# Why are top-mount joist and purlin hangers shorter than actual lumber dimensions?

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, 2x4 purlins are 3 7/16" high and 2x6 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?

• Go measure an actual 2x4. Jun 1, 2016 at 16:01
• @ChrisCudmore I'm familiar with the actual/nominal size distinction if that's what you mean. A 2x4 is 3 1/2" wide but the perlin hanger is 1/16" shorter. Jun 1, 2016 at 16:10
• I think the point is that you want the joists at the same height as the header, and if the purlin were the same size as the (nominal) wood dimension, the purlin could interfere with the underlayment. You place the purlin so the top of the joist is exactly at the level of the headers. Jun 1, 2016 at 16:13
• "you want the joists at the same height as the header" -- correct. I'm not sure what underlayment has to do with this since there's still a subfloor going on top of the joists and header. "You place the purlin so the top of the joist is exactly at the level of the headers." I don't see how this is possible. If the hanger is shorter than the lumber, the lumber will be higher than the header. Jun 1, 2016 at 16:19
• I didn't realize that we were talking specifically about top-mount hangers. My mistake. Answer updated. Jun 1, 2016 at 18:12

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).

• "The hangers that wrap around the joist will need room below the joist (just the thickness of the hanger - not very much)." So shouldn't one expect the hangers to be longer than the joist height? "Where joists are the same size as the header..." I'm not sure how the header size is relevant. Jun 1, 2016 at 16:27
• @RyneEverett I don't know what you are wanting or not understanding. The hanger doesn't need to be longer; it needs to be the same size or shorter. If the hanger wraps around the bottom of the joist it might stick out below. Sometimes joists are smaller than the header. If the joist is smaller, then there's space below the joist... the hanger won't be below the bottom of the header... unless the joist is the same size, then the hanger will be sticking out, below. Perhaps this is not relevant to your project... but if it is, then use an L-angle hanger that doesn't wrap under the joist. Jun 1, 2016 at 17:19
• We both missed that the OP was asking about top-mount hangers in particular. Jun 1, 2016 at 18:15
• Good idea on notching the end. Much easier than planing the whole thing. Jun 1, 2016 at 19:15

Answer revised after realizing that the OP was asking specifically about top-mounted hangers.

In the case of perlins, it's not usually a problem that they align slightly above the beam. Any subfloor or roofing should span that small gap without a problem.

Why are the hangers not exactly 3-1/2" high inside? To give a slight margin for error with respect to lumber dimensions and other oddities. In the world of high-production framing, you don't want to have to achieve perfectly flush joints in cases like this. It's better to have a little fudge room built in that doesn't affect the final product.

Also, if the dimensions were exact, the hanger would create a slight bulge in the roof or floor, not just from the hanger but from the nail heads. By keeping the perlins or joists slightly high, this problem doesn't occur.

• Thanks for taking the time to understand my question. Although the products I linked are simply called "purlin hangers" the top-mount aspect must not have been getting through to anyone. Jun 1, 2016 at 18:24
• I am surprised by your answer though -- that the weight of the walls and roof above can be supported by the subfloor span. Jun 1, 2016 at 18:25
• Obviously in cases where critical loads occur this wouldn't be valid. Jun 1, 2016 at 18:38
• I suppose that could explain why the discrepancy only holds consistently for the low-dimension hangers which only hold joists with relatively low load allowance anyway. Jun 1, 2016 at 19:19