Concrete work for RC stairs

• Concrete specification should be very strong , otherwise concrete will flow over the formwork riser boards . 
• Thickness of concrete required is dependent on the loading and span , but is not generally less than 100mm or more than 150mm measured across the waist , which is the distance from the soffit to the intersection of tread and riser . 
• To achieve a greater tread length without increasing the actual horizontal going , it is common to use a splayed riser face giving a 25mm increase to the tread length . 
• Concrete cover to reinforcement is at least 15mm or the bar diameter , whichever is the greater . 
• This cover is sufficient for one-hour fire resistance in most buildings , but will need to be increased for buildings in certain purpose groups .