Looking at the load tables for various products, purlin hangers are often shorter than the actual size lumber they're supposed to hang:
The size difference seems more consistent at lower dimensions. For instance, 2×4 purlins are 3 7/16" high and 2×6 purlins are 5 3/8" high.
Why is this? Won't the joists end up higher than the header? Is something supposed to go on top of the header to offset this so the subfloor is level?
Edit
I'm using purlin hangers to hang floor joists inside my walls, which seems to be a suggested usage of these products.
Perhaps others can put this problem better than me. Check out the reviews and question on the USP JPF at Lowes.
Edit 2
Just to be sure we're all on the same page, this is a purlin hanger being used to hang floor joists. Only difference is I'm using solid sawn joists rather than i-joists.
Perhaps the foam tape supposed to make up the difference?
Best Answer
Besides safety and practicality mentioned by isherwood... part of the answer is money... the size of the hanger is related to the cost of the hanger. If you make or sell hangers in bulk, then every cent matters (customers like walmart and lowes will buy the hangers that are one cent cheaper per 100 boxes). If you only need to hold a certain weight, then there's little benefit to oversizing the hanger.
Another other part of the answer is that purlins are part of a (steel) roofing system. You cannot have the hanger sticking through the roof.
But If we are just talking about joist hangers in general, tops of the joists will be level with the header if you install them level. The hangers that wrap around the joist will need room below the joist (just the thickness of the hanger - not very much).
Where joists are the same size as the header, hangers that wrap under will be protruding at the bottom (if the top is level). Sometimes that doesn't matter, or if it does, then using hangers that don't wrap around the joist are necessary.
EDIT- After seeing the second edit to the question, I now understand the issue. I think you could notch the joist (1/8") to fit in the hanger (or use a different hanger).