RHS Limited
226-228 Holbrook Lane
First we have to define what we mean by a trapezoid or we will all be confused! This is not as simple as it sounds; I blame the Greeks (probably Proclus and Euclid) for not getting their act together. The result is that a Brit like me thinks a trapezoid is an irregular quadrilateral i.e. a four-sided figure with no parallel sides, where as my American cousin The Elevator Doctor thinks it has two parallel sides.
As my middle name is John Bull I am sticking with the English version in this blog, so there! So we have this odd shaped four-sided lump of wood, but what has it got to do with a spiral staircase. Well a typical landing to any spiral staircase, and particularly a kit spiral stair, has a triangular shaped top tread that acts as both the last step and a landing area at the top of the spiral stair. It also has the function of securing the centre pole of the spiral staircase to the top of the structure, so it is an important element in spiral stair construction.
The triangular landing tread usually has 60 degree angles so in practice this makes it about twice the size of all the other treads. This in turn helps to make the whole spiral stair structure more stable because it allows for more than one fixing point to the structure at the top of the spiral stair. It also makes the landing function of the top tread big enough to work as landing. The down side of a triangular landing is that if you have a square structural aperture in the upper floor, the spiral stair landing can only be fixed in the centre of any of the four sides, so there are only four exit points 3,6,9 & 12 o’clock. You have to take my word for this or draw it a scrap of paper and you will see that you have to change the triangular shape if you want to step off anywhere else, like say at one of the corners.
A point worth noting is that if you make circular aperture this does not apply you can finish the landing anywhere with a triangular landing as long as you scribe and cut the outer edge of the triangle to suit the shape of the circle. Therefore you will end up with a segment of a circle landing instead of a triangle, but lets not go any further down that alley, it is really not what this blog is about, and circular apertures are more difficult to construct than square ones. So the solution if you want step off your spiral staircase in the corner rather than the middle is to use an odd shaped landing, which we Brits and most Europeans call a trapezoidal landing and our colonial cousins call a trapezium.
If you are concentrating you will by now be thinking why does it have to be an odd shape, why can it not be just a square that fills the whole corner of the aperture? Well in some circumstance you are right you can use a square landing but in most case it is fraught with danger. The problem with a square landing is that it projects more than any other landing shape over the lower treads, and this can create serious head clearance height problems as you ascend or descend the spiral.
The solution to this is to cut the corner off the square to create more headroom, and what you are left with is a trapezoid shaped landing. So using a trapezoidal landing gives you 8 (2 at each corner) more options for stepping off positions at the top of the spiral stair. You will also have more variations for stepping on points at the bottom of the spiral stair. So by choosing between a triangular and trapezoidal spiral stair landings you will have a dozen variations, one of which hopefully will suit your project.
Bear in mind that when a spiral stair kit manufacturer produces a range, they have to consider how many variations in parts that it is economic for them to produce and stock. The result of this calculation is that usually the cheaper entry-level spiral staircases such as the Arke Klan are only offered with the more common triangular landing. Where as the more up market spiral stair ranges such as the Albini & Fontanot Genius range offer both triangular and trapezoidal landings.
Another word of warning, some manufacturers have scratched their heads and attempted to design a universal landing that is all singing and dancing for both triangular and trapezoidal landing situations. In my opinion no one has yet managed to cheat the geometry and make one work without compromising the width of the exit point. Personally I stick to the ‘either or’ systems I know that they do what it says on the tin.
The Staircase Doctor
Oh dear I wish it was the first time that I have been asked that question! In these modern times everyone is trying to create more space within the confines of the box in which they live.
It is no longer acceptable to have your clothes in a wardrobe in the corner of the bedroom, one just has to have a separate walk in dressing room. I am being cynical, there is nothing wrong with making use of the unused space in our castle, after all we have mortgaged ourselves up to the hilt for it, so why not use it all I say.
The problem starts when you cast you eye around looking for unused space on the ground and first floor of your palace, you then realise there isn’t any. Eventually your eye falls on that total waste of space the hall, stairs and landing. All we have to do is plonk a spiral staircase in there, save loads of space, and put the snooker table in it.
To understand why life is never that simple, we have to first consider the past. Most of the housing stock in the UK is less than 150 years old, during the whole of that period designers of houses were striving to make the best use of space within the properties that they were designing, it is not a new idea to get the best use out of limited space.
The point I am making is that you are almost certainly not the first to try to do it in your property. The chances are that the first guy knew what he/she was talking about and did as good a job as is possible. OK in older designs there may be too many internal walls creating boxy little rooms that were the fashion then. Removing a few of these will be no problem, but the staircase is a different matter all together.
Other than in ’space no object’ grand designs, stairs were never a selling point and so designers always did their best not to waste space with them. The geometry of a safe building regulation compliant staircase is such that you cannot make it smaller in size (that is also true of changing a traditional staircase to a spiral staircase). You can change the shape and reposition it within the building, but you will not gain any extra space as such.
If you are thinking of repositioning the staircase to make better use of your space then it probably will only become viable if you are ripping out all of the internal walls on both floors to totally change the room layout. Stairs have to pass through a hole in the upper floor, and the shape of that hole is very specific to the shape of the stair.
A change of shape or position means serious alteration to upper floor structure. If you are prepared for that sort of upheaval then the world is you oyster, modern open plan flight stairs or spiral staircases are works of art, they look superb in open plan living areas that are in vogue right now.
You could end up with your dream home in that former boring sixties semi, it will cost you, and you will not have gained an inch of space, but it will feel like it has.
The stair doctor.
The answer is that you can but it almost certainly will not work in your property. The problem is all a question of the size of your feet! In order to be useable the centre of each tread of a spiral stair needs to be big enough front-to-back to safely take your size twelve's.
Although I do not know how they did it, the nice people at the British Standards Institute have clearly defined how much tread you need at various points on the width of the tread in order for the spiral staircase to be safe to use. Once you accept that fact, it will determine how many degrees of rotation each tread will require in order to provide this safe footfall dimension.
This is not a fixed number of degrees it will vary depending on the diameter of the spiral staircase. The bigger the diameter of the spiral stair the smaller the number of necessary degrees of rotation will be needed to provide the magic minimum dimension (think about it and you will understand why).
"So how does this all affect my need for a half a circle spiral" Well a typical height domestic spiral stair will need 13 treads to safely reach the upper floor, any less and the step between each tread will be too much, not to mention being over the Building Regulation maximum tread rise of 220mm.
Each of these spiral stair treads will need to rotate between approx 30 degrees (1200mm diameter spiral stair) and approx 27 degrees (1600mm diameter spiral stair). Multiply by 13 and you will see that for a typical spiral staircase your stepping on and off points will be about a full circle apart.
Were you to attempt to cram 13 treads into half a circle rotation it would be possible but you would not have enough tread to stand on. Obviously things change if you have a very low or very high floor-to-floor height that requires more or less treads, and as already said larger diameter spirals will need less degrees of rotation. However as a rough rule of thumb you can think of a spiral stair rotation as the hands of a clock each treads rotates 5 minutes giving 12 treads to a complete circle (bigger spiral stairs go up to 16 treads or more).
There are other things that a spiral staircase manufacturer has to consider when considering the geometry of a spiral stair, but the main purpose of this blog is to dispel the idea that you can make a spiral stair do anything you wish as regards the stepping on and off points. Unfortunately you cannot cheat on the geometry just because you want to.
The Staircase Doctor.