This question was asked at the website below. Very nice answer.
Can I wash waterproof clothing in pure soap flakes without proofing it again?
The name is Gear, Richard 'Clean Machine' Gear and I'm a demon for detergent, erm. Well, not really. In short, the answer to your question is yes, but... If you use pure soap - or a soap-based technical cleaner from the likes of Grangers or Nikwax - then it shouldn't compromise the factory water-repellant treatment on your waterproof clothing.
Normal detergents, both biological and non-biological and including Ecover and the like, contain agents which can mask the effectiveness of the original DWR. pure soap doesn't do this.
Unfortunately there are down sides - soap flakes don't clean as effectively as washing powders and liquids from the supermarket, so if your jacket is very dirty, soap alone may not shift all the grime, which can in turn affect performance. [Note: I'm not sure this is always the case. I've had reports of very dirt jackets being cleaned just fine using Soap Flakes. But try it and see. --Mike]
This is why eVent suggest that you use normal detergent followed by a soap cleaner and a double rinse to first get things clean and then to remove any residue from the detergent.
The other issue is that DWR degrades with use, say with friction from pack straps or rubbing as you walk. I would try washing with pure soap flakes - I've found the easiest way is to make a soap solution with hot water - then rinse and dry before checking to see if water beads off the surface of the garment.
If it does then that's all well and good. If it doesn't, try heat treating the garment using either a cool iron or a tumble drier at a low setting for about an hour. This may be enough to revive a tired proofing treatment. If that fails and water splashed onto the fabric doesn't bead and run off, you'll have to re-proof the garment either with a wash-in treatment for three-ply garments or a spray-on for two-ply ones.
You may then have to heat treat the garment if you use Grangers products, if you use Nikwax, that's not necessary.