If you are fixing something to a wall the first thing you must do is to ascertain how heavy it is and what is the wall made of, eg brick, plasterboard, thermal block etc. Experience does go a long way to help you decide what size / type of screw and plastic rawlplug you should need and the more DIY you do the better your judgement will be.
The following examples of screw sizes should only be used as a general guide.
Fixing brackets for kitchen cupboards to a plastered brick / block wall – use 2 1/2 ” x 10s countersunk cross head screws with Brown plastic rawlplugs and penny / mudguard washers (these steel washers are 1″ in diameter with a 1/4″ hole). If the wall is dot and dabbed, as a lot of new build properties are, (plasterboard stuck to the wall) then you may need screws 1″ longer to allow for the void created by the plasterboard.
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Fixing hinges to a softwood frame for an internal door use 1 1/4″ x 8s countersunk cross head screws in steel or brass depending on the hinge material. If you are fixing to a hardwood frame drill a 1/16″ pilot hole to the depth of the screw first.
Fixing a 3 foot long shelf to a brick wall, if the load on the shelf will be light, eg ornaments, use 1 1/2″ x 8s countersunk / roundhead (depends on the bracket) screws with red plastic rawlplugs. If the shelf is in a garage use 1 1/2″ x 10s screws with red rawlplugs as the load will no doubt end up heavy.
Hanging a 12″ x 8″ picture frame on a brick wall use a 1″ x 6s roundhead screw with a yellow plastic rawlplug.
Whenever you use a screw fixing you should always ‘feel’ the screw tighten up as you screw it in, if it feels loose, remove it and increase the screw gauge say from 6 to 8 using the same rawlplug, if this fails to improve the fixing you will have to increase both screw size and rawlplug.
Choose the appropriate length of screw for the job, if you are fixing a piece of timber to a wall there should be at least 2/3 of the screw in the wall.
Does the screw need to have an anti-corrosive coating for exterior work.
Will the screw be used as a fixing into hardwood, if so drill a pilot hole so the wood doesn’t split and the screw head doesn’t shear off (more common with brass screws).
A popular screw with both DIYers and professionals is the Phillips or cross head screw. They are ideal for use with battery drill / drivers and if the screw is being driven in by hand there is less chance of the screwdriver slipping and damaging the surface of whatever you are fixing in place.
Don’t be a cheapskate and buy the cheapest fixings available, good fixings cost a little more but will save you a lot of time and frustration. Most of the big DIY stores such as B & Q, Wickes, Focus etc, have vast ranges of good quality fixings.