The guide shows you which anchor is the right choice for which task. Find out more about the different types of dowels and their respective uses.
Which dowel for which screw?
The right dowel must match the substrate or the building material, as well as the drill and the screw. So that the dowel expands properly when the screw is screwed in, the length of the screw should be at least equal to the length of the dowel. Regarding the diameter, choose a dowel that is slightly larger than the diameter of the screw. For the most common sizes of dowels and screws, this means:
- For a 6 mm dowel, choose a screw with a diameter of around 4 mm to 5 mm.
- For an 8 mm dowel, choose a screw with a diameter of around 4.5 mm to 6 mm.
- For a 10 mm dowel, choose a screw with a diameter of around 6 mm to 8 mm.
If you want to turn a screw into building materials in which they cannot hold alone, you need a dowel. Typical areas of application are masonry, concrete and plasterboard as well as building boards, aerated concrete or similar building materials and suspended ceilings.
While the type of dowel depends heavily on the planned use and the substrate, you have a little more freedom when choosing screws. It is important that the screw has a suitable diameter, is not too short and holds well in the dowel. The screw and dowel must be suitable for the planned load.
Common dowel types at a glance
Dowels are made from steel anchors, chemical compounds, and plastics such as high-quality nylon, among other things. In many installations, the universal dowel is mainly used. Universal dowels are available in various designs, sizes and lengths. The all-purpose anchors are suitable for a variety of building materials, but are mainly used for assemblies on masonry and on concrete walls and ceilings.
You also use expansion anchors for installations on walls and ceilings. They work in a similar way to universal dowels, but are somewhat more resilient. This is why you use expansion anchors for installations with a higher load, such as when attaching wall units in the kitchen. There are expansion dowels made of plastic and metal and in various sizes and lengths.
If your walls are made of aerated concrete, universal and expansion dowels do not offer sufficient support. In that case, you dowel the wall with special aerated concrete dowels. These dowels are specially adapted to the nature of the subsurface.
You also use special dowels for drywall made of plasterboard. Plasterboard dowels literally cut into the building material and thus provide optimal hold in the comparatively soft subsurface. It is sometimes advisable to pre-drill so that the dowel fits perfectly into the material.
Alternatively, you can use hollow wall anchors for hollow walls. In order to insert the dowels, first pre-drill as well. In the case of more massive building materials, you dowel with cavity dowels, which expand when the screw is screwed in, similar to universal and expansion dowels.
If you want to install lamps or the like on a suspended ceiling, spring clip anchors provide the right hold. First you drill holes so large that the compressed spring flaps just fit through. Then you insert the hinged dowels. For most installations you need washers, which are often already included with the dowels.
Chemical dowels and liquid dowels are a specialty. You inject this into a drill hole that is too large. The agents harden almost immediately and can be used in many different building materials. After drying, the material partly behaves like wood, so you can screw screws directly into the dowel mass. With other masses, you quickly press a dowel in during the drying process so that you can screw the screw into it later.
In addition to these frequently used dowels, there are various other special dowels. These include, for example, plugs for sanitary installations, insulation plugs and window frame plugs.
Before choosing the right dowel, ask yourself: What should be fastened and which forces act on the dowel and screw. Tensile, transverse and / or oblique load? What is the weight of the parts and objects to be attached? In which substrate should the anchor work: is the material porous? Does it have voids? Do I need a hammer drill and which drill do I choose?
The substrate: which dowel fits?
The choice of the right dowel depends heavily on the building material or the substrate in which the dowel is to hold. Pressure-resistant, compact materials such as brick and concrete are good for anchoring a dowel.
But not all concrete is the same. So-called lightweight concrete or aerated concrete contains aggregates such as pumice and expanded clay, which make it less pressure-resistant and a greater challenge for the anchor. Use a dowel with a long expansion zone to increase the force of the dowel. In normal concrete you drill with a hammer drill and use expansion or universal dowels. A special dowel is necessary if a dowel is to be inserted into aerated concrete. The aerated concrete anchor with its spiral-shaped outer ribs is the best choice here.
There are also differences when it comes to masonry: Is it solid or perforated brick with a dense structure? The former are pressure-resistant and therefore unproblematic. All-purpose and expansion anchors are also suitable here. If, on the other hand, you drill into a perforated brick, a cavity dowel may be necessary. These dowels spread wider than conventional universal dowels.
Building materials such as plasterboard, chipboard or plywood often only have a low strength and thickness and are therefore not without problems when it comes to anchoring a dowel. Cavity dowels are used here, which anchor themselves in the cavity on the back of the panel.
Dowels and assembly
So that the dowel fits into the wall, you pre-drill with a drill of the same size – unless it is a so-called hammer or nail dowel. You drive these dowels directly into the wall.
If you don’t know what the subsurface is like, you can drill carefully. Hold the drill at a right angle to the wall and do not use any impact at first. If the ground is very solid, you increase the pressure or drill with a punch.
However, if the drill sinks directly into the wall, caution is required. The hole should never be too big. In the case of particularly soft surfaces, choose a smaller drill size (-1 mm) so that the dowel sits firmly in the hole. This increases the load-bearing capacity and security against rotation.
When installing with dowels, a distinction is made between push-through installation and pre-positioned installation. With push-through installation, the dowel is inserted through the component into the borehole and expanded. With pre-positioned installation, the anchor is flush with the surface of the anchoring base. The borehole is larger than the hole in the connection component. A third variant is stand-off installation, which is used, for example, on facade elements. Here you fix the connection component at a distance.
Note: Clean the drill hole with a vacuum cleaner after drilling, as dust and drilling dust impair the functionality of the anchor and thus the fastening.
Tip: If you don’t have a helping hand for the holes to the side, stick a piece of paper, bent upwards, to the wall below the point to be drilled to catch the drill dust trickling down from the hole.
Load capacity of the dowels
As a rule of thumb, the bigger the anchor, the more it can carry. Since the subsurface contributes significantly to the hold of the fastening, manufacturers rarely give precise information on the load on dowels. If in doubt, use a slightly larger and longer dowel to increase the load capacity. Note that you can only achieve the highest load capacity if you choose the right screw diameter.
Depending on the installation, several forces act on a dowel, which it has to transfer into the wall. For example, the pulling force pulls the ceiling lamp and dowel vertically downwards. The transverse force acts on a wall mirror and the dowel placed horizontally to the floor. The diagonal pull results from tensile and transverse forces and plays a role especially with heavy wall mountings such as washbasins and wall units. The anchor manufacturer’s load tables provide information about the weight of the anchor in question – in Newtons and kilograms.
If a dowel was installed or selected incorrectly, it can easily slip out of the wall. For these torn holes there is quick help in the form of repair fleece. You wrap the fleece around the dowel and put it back into the drill hole without drilling again. Alternatively, you can use repair mortar or similar products to fix the dowel in the hole and obtain a secure attachment.