Broken Stair Tread

[manbearpig]

I've got a bit of a problem with a cracked stair tread.

Photos speak a thousand words, so here they are!

First tread on the right of this photo is the cracked one.

Ive opened up the area under the stairs, and I cant access the tread, as you can see.

Are the two thick bits of wood under the tread structural? What would be the best course of action?

Thanks!

 

[dextrous]

Easiest thing to do would be a few bits of timber at right angled to the crack, glued and screwed through from the top of the thread

 

[gregers]

as dextrous has said,or cut a piece of ply to size required(underneath)copious ammounts of pva,couple on small pins to hold it in place and then screw it from above.

 

[big-all]

normaly you would expect the tread to be wedged into the rebate on the string
on your top picture the winder[triangular step] seems to sag at the side
on the bottom picture there dosnt appear to be anything holding it

if the edge isnt properly supported would personaly push the tread up from underneath untill level with the rebate measure 14mm down from the bottom edge off the rebate screw and glue a batton along this line into the string[side off the stairs]
then using a tread shaped 12mm ply triangle slid onto the batton and pu[foaming glue ] or normal wood glue if you already have it screw to the bottom with4.5x 30mm screws or4.5x 25mm screws if you countersink or no8 by 1 1/4" or 1" iff you countersink

 

[masona]

Also you will need to put in wood blocks with glue underneath, very important!