[FiB]

Got a hairline  crack in the top corner of one of our loft water tanks.  Not sure which plastic the tank is made of but its black and lidded, so looks loik a proper potable water tank.  I've tried an external repair with milliput (which has served me well for plumbing repairs in the past), to no avail, so now have to consider internal repar but not sure which materials are safe for use with potable water.  Help!!! Please 
Replacing the tank has to be a last last resort as we'de have to make a hole in the roof and do it that way!

[robert waddell]

plastic welding rods     and before you all fall of your seats and injure your brain       there are specialist company's that do this you can also do this yourself                            a plastic rod fitted in a high speed drill and switch on the friction creates the heat that melts both rod and plastic tank and you have a plastic weld       you used to get toys that used this method of construction

[digit]

You could try Q-Bond sticks and repairs most things

[chrismahon]

The tanks are polypropylene and can't be mended. There are many reasons for failure. Not cutting the holes correctly and leaving a crack propagation point, poor support of the tank on the base (twisted), age cracking, using even the tinyest amount of recycled plastic in the mix (brittle), impact damage when fitting.
 
With a crack in the top corner the cheapest solution is to lower the water level in the tank by refitting the Ball Valve lower down and moving the overflow down with it. Obviously you need to drain the tank and check the stability of the supporting base. If you can, drill a small hole below the crack in good material to stop it growing -stress is distributed around the hole.
 
If the crack continues to grow and leak and you need a tank with the same capacity as the original you can fit two or more smaller tanks connected in SERIES. The flow from the valve inlet must pass through all the tanks and exit from the last one. Presumably you have a gravity mixer shower or an unreliable/ poor mains supply.
 
Hope that helps FiB.