Hey guys,
So we're in the home stretch of doing our hardwood flooring reno.
If you read my other post, you'll know we failed to think this through fully, and are working outside in as we lay the hardwood on our landing.

The nosing for the landing/stairs have been installed.
the hardwood floor is being run perpendicular to the hardwood floor already laid in the bedroom opposite the stairs.

In one section (about 3 feet wide) Where the floor meets the nosing, the floor dips some (about 4mm at the deepest)... this is far too noticeable and will lead to damage of the floor I'm sure....

It's too late to re-lay the subfloor...what can I use shim up the hardwood floorboards? What can be used as shims and how best to lay them?

I should mention:

It was recommended to NOT use PL at one point for laying the nosing, as it dries hard and thick - could I use a bit of PL to essentially boost up the floor where it meets until level?

Also, when I asked about roofing shingles at homedepot for this purpose they looked at me like I had 4 heads, and wouldn't what asphalt inside my house. Pro/con with these?

4mm? That's less than 1/16 inch. What are you using for a vapor retarder.. Aquabar B, roofing felt? Add some additional pieces to build up the dip. Lay them down ala a topical relief map. I would take the advice given by most HD employees with a grain of salt. Roofing felt and asphalt shingles are commonly used for this purpose. Even Aquabar B has asphalt sandwiched between layers of kraft paper. Asphalt inside the house doesn't sound good by has been used forever. Aquabar B literature says there may be an odor.

Asphalt products used for roofing make accurate shims. They will not compress like cardboard can. They come in different thicknesses, roll roofing is about 1/8th inch, 40 pound roofing felt 1/16th, 30 pound a little less and tar paper even thinner. I use latex sub-floor adhesive to glue the asphalt down, cause it sticks pretty well. You can also use a stapler to hold the shims in place after they are cut to shape filling the low area.
Once covered with flooring you will never know that there is any asphalt in the house. In the old days asphalt was used to glue both hardwood flooring down and under asphalt 9X9 inch tiles. Maybe the older homes had so many leaks that no one noticed. Newer homes are sealed up more, but when covered by hardwood asphalt should not be noticed.
Using a thicker layer of glue is folly if you have a low spot.