Housing treads and risers in closed string .

• Closed string is a string which encloses the treads and risers it supports . 
• Made wide enough to enclose the treads and risers 
• Top edge projects above the line of nosings to form a 50mm margin . 
• Grooves are cut 12mm deep into strings and tapering slightly in width to accommodate treads , risers and wedges . 
• Ends of treads and risers are glued and wedged into shallow grooves cut in closed string . 
• Wedges are driven into shallow grooves below the treads and risers .