As a deck builder and timber stair builder in Durban I have had to learn how to build timber stairs the hard way, through trial and error (and a bit of Googling). Timber stairs have always been a tricky one for me and I have battled to build them in the past. At first I Google’d ‘building timber’ stairs and found a few videos on the topic which explained it quite nicely. Most of the videos online though are American and they therefore explain it in inches. But after converting it the technique and calculation used to work out your runs and risers becomes quite clear.
The riser is the vertical piece of the step and the run (or tread) is the horizontal piece. Building regulations in South Africa state that the riser can be no more than 200mm high and the tread or run can be no shorter than 250mm. Timber, either in pine or balau, will come standard at either 140mm wide or 220 wide. One should therefore use two pieces of 140 wide stock in order to get a tread of 280mm wide. I have seen stairs built using 220 wide stock which although is not completely according to building regulations can work if a gap is left between the front of the top tread and the back of the tread below that. The gap should be at least 30mm to get back to your minimum of a 250mm wide tread, although building regulations state that this gap should be no bigger than 20mm. Because the stairs are open, one can normally get away with this as your foot will be able to move past the back-end of the tread as you climb the stairs. Be sure to check with building regulations before designing your stairs to ensure you are compliant or that your contractor is adhering to these regulations.
First you need to calculate the exact height of your riser and the exact width of your tread. I will use 1,1m as the height of the stairs for the example below.
Assuming a riser of about 200mm or less divide 1, 100mm by 200mm. You get 5.5 risers. Increase this to 6 risers because if you take it back to 5 risers you riser will be more than 200mm.
So 6 risers and therefore 5 treads. The top tread of course being the deck itself.
1, 100mm divided by 6 risers give you an exact riser height of 183.3mm. 5 treads of 280mm each will result in a total floor length of 1, 4m. From this you can work out the length of the stringer. a²+ b² = c². Remember him? Yeah!!! The stringer by the way is the piece that runs down diagonally from the deck to the floor and to which the treads are attached.
Now lay the stringer flat on the ground, starting your markings a comfortable distance from the end to allow for cutting to the correct angle and possibly notching the end to accommodate any lip that may exist on the deck. Deckboards can either be set flush with the joist or they can hang slightly over which will dictate whether or not the stringer needs to be notched so that it can be fastened to the joist or beam.
With the stringer flat on the ground, take a large square with markings on both sides (the big black square) and slide it along the one side of the stringer so that you reach the 183.3mm mark on one side and the 280mm mark on the other side. Of course you would round this to the nearest millimetre. One can buy little clamps that clamp to the sides of the square to make this job easier. In fact sophisticated stair stringer squares can be bought but the big black square works just as well. With your riser height and tread width marked on the stringer you can now draw a line around the square at the corner. This is where your riser and tread will be. Move the square down to the end of the marks and repeat to mark the next one. Continue until the end of the stringer. Cut the top end of your stringer off parallel to the riser at the correct distance to attach to the joist. The bottom of the stringer will be cut, either parallel to the riser or tread depending on whether you want a vertical or horizontal end which will either attach to the concrete ground (horizontal) or be set into the soil using a post and concrete (horizontal or vertical).
If you have as much difficulty understanding this as I did, then you are not alone. Try it and you will find it becomes second nature.
Now it’s just a matter of attaching your treads and job done. If you have a balustrade running down the sides of your stairs this can be attached now. Care should always be taken where the balustrade capping and cross supports, on the stairs, meet the same on the deck as they are running at different heights. It is better to take the balustrade up, level out whilst still on the stairs and then meet the horizontal.