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Which Screw to Use Where and Why?

By: Chris Hogan MSc - Updated: 3 Dec 2012 | comments*Discuss
 
Screw Thread Pitch Head Heads Crosshead

When you're starting out in DIY you are likely to think that the only thing you need to get right about a screw is the length, apart from that anything will do. But although you can get away with a lot, knowing how to choose the right screw for the right job will make your life a lot easier in the long run.

Aspects of Screws

For the purposes of this article we're going to restrict ourselves to the screws that you will need for basic DIY jobs around the home. This basically means screwing into walls or wood.

There are about seven things to take into consideration when choosing a screw

  • length required
  • diameter of the screw
  • material being screwed into
  • material the screw is made of
  • thread type and pitch
  • type of drive – crosshead or simple slot
  • profile of the screw head
This might look daunting but most DIY stores help out by labelling the packets by the use, for example 'chipboard screws', or 'wood screws'. Let's look through the rest of the screw maze.

Length and Diameter of Screws

Getting the size right is important. Imperial sizes are numbered 1, 2, 3, 4 etc, the bigger the number the thicker the screw, or you can buy metric screws which simply state the diameter in millimetres. If you are fixing into a wall then the diameter will be determined by the wall plug you are using. Most of them will have the screw sizes printed on the plastic tab they are attached to and many will tell you the size of drill bit to use too.

If you're joining two pieces of wood the screw length should ideally be three times the width of the top piece, but never more than two thirds of the total width of both pieces. If you are fixing a piece of wood to a wall, at least two-thirds of the screw needs to be in the wall. Some long, narrow screws are used for joints in chipboard furniture as they can go deep into the edges of panels without splitting them.

Get the Right Material

You need to be sure that the screw is strong enough for the purpose and will not damage the material. This is more important with screws that are being used outside as steel will rust and corrode, leading to unsightly stains and ultimately breakage. You will never be able to remove them ether, as they will snap off.

Inside the house brass screws are often used to screw fittings into doors, as long as they match the fittings, because they don't rust. Otherwise steel is fine except perhaps in bathrooms, where there is a lot of moisture in the air. Make sure you drill pilot holes in wood, hardwood in particular, to prevent it from splitting when the screw is tightened.

Thread Type and Pitch

There are many aspects of threads but for the screws used around the home it's really only necessary to be aware of two things, shanks and the coarseness of the pitch. If you are joining two pieces of wood then the smooth shank (the part just below the head) will allow the two pieces of wood to be drawn together in the final tightening.

Most wood screws will have a medium pitch (the distance between the peaks of the screw thread) but some screws, such as the chipboard screws mentioned earlier, have a coarse thread. This means there is a larger distance between the peaks of the thread and the screw will go through the soft material a lot quicker. If you use these for proper wood you may well split it. Wood screws are fine for fixing into a wall plug.

Different Screw Drive Types

Traditional wood screws have one slot in the top for an ordinary flat-bladed screwdriver. These should be used when the head will be visible as they look better than crosshead screws. Crossheads were developed for machine driving, because they don't slip so easily they can take much more driving force.

But make sure you use the right screwdriver or bit for the right type of head. There are two main types, Phillips crossheads and Posidrive, and you need to make sure that the bit is the correct type and the right size. Phillips heads have four cross points and a Posidrive crosshead looks like an eight-pointed star. A Phillips crosshead screwdriver can be used on a Posidrive head but not the other way around.

Screw Head Profiles

Finally we'll look at the screw head profile, which means the shape of the screw head when you look at it side on. The most common for DIY is the countersunk screw head, tapered so that it finishes level with, or just below, the surface of the wood. This is useful for areas where you may want to paint over the screw heads, perhaps after skimming a bit of filler over them, to conceal them.

You will need a countersink bit to drill the shallow depression for the screw head to go into. If the screw heads will be seen or you can't countersink, choose a round head or a raised head screw. There is also a bewildering array of different decorative screw heads to choose from for those that have to remain on view, including some that come with caps to conceal the head. As long as the thread and material are right, just choose the type you like the look of.

Don't Worry Too Much

Phew! There's a lot there but don’t worry too much. When you buy screws make sure you know what job they are for and buy the most appropriate screws you can get hold of. They don't have to be perfect but the closer they are the easier your jobs will be.

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