through the plaster and into the wooden post behind it. You find the wooden post behind it by using a magnet to detect the nails.

First, you don’t screw it into the wall, you screw it into the wooden studs within the wall. Screwing into drywall or plaster alone is insufficient for anything but the lightest loads.

Second, if you’re dealing with any sort of significant loads you don’t want to use regular screws you want to use lag screws, which are basically a cross between a big ass bolt and a screw. Normal screws just can’t handle the sheer forces or pullout forces a lag screw can.

If you’re screwing it onto the drywall or plaster instead of the wooden studs, then yes, it would eventually fall.

Screws, bolts, and nuts produce an incredible amount of clamping force. A screw is basically two very low incline ramps or shims being pushed against each other. It takes very little force turning the screw (torque) to create a huge force in the axis of the screw or clamping force. For example the force holding your wheels to your cars is over ten times the weight of the car PER WHEEL. Just from 5 little nuts torqued to ~100ft/lbs or someone pressing a foot long wrench with 100lbs of force.

Edit: Misread the point of the question, yes typically heavy things are not screwed into the drywall but into the frame of the house itself.

You guys have plasters and wood for walls? I got concrete.

Most of the force applied is pulling DOWN on the wall, not out from the wall, as long as the bolt/anchors are sufficiently deep into the wall. You can easily place a water tank or your body weight on top of a wall without worrying about whether the wall will be crushed, and that’s essentially what’s happening when you mount something properly. You’re creating a vertical force (mostly) at the mounting site. Note that the farther you place the center of mass of whatever object you’re attaching to the wall from the wall itself (like a really long platform), the more you’re going to need to drive the anchors/bolts deeper or add some additional support (like an angled support bracket) to transfer more of the horizontal load to the wall for vertical support.

Screws in bricks/concrete are basically friction fit: you put in the plastic sleeve which is already snug in a hole if properly drilled, then put the screw in. The screw is like an inclined plane as somebody else said it and has immense mechanical leverage. It expands the sleeve and crushes it into the rough surface of the hole with extreme force. A well chosen fastener will not be that much weaker than directly casting that screw into the concrete/brick. Nevertheless there are supplementary products than can be sprayed into thehole to make extra sure things stay in. The big ones used for mounting water heaters plus the epoxy hole filler get you a screw that [takes more than a ton to pull out](https://link.springer.com/article/10.1617/s11527-020-01536-2#Tab4).

A lot of people have added stuff but another major factor is most force is going to be towards the ground and so perpendicular to the fastener. Things that are designed to be attached to the wall generally have the most weight placed as close to the wall as possible so there is the least amount of pulling away from the wall.