Over the years, you may find that some of your stairs work loose from their fittings. Or they might dry out and shrink slightly, causing creaks and squeaks when you climb them. Here are some of the steps you can take to repair them.
The way you fix creaking stairs really depends on whether you're able to repair them from below or above. It's worth checking if you need to take some plaster off before getting at them from underneath. If you do, it might well be quicker and easier to do the job from above.
Start by identifying any loose steps. Get someone to walk up the stairs while you listen from underneath, and then mark any you find with some chalk. If the tread or riser has become loose in its string holding, the glued wedge, that secures the joint, might have worked its way loose. If this is the case, simply remove the wedge, give it a clean, squeeze on some PVA wood adhesive and then replace it.
Next, check the triangular blocks that fit in the angle between the treads and the risers. If they're not glued on securely, remove them and scrape off the old adhesive. Then prise open the tread/riser joint slightly and inject some glue.
After this, apply adhesive to the block and press or screw it in place. If you discover that no blocks were fitted to the stairs, make some (each should be about 75mm long), glue and screw them in place.
If you need to strengthen the joint between the back of a tread and the bottom of a riser, squeeze some PVA glue into the gap. Then drive three evenly-spaced screws up through the tread and into the edge of the riser.
If you need to remove some plaster before trying to repair your stairs from underneath, it's better to fix them from above.
One way you can fix creaking stairs from above is by screwing the front of the tread onto the riser. You can do this by drilling clearance holes for the thick part of the screws (called the shank) through the tread and thinner pilot holes (for the point of the screws) into the top of the riser.
You can then drill the clearance holes with a countersink bit, so the screws will be just below the surface when you screw them into place. Next, you'll need to squeeze some PVA wood adhesive into the holes and put the screws in tightly. If you don't have carpet, you'll need to cover the screw heads with wood filler.
If the loose joint is at the back of the tread, you can make it stronger by gluing a 12mm x 12mm length of triangular moulding into the angle between the tread and the riser. When you're doing this, remember that the tread can't be narrower than the regulation 220mm.
Another way you could do this, is by screwing two thin metal brackets into the angle between the tread and the riser. To get a flush surface, cut out a space for the brackets using a sharp chisel and mallet.