This is a common question that I hear a LOT. A LOT. And it's still debated among siding professional to some degree. Mostly that debate centers around maintenance, and the expectations of what the homeowner will and won't do.
JamesHardie recommends that the siding be nailed at the TOP, or blind nailed as it's commonly referred to. You can't see the nails on the siding once it is installed in this method, and thus where the term "blind" nailing comes from.
Visible nail heads in HardiePlank siding is not the preferred method of installation (currently), but it was a very common practice a few years back in my local area. And you will (or should!) have visible nail heads at the bottom of every butt joint in the siding, as the joints will move otherwise and you will have busted caulk lines at each of those joints. We repair busted siding caulk joints all the time for home re-sales. (Butt joints are where the ends of two siding boards meet each other.)
Any visible nail heads, as long as they are covered with a thick coat of paint (or preferably 2 coats) should not be a problem. You have visible nail heads on your roof in many places I'd bet! And hopefully they are caulked/sealed with a strong roof sealant. And certainly your roof is going to get a lot more water on it than your siding will.
I actually prefer nailing the siding boards at the bottom, specifically because it holds the lap siding in much better than blind nailing (i.e. nailing at the top). The siding does not move at all with nails in the bottom of the board, as you have effectively nailed the board at the bottom AND top once the nails go in the bottom of the next board above.
However, this method can fail because of a few things:
- If the contractor left a gap in the butt joint, as caulk WILL fail. The butt joints should be tight in your siding. This is a more recent installation practice, and you may have 1/8" gaps between your siding joints. If so, keep them caulked!
- If there is no flashing behind the butt joints. I've actually yet to see this done in our area on any HardiePlank installation, even though it's recommended best practice. But maybe it's because we're only fixing the bad installations. Let's hope so.
- If the homeowner doesn't maintain paint over any exposed nail heads.
In today's fast paced world, not many folks want to have to maintain things. And thus, blind nailing/nailing at the top of the siding is the preferred method in that case, as it removes the maintenance issues from the homeowner.
So bottom line is this: if you're going to regularly paint your house (like you should!) then I think the bottom nailing offers many advantages in HardiePlank. If you're not going to maintain your paint, then you should have it blind nailed, so as few nail heads as possible are visible.
UPDATE 7/1/11: Thanks to Robert Butler for pointing out a wording error: labelling visible nail heads as exposed nail heads. His comment below is correct that exposed nails would be a fault, but visible nail heads that are covered with 2 coats of paint on your siding is not a fault. I confirmed this with the James Hardie regional warranty rep last year after repeatedly having to repair busted caulk joints on homes with blind nailed siding.