How long should Nails be?

How long should nails be?

Hired a roofing company last week. There almost done with job. There going to nail down some 1 inch poly-iso roofing insulation. To adjust the pitch a little on the low slope roof. But the nails they will use are 3" why so big. Do the nails really have to be 3" long. The deck is only 3/4 “ thick the insulation is only 1" and the cap sheet with the base is less than 1/8" . Which is torched down. That is a total of 1 7/8 inch.

That leaves 1 1/8 inch nail penetration through the deck. Is this right? Should nails penetrate the deck at all on a low slope roof.

Thanks. I just need to know before they start nailing this stuff down. If It should be different I will let the roofers know and have them make the changes if needed.

Thanks everyone for the help. Roofing…com is the best. I saved a little over $ 3000.00 on a tear off. Did it my self and had a roll off dump to help with the waist.

will the under side be exposed?

The answer you seek has already been posted. Look for Bread’s post on nail length.


no, the nails will not be exposed.

Tar monkey. that is one gross picture. you have been surfing the internet way to long. haha…

beard’s post: it looks like 1/4 inch penetration through the deck, but then again it was for shingels
maybe I have to dig further into his posts.


yeah ide prefer to see somthin about 2" in your caes , but 3 should work.
trust your roofer.



Fastners need to extend through the deck a minumum of 3/4".

The length is fine.

The thing I would question is why they are nailing and not screwing the insulation down.

Thanks Lefty— you beat me to it.
And may I add: What kind of nails are they besides 3" long? (As in do they come w/ a really large fender washer?) :mrgreen:

Thanks lefty, good info. They did a few rows today, but not the entire roof. Rain today. The nails they are using are standard 3 inch roofing nails and have a 3" tin fender washer on them as Twill59 was pointing out. I did ask one of the crew members why nails and not screws also.
he the roof crew member said that “when the roof needs replacement it be will easy to remove”. my point is. the wind can do the same for me a lot sooner. he told me the boss will call me. Anyway the 3" nails are coming through the deck at 9/16 of an inch. that’s what I measured on about 10 of them in random places.

I assume screws are better than nails any day of the week for 4x8 insulation sheets.

I don’t know what the point is at being easy to remove using nails vs screws. maybe when the roof needs a replacement the screws will tear up the deck. when being pulled.? I assume maybe that’s what he means.

Im am concerned about this, that is why I was wondering how far the nails should go though the deck. Because of the 4x8 sheets of insulation. I don’t want the cap pulling up from wind getting under it if a parapet corner or edge comes up and taking the roof with it.

IMO you should have the insulation and the decking screwed on. We get Chinook winds that come off the mountains at 100+ mph. On one job that I worked on the wind removed 4x8 sheets of plywood with shakes completely off the roof. the sheeting and ISO board was hand nailed with 16 penny nails. If the decking had been screwed on I feel that the roof would not have lost any material.
I hope that this helps in your decision.


“I don’t know what the point is at being easy to remove using nails vs screws. maybe when the roof needs a replacement the screws will tear up the deck. when being pulled.? I assume maybe that’s what he means.”

So when you get new tires they should only use 2 of your lugnuts? That’s the dumbest excuse I’ve ever heard. We don’t pull screws, we take them out same way they go in, with a drill/screwgun.

Hi stevenje98,

That is why the insulation is screwed down. The nails will pull out with ease. Have them screw it down.

dont have them rip it up ,have them go over it and put screws in

I talked to the main boss of the roofing company this morning and mentioned the 3" nails and maybe going with the screws other than the nails. he told me. " Why is my crew putting in 3 nails? , I bid your job with 3" screws with caps." he came down to the job site this morning with 4 very large boxes of screws and caps. He thinks their was a mix up on the job order . Typo maybe.

Thanks everyone for pointing this out. otherwise I would have paid for screws and received nails. Im glad you pros caught this otherwise I would have thought everything was ok with the nails.

Once again the professionals have come to the rescue.

[size=75]sounds to me like he got caught cutting a corner and blamed it on the crew[/size]…hmm maybe not :?