Using screws is simple enough, but there are many points that can avoid problems & make for a more satisfactory experience.
Choosing a size
Size choice tends to come from a little experience. Its hard to give exact recommendations as material conditions vary, degree of security of fixing versus speed varies, and the weight supported varies.
As a rough ballpark to start from, and these figures will inevitably be wrong in some situations:
- fixings into masonry wall: typically 2.5" screws
- fixings into wooden uprights in plasterboard walls: 1.5" - 2" screws typical
- fixing small light items to wooden uprights in plasterboard walls: typ. 30mm or 1.25" screws
- heavy items to masonry walls: typ. 3" screw
Thread widths are measured either in mm or in woodscrew sizes. Woodscrew sizes 6,8 and 10 are the most common in DIY.
- Thicker threads give more strength to the screw and more strength to the screw-wood bond.
- Thinner screws are more often ok with no pilot hole
- Thinner threads take less physical energy to insert. This makes for easier driving, more insertions per battery life and less tiring.
- Thinner screws usually cost less, but not always
- Coach bolts are mega-fat wood screws, for when an ultra strong large joint is needed.
For countersunk screws the quoted screw length is simply the physical length of the screw. For raised head types its not, the head is omitted when quoting screw length. The idea is that in all cases the quoted length equals the distance from the surface of the workpiece to the tip of the screw.
Head Recess Types
Slotted screw heads are the oldest of all today's head types, and have the poorest performance of all major screwhead types. There is no misalignment tolerance, and heads are easily and often damaged in use. The bit gives no support to the screw, which must be supported by fingers in nearly all cases. And conveniently the driving bit has sharp corners, just what's wanted when the bit is liable to slip off the screwhead onto your always required fingers.
The advantage with slotted head screws is they look nice, and fit aesthetically with old fashioned goods.
Slot heads also have a minor advantage where access is especially difficult, which is that a slotted tip can be ground at any desired angle. However screws are hardly suitable when access is this difficult.
Philips & Pozidrive
These give some amount of support to the screw while screwing, provide more grip, and more tolerance of minor misalignment. The bits don't have sharp corners and don't slip out of the head as easily, reducing finger injuries.
Pozi has slightly better characteristics of the two for most tasks. Philips is preferred for plasterboard screws as the bit disengages from the head more readily when driving torque limit is exceeded (ie it cams out).
Bits are available that can drive these screws at upto 15 degrees out of line.
Pozi size 2 is by far the most common screwhead in diy use, size is 1 used for small wood screws. Size 3 is mainly used in flatpack fixings and large screws, eg 6". Sizes 0 & 00 are used on small appliances, size 4 I've never seen in DIY.
Philips and pozi are to a fair degree interchangeable, but often using the wrong driving bit causes poor driver performance.
Sometimes screws that appear to be one type turn out to fit a screwdriving tip of the other type much better.
Pozislot heads will accept both pozi and slot screwdrivers.
Allen & Square
These have greater interlocking of bit and head than pozi, they mostly eliminate bit slippage, and hold the screw firmly inline with the bit. Square is more wear tolerant than hexagonal, and is also known as Robertson.
A drawback with allen key driven heads is that attempting to drive them using the wrong size bit can cause rapid bit damage.
Hexagonal ball ended bars are available that will drive these heads at an angle. These are less robust.
Torx appear hexagonal, but with a star shape. They have better wear resistance, and are intended to prevent cam out, unlike philips. Torx sizes T5 to T25 are in common use.
Multitiered hexagonal recess
- These heads aren't popular
- The advantage is that one bit fits all sizes of screw
- The disadvantage is increased wear.
- The heads are hexagonal, and a socket or spanner is used.
- Sometimes they also have a slot or pozi recess.
- These enable high driving torque
- the screw head is drivable over a range of angles from around 45 degrees to 90 degrees as well as 0 degrees, occasionally making them usable in difficult access situations.
- Spanner heads can't be driven by a spanner
- They have 2 holes into which the bit is inserted to drive them
- Sizes 4mm, 6mm, 8mm, 10mm
- The heads and bits do not have as much strength of many other head types
- If necessary a slotted screwdriver bit is easily turned into a spanner head with a die grinder.
Security type heads
Security bit sets are readily available at builder's merchants and DIY sheds.
Many of these heads have a central pin. If caught without the right tool its often possible to insert a screwdriver and turn it to snap off the pin.
Hex tamper resistant
- Has a central pin.
- Requires a TR (tamper resistant) hollow hex bit.
- Has a central pin
- Requires a hollow torx TR (tamper resistant) bit.
- There are also other variations on the Torx theme, such as external torx, torx plus, and tamper resistant torx plus with 5 lobes instead of 6.
A 3 lobed version of pozi.
- Triangle shaped head recess
- Can be undone with the right size of slot bit if they're not too tight.
- A triangle shaped head (not a triangular recess)
- These can generally be undone with pliers if they're not too tight
- Waterpump pliers, which have a ribbed surface to the jaws, can grip them better
- Used on gas meter cabinets
- These have a one way only slotted head.
- These can't be unscrewed, even with the right screwdriver bit.
- Not one of the higher security types though
- Look like philips but with offset slots.
- Designed to cam out when sufficient torque is reached.
Shear head screws
- The driving heads shear off when tightened to torque, leaving only a smooth cone shaped head behind.
- Especially difficult to remove, they are used as anti-vandal screws.
- Removal requires drilling or cutting them out.
These types offer more security than standard non-security heads, but they don't give as much security as proper security screws, which have hardened heads to make drilling & cutting difficult.
Drilled head screws
Crosshead screws are sometimes used as basic security screws. After fitting, the head recess is drilled to make screwdriver use impossible.
If you only need semi-security screws once a century, saving a few rejects and hammering them in often works.
If screwing gets too tight, lubrication and/or a pilot hole is needed.
Recommended Lubricants include:
Handy non-ideal lubricants good enough for many purposes include:
- the oil from some types of pickles
- very greasy food
- waxy furniture polishes
- silicone furniture polishes
- handwash, shampoo, hair conditioner
Holding slotted head screws while driving is a recipe for minor injury. Alternatives include using pliers to hold it, or using a screw with another type of head recess. On soft surfaces such as plasterboard, its also possible to tap the screw in a bit first to increase the odds of it staying in line.
Occasionally a screw needs to be fixed to the screwdriver bit to make it possible to insert it. Blu-tack is an effective sticky, but even jam can work.
Pilot hole sizes
A good rule of thumb for softwood is to pick a pilot hole drill the size of the inner diameter of the screw shank, and a clearance hole that just clears the outer diameter.
No pilot hole
Small screws can usually be inserted with no pilot hole. When neat results and maximum strength are not important, and occurrence of very small localised splits is acceptable, medium screws may be inserted into softwoods without a pilot hole. Medium screws into bigger timbers such as joists are generally fine without pilot holes.
When going pilot-hole-less, the thinner the screw, the better it will behave, and the easier it will be to insert.
Wood is prone to splitting when screwed with no pilot hole. Blunting the point of the screw reduces the risk of this. Wood is made of hard fibres with a softer infill between them. A blunted screw will not penetrate the hard fibres, but slides off into the softer material, which will give sideways as its screwed in. A sharp pointed screw can stay on the hard fibres and penetrate there, levering a distance of hard fibre apart, thus cracking the wood.
Wood is more likely to split when screwed into near the edges. A pilot hole is necessary when screwing near edges.
Screwing into Knots
Knots may be screwed into perfectly satisfactorily as long as a larger than usual pilot hole is used. Knots contain mainly hard fibre and little soft infill, so will not deform around the screw as much as the rest of the wood. Quite shallow penetration of the knotwood by the thread will give high pullout resistance due to the knot's hardness.
Attempting to screw into a knot with a standard diameter pilot hole or no pilot hole will almost certainly cause a split.
If you're countersinking into a knot, making a recess hole for the head first is essential.
All this assumes the knot is firmly fixed in the wood, if not there's little point screwing into it.
A clearance hole is marginally bigger than the outer diameter of the screw shank.
Where slight wood movement is to be accommodated, using an oversize clearance hole and a thin screw, and not doing it up tight sometimes suffices to permit movement.
Countersunk screws sometimes make their own countersink recess in the workpiece. Whether they will depends on the softness of the workpiece, the screw head shape, whether the head has cutting ribs, and the force applied in screwing it home.
Ways to drill a countersunk hole:
Plasterboard screws have a bugle head shape to maximise their ability to self countersink with minimal force and material damage. These often self countersink when other screw types won't.
All coarse thread screws can be used in wood, including types not designed for wood use. The differences between them lie in optimisation rather than essential function.
Most modern woodscrews have parallel sides, single thread and are threaded full length. Most have pozidrive heads.
Traditional Wood Screws
- Traditional woodscrews have slotted heads, tapered shanks and a threadless top section. The tapered shank likes to split wood if used near the edge.
- Parallel sided
- Threaded full length
- Various thread patterns available, such as dual thread with one deep one shallow thread, or single steep thread
- See also confirmat screws for a stronger fixing
- Multipurpose low cost screws
- Shallow bugle head to avoid rucking up of board
- Thin size to ease pilotless fitting and reduce cordless tool energy consumption
- Coarse wood-type thread
- Threaded full depth
- Parallel sided
- BZP & Black phosphated anti-corrosion finishes common
- Sharp tip to pierce steel studs
- Philips head to cause cam-out and so help avoid overtightening
- A huge improvement over plasterboard nails
- Available in single thread and twin thread versions
- Available with built in drilling tips for thicker steel, these drill and screw in one operation
- Good for lots of uses other than plasterboard
- Drywall screws are often the cheapest type of woodscrew available from suppliers
- Drywall Screw Eulogy
Cupboard Hinge Screws
- Short extra-fat chipboard screws
- Used for screwing into the flat face of chipboard, where very little depth is available.
- Mainly used with recessed hinges used for chipboard (typical of kitchen units)
- Extra fat screws give more strength of attachment to sheet chipboard
- Flat screw tip enables maximum depth penetration into sheet chip, but pilot holes are necessary.
- Use where strength of attachment to sheet material must be maximised, eg attaching kitchen cabinet door hinges.
- Available in short lengths
Drawer runner Screws
- Short extra-fat chipboard screws
- As above, but with minimal low profile heads
- Used for fitting drawer runners to chipboard
- Also used to fit computer PSU fans
Wafer head screws
- For fixing light steel frame
- Can be used to fix roofing felt
- Good for general fixing of very thin Sheet Materials
- Please don't do this!
- Hammer in
- Supplied with plugs
- Can be unscrewed
- The whole fixing can be inserted after drilling one hole through the lot
The top part of the thread is designed to grip the item being fixed in place, so that backing out the screw a bit moves the item being fixed away from the wall etc.
These have a screw-on chromed cap. The mirror is screwed to the wall then the cap screwed on to make it look nice.
These are used by fast automatic screwdrivers which feed the screws in on a flexible plastic strip. Collated screws cost more but the tool greatly speeds up repetitive work.
These are double ended screws with no head, each end has a point and thread. Used to invisibly join items - at least one must be rotated to joint them though.
Large screws that used to be used in coachwork in the horse & buggy days. The one shown is 6" long.
These are used to fix lock cylinders in place. They have narrowed sections to enable them to be cut without causing a thread damage problem.
Despite the name, uk.d-i-y feedback on attempts to snap these screws is mixed. Sometimes snapping is successful, but often the screw just bends, and is ruined. Can be cut with angle grinder, bolt croppers, or more slowly with a hacksaw.
Pointed Self tappers
- Coarse thread screws designed to self tap into metal.
- Sharp pointed & flat tipped version both available
- The sharp points are very short, with a steeper taper than all wood screws
- Pointed ones work fine in wood (& wood products)
- Widely available in stainless steel
- Worth getting when the screws you want aren't available.
- For use in metal they normally require a pilot hole, though its not impossible to put them into thin metal without one.
- Flat tip instead of a sharp point.
- Pointless screws are self tappers, some are intended to screw into holes in metal, some only into plastics
- Much used in appliances
- Not much use for diy, but can work in a predrilled hole if you really can't find a more suitable screw.
- Top surface lies flush with wood.
- Countersinking usually needed, though not always
- Domed shape
- Sits pround
- more prone to catching clothing than other head shapes
- Partially recessed, partially raised
- UK electrical accessories mostly use raised head screws
- These are M3.5
- Old socket backboxes (1970s and before) may use a slightly different size
- Shallower than roundhead
The shape of a cheese. A lot more popular on machine screws than wood screws
- Shallow slimmed countersunk head used on plasterboard screws.
- Prevents damage to plasterboard
- Enables screws to countersink into softwoods with no need to predrill a countersink recess
- Strength is reduced, and occasionally this can be an issue, a head can snap off
- These have a wide collar under the head
- fit a large hole in the top layer of material
- this gives the timber or steel increased strength against lateral movement & resulting material damage
- Also called cylinder head
Most coarse thread DIY & construction screws have a single thread tapered to a point. Other features give various extra properties that are sometimes useful.
Self cutting tip
- Self cutting tips drill the wood as the screw goes in. Self cutting screws won't drill steel, but self cutting self tappers will create their own thread in steel if given a ready drilled hole.
This self cutting action means less need for pilot holes in timber.
- Drill tips enable the screw to drill and drive in one operation.
- Quicker than drilling and driving separately
- Useful for driving into steel
- These can drill their way into thick steel, self cutting tips can't.
- Plasterboard screws have sharp tips
- can penetrate thin 0.5mm steel, eg steel partition wall framing
- can penetrate fingers too!
- These have a bit more ability to cut a thread into the workpiece than plain screws. Useful for some tough materials. Not effective on hard materials like steel etc.
Lack of a point makes these screws unable to penetrate undrilled wood. These are intended as self tappers for metal or plastic. If really necessary they can be used in wood in a predrilled hole.
Where the possible hole length is small, a flat tip in a predrilled hole gives a bit more grip on the wood than a pointed tip. See ?? screws.
- Extra fast driving
- Reduces energy use per insertion in soft materials such as chip
- More screws per charge with cordless tools and soft materials
- Picture shows twin thread on left vs single thread
Steep thread pitch
- for chipboard
- they damage chipboard less on insertion, so the fixing's not as weak as it looks
- very fast insertion
- These have a section of non-spiralling thread under the head. They can be screwed in then backed out, and will raise the top wood sheet (etc) up. Used to level items on non-level backgrounds, adjust frame position and so on.
These have a short section of reversed head near the head end of the screw. This thread creates a tightish clearance hole from what was a pilot hole. This means you can just pilot hole both pieces, and the screw should still pull the 2 items together when inserted.
Ribs give a degree of cutting action, reducing the need for pilot hole drilling.
Ribs under the screw head drill a countersunk recess in the workpiece as the screw goes home. In practice this is only partially drilled, but is enough to usually omit the separate predrilling of the countersink.
If you're using a lot of screws with washers, screws with captive washers make the job quicker.
Its seldom worth cutting down woodscrews. After chopping woodscrews, either:
- file the tip to a point
- or just file down damaged thread and use a pilot hole
Fine threaded screws
When chopping fine thread screws (that take nuts), the thread gets mangled where its cut. There are 2 ways to deal with this.
1. Put 1-3 nuts on the screw before cutting. Unscrewing a nut usually makes the thread usable again, though it will be stiff. The first nut off gets ruined. Expect a failure rate. Using 3 nuts makes nuts to run smoothly on the shortened screw. Only the first nut off gets ruined.
2. Simply grind the thread down where its been cut and mangled.
Cylinder lock barrels come with snapoff screws intended to be cut to length. The screws are meant to be snapped, but often they're unhardened, and an attempt to snap ruins the screw by bending the threaded portion that will be used. These can be cut with angle grinder, bolt croppers, cutoff saw, die grinder or hacksaw.
Hammering Screws in
Screws as nails
- higher resistance to hammering than nails
- more physical energy use & higher shock forces on insertion
- higher pullout resistance than nails, lower than screwed screws.
- Less damaged screws when inserting than nails
- Removal effort is harder than nails, easier than screwed screws
- material cost as screws, labour cost as nails.
- not suitable for hammering into hardwood
There is a breed of site-animal which asserts that any screw can be driven by a hammer. This is, strictly speaking, true, but the unfortunate soul who attempts to remove a screw thus driven is rarely grateful to the former.
Screws as frame fixings
Instead of the traditional slow process using a wall plug in masonry, there is a quicker method. A hole is drilled through both wood and masonry in one go, a plug and screw inserted, and the screw hammered home. Where marking the front wood needs to be avoided the screw is hammered most of the way and screwed home.
The frame fixing method is quicker, but leaves an oversize hole in the front piece of timber, allowing movement to occur in some cases. This can be used for final alignment. Wallplugs without lips are required.
Removing a Damaged Screw
There are many ways to remove screws with damaged heads. Its hard to think of situations in which none of the following methods will work. Failure to remove normally comes down to lack of patience, equipment or knowledge.
Screws with heads with no grip left and jammed screws can be removed with the following methods:
- Wirecutters sometimes get a better grip than pliers, squeeze them hard to dig into the metal a bit before turning.
- Tapered screw removers - these screw into a drilled hole in a stuck screw. They are prone to expanding the screw, burying it into the workpiece, then breaking in situ, creating a second and bigger problem. Common but not recommended.
- Left handed drill bit - these either dig into the head and unscrew it, or if it can't ever get a grip it will drill the head off. These are a good choice for general problem screw removal. Also a good choice for clutch head screw removal.
- A hacksaw can often cut a slot into a head, enabling unscrewing.
- Grind or file 2 flats on opposite sides of the head. Undo with waterpump pliers or spanner.
- Weld a piece of metal to a screw head, with which it can be undone. The heat often helps free the screw.
- Weld a blob of metal on the head that can then be slotted or filed for grip, or simply undone with pliers.
- Dig under the head on 2 sides and apply pliers
- Hammer the head sideways a little in a few different directions (using a junk wood chisel) to loosen the screw.
- Hammer, nail remover or prybar can all be used like a nail puller, getting a grip under the head and pulling out. This does cause some wood damage, and is ill suited to any but small screws.
- Angle grinders can grind the heads away or cut them in two. Both involve some damage to the workpiece.
- Applying a soldering iron to the screw head can sometimes loosen it enough
- Sometimes a hammer blow straight down on the screw will loosen it enough to unscrew. Use a poundthrough screwdriver if you want it to last, ordinary screwdrivers are not designed for this.
- A Die Grinder with grinding disc can cut a new slot in the head.
- Use an impact driver (of the manual hammer operated type)
- Use an impact driver of the power tool type.
- Electrolytic rust removal
- Spark erosion (not cheap)
- Apply red hot poker to screw until wood starts to smoke a bit.
- Pushing really hard on a screwdriver bit in a brace & bit can work. Use your whole body weight where possible, long as it won't break something.
- Hammer a (hardened) philips screwdriver bit into an (unhardened) slotted screw head.
Paint Clogged Head
- Paint clogged heads can be cleaned with a knife in the case of slot heads, or a sharp pin for other types.
- A screwdriver heated with a blowtorch will melt through paint very quickly, enabling fast work with paint clogged heads. Its a good way to ruin screwdrivers though.
The strongest and cheapest screw metal, hence most screws are steel. Usually Bright Zinc Plated (BZP) for basic rust prevention. Many yellowy screws are coated steel.
Decorative and somewhat corrosion resistant. Brass is much softer than steel, and it is generally recommended to drive a similar steel screw first, then remove it and use the hole for the brass screw.
Brass coated steel. These look like brass, but rust readily.
Excellent corrosion resistance in most applications. There are different types of stainless with differing levels of corrosion resistance. Note that stainless needs oxygen to remain uncorroded, so these metals aren't good for all applications.
Common A2 stainless steel is rather softer than steel and screws are more prone to mangling of heads or shearing of their shanks, so care must be taken to drive squarely with a good bit and not to attempt to force a screw into too-stiff material or over-tighten. Resists corrosion by fresh water, but not saltwater.
A4 stainless resists corrosion by saltwater as well as fresh water. Good for marine use.
Plastic screws are used for their non-corrosion, non-conductivity and non-scratch soft faces. They are very much weaker than metal screws, thus are of limited use, and not often found in DIY.
Steel screws are coated to prevent corrosion, or for lubrication.
- BZP - bright zinc plated - Brightish almost chrome-like appearance, but not mirror finish. Most common finish
- Zinc plated - as above but matt finish
- Zinc plated & yellow passivated - a yellowy decorative finish for indoor use
- Galvanised = zinc coated
- Sherardised - another zinc coating process
- Phosphated - tough matt black rust resisting coating
- Wax - lubricant, clear.
- Brassed steel - a cheap alternative to brass screws, they rust readily.
- Net-coat - brown or green, for outdoor use (pictured)
- Green corrosion resistant - a tough anti-corrosion coating for exterior use, resists freshwater corrosion
- Blue ruspert - blue, used on masonry screws
- Oxide - black
- Japan black - a durable gloss black paint
There are other screw coatings too, that aren't generally used in DIY.
Wallplugs are often called rawlplugs (a brand name).
The above rule of thumb can of course be ignored. An effective ploy with long screws (which give good support in weak masonry) is to use 2 small wallplugs in one hole to give a plug of double the length. Small ones are often used because rated size plugs usually end up too long.
Ordinary interior fillers such as polyfilla etc are good enough in almost all cases. If high strength is required, adding a small amount of pva or cement to the filler toughens it up.
Hollow Wall Plugs
- For fixing to hollow plasterboard walls.
- Toggles have heavier pull-out rating than other types.
- However these don't behave so well with intermittent loads at 90° like coathooks, as load is applied and removed they tend to very slowly break up the plaster below them, eventually coming out entirely after a couple of years.
- For fixing to hollow plasterboard walls.
- Very popular
- Available in plastic & metal
- Quick to fit
- No drilled hole needed, just whack it in
- Cost more than traditional types
Screw caps cover the screw heads after insertion, providing a plastic finish to match the workpiece. Not a quality finish, but often an improvement over bare screw heads on finished furniture.
There are 2 types of caps
- caps that press on after screw insertion. These grip the pozi recess and have a habit of falling off.
- 2 part caps have a plastic skirt that goes under the screw thread. The cap then snaps onto this skirt. These are bulky and need prefitting, but don't fall off.
Cups are 3 dimensional washers. They are curved and look reasonably pretty, and are used with countersunk screws. Usually brass or brassed steel.
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Still to come
multitier hexagonal recess name hex drive hex slot head hex pozi head self piloting app table twinthreaded points