Purlins for a metal roof

GW · July 06, 2008, 01:06:33 PM

Hi, I'm at the roofing stage on a small studio cabin I'm building. I had planned to sheath the roof with 7/16 osb, then 30# roofing paper then install metal roof. During one of the building inspections I was talking to the inspector and he advised against the osb sheathing with a metal roof, he suggested I use 2x4 purlins which would hold the screws a lot better. I already have the osb but its cheap so I can probably use it elsewhere. My question is what spacing do I install the purlins? The inspector suggested that I place the screws at 24" oc, following the manufacturers instructions on screw placement. Would purlins at 24" oc be strong enough? I'm planning on using 2x4's because they are cheaper than 1x4's for some reason.

Redoverfarm · July 06, 2008, 01:46:55 PM

GW I used 5/8" sheeting on the roof of the cabin(10/12) and 1X4 (4/12) purlins on the porch. A friend helped me which is a metal roof installer. The main reason I used sheeting is that mine is in the woods and wanted to prevent a place for bats, flying squirrels and bees to nest. He said there was not a problem with doing it that way. The screw placement was every 36" horzontal with the exception of the bottom and top which were placed midway center of the flats. He has put on several hundred and never had any problems. The main problem is the drip edge end and wind. The more holes that are put in the roof the more chances for leaks. He stated that when home owners put it on they put way too many screws. The metal will expand and contract with the weather and too many will not allow this to occur and end up with a bad looking roof.

Just another piece of advice. Pre-drill the screw holes. Using a drywall square mark the horizontal position of the screws which correspond to the location you want them to appear on the roof(from the bottom edge up) either on the sheeting or midways of the purlin. You can actually drill 2-3 sheets at a time. When it is all done the whole horizontal row of screws are lined up. They advertise the screws are self tapping but you will end up with a lot less scratching trying to get them started. Then used a cordless drill with a torque adjustment to put them in. Only draw them down until the washer is tight. Don't draw them down to where the washer buldges past the outside of the screw or they will split and dry rot and leak.

If you have trees you might also check into the low profile screws rather than the hexhead. The low profile are less noticable and there is a Torx # 20 fitting in the screw head. This allows leaves, pine needles and the like to slide over them and off the roof rather than hang on the screw heads. They are color matched to the roofing and are only like $2.50 more per bag than the others.

Good Luck.

John

July 06, 2008, 09:46:40 PM

I have heard from multiple sources, including this forum http://countryplans.com/smf/index.php?topic=3940.0 of problems with interior condensation from metal roofs with no vapor barrier. If your using purlins or sheeting, it looks like you will want a vapor barrier under the metal. It seems like putting a vapor barrier over purlins could be a challenge. Especially if its windy.

Redoverfarm · July 06, 2008, 10:36:02 PM

GW I did have "titanium flet" on the sheeting. I forgot to mention that. But like I previouslt stated I wanted to isolate the insulation area which would be prone to bats, bees and flying squirrels. With just purlins you would leave that area open between the roof metal and the insulation. I believe they make a insect shield fiberglass inter-tangled product similar to ridge guard but the cost is pretty high. It wouldn't be a problem putting a barrier on the purlins. Just cut the material in the width of the purlin, start on one end and attach as you unroll to the other end.

glenn kangiser · July 06, 2008, 10:42:47 PM

Looks like the vapor barrier is needed - remember Christina's problem - we decided felt was a minimum I think.

I used 2x4's 2 foot centers ( I was using scrap corrugated and home sawn lumber on the garage) although the sheeting on steel buildings is rated for 4 feet in most apps without heavy snow loads.

July 06, 2008, 10:44:04 PM

I have 3 rolls of 30# roofing tarpaper, would this not work? I thought that I could put it over the purlins but under the metal.

Redoverfarm · July 06, 2008, 11:00:04 PM

Unless you have an extremely large building that would be more than enough to just do the purlins. Take one roll and figure out the width of the purlins or something a little larger what ever will go into 36" . Use an old handsaw and cut throught the whole roll while it is still rolled up. Then unroll one at a time on your purlins and attach as you go. I forget how much is on a roll as i have switched to "Ice Guard" for the most of my work. There is 66-2/3 feet on a roll of it. Maybe 100' on a roll of #30 not sure.

glenn kangiser · July 06, 2008, 11:41:51 PM

Yeah - I used 30 lb on my roof - there is also #30 - the new replacement which iss much thinner but may be all you find.

You may want to adjust your purlin spacing to support the edges of the felt better -

JRR · July 07, 2008, 10:31:12 AM

Depending on the style of metal roofing that you are using, you may find it easier to pre-attach the barrier to the underside of the metal. I would expect one of the PL brand caulking-gun roofing sealant/adhesives to work pretty good for attaching felt to metal.

Thus far, I have only used the corrugated style, and have always depended on the air space underneath without vapor barrier. And we do have a lot of wasps move in.

July 17, 2008, 09:49:16 AM

I built a 3 bay garage and put an apartment in one end to live in while building my house. I used metal on both and have had no problems at all. On the garage I used 1 X 4 purlins spaced 3' on centers bottom to top. I then had the bottom sprayed with Icyene foam and that's it. Of course I can see the foam from the garage floor and have never seen any condensation, leaks or insects of any kind. The garage is now 5 years old.
On the house, I went ahead and used 4X8 sheets of OSB and covered it with 30# felt before screwing on the metal. Again, underneath I sprayed in Icynene with no vents and have no penetrations anywhere on the roof.
As to pre-drilling the holes for the metal screws, it seems a total waste of time and energy to me. I had no problem with the garage or house with the self tapping metal screws with rubber washers. I did miss an occasional purlin on the garage and left the screw in place even thought it was loose. After installing the Icyene, the screw was tight in the hole and none have ever leaked.
That's my story and I'm sticking to it!
Okie Bob

glenn kangiser · July 17, 2008, 10:41:29 AM

There ya go BoB. Never give an inch.

Rather than pre-drill I did like to pre-punch. I did hundreds of buildings pre-punching.

The reason is if you pre drill and are very careful it is not a problem but if you boo-boo then you have to plug each hole with a screw. With pre-punching the pile must be straight - lay out the holes carefully then hit it with a hammer and center punch.

The marks go down through about 5 sheets and if for some reason your purlin was down a little etc, then most of the sheets will only have a little dimple in them - not a hole. You can continue the punch marks every few sheets from the dimples on the previously punched sheets.

GW · July 17, 2008, 03:02:26 PM

I had already made up my mind to punch instead of drilling. I put the Purlins on last week, I did 2x4's on 24" centers. I noticed that some of my trusses in the middle are leaning forward a bit. I am being kept awake at night thinking that the roof is not square and the metal will look all rippled. Here is a picture that I took with my cellphone, I forgot the camera.

OldDog · July 17, 2008, 05:41:45 PM

GW

If you find that your roof is not square.

Stretch a string end to end where the drip edge of the roofline will need to be and roof to the string.

The end gable trim will cover a lot.

It might take a few minutes to block out for the string but it will work.

Will look better than cheating on the roof and ending up with a "saw tooth" edge.

firefox · July 17, 2008, 10:13:10 PM

This is in reference to putting felt or something on the underside of the roofing before
installing the roof. I don't know what you call the stuff, but the stuff that is sprayed on the bottoms
of metal sinks. I believe it is put there to reduce noise, insulate, and to keep the sink from sweating.
Feel free to correct me if this is wrong. I was thinking that it would be a good idea to spray the underside of the roof with this stuff prior to installing it. Maybe it would provide a good adheasion surface when spraying in the insullation after the roof has been placed.
Bruce

July 18, 2008, 01:42:07 AM

It would really be hard to do compared to normal roofing.

If you are on 2' centers then you will have to put a pretty good lap to get to where the felt doesn't sag due to non- support

You could also attach a stop to your rafter tails then let the sheets ride against it as you line them up and screw them. Screw a temporary extension onto the rafter tails for the stop if necessary.

GW · July 18, 2008, 07:32:00 PM

Quote from: glenn kangiser on July 18, 2008, 01:42:07 AM
It would really be hard to do compared to normal roofing.

If you are on 2' centers then you will have to put a pretty good lap to get to where the felt doesn't sag due to non- support

You could also attach a stop to your rafter tails then let the sheets ride against it as you line them up and screw them. Screw a temporary extension onto the rafter tails for the stop if necessary.

The temporary stop is a great idea, then I could make sure all the sheets line up on the bottom and they won't slide off. I am planning to do a 1' overlap of the roofing paper.

Okie_Bob · July 24, 2008, 08:27:32 AM

Yep, Glenn, never let it be said that I let my ignorance overcome good sense!!!
I have to admit your idea about punching is great for you pros but, I have to admit that I didn't worry a whole lot about
keeping my purlins as straight or as evenly spaced as you guys. So, it was much easier to screw each sheet as I went
as the purlins would have been missed otherwise.
Also GW, it's difficult to tell from the photo but, I can't see the diagonal bracing on your trusses. Please check out a good framing
book on how to install 2X4 bracing inside the scissor trusses. That is where you really get your strength. Wish I could recall the name of
the book I got that from. It's easy to do and only takes about twice the length of 2X4's as the length of your roof.
Okie Bob

GW · July 24, 2008, 10:38:36 PM

Quote from: Okie_Bob on July 24, 2008, 08:27:32 AM
Yep, Glenn, never let it be said that I let my ignorance overcome good sense!!!
I have to admit your idea about punching is great for you pros but, I have to admit that I didn't worry a whole lot about
keeping my purlins as straight or as evenly spaced as you guys. So, it was much easier to screw each sheet as I went
as the purlins would have been missed otherwise.
Also GW, it's difficult to tell from the photo but, I can't see the diagonal bracing on your trusses. Please check out a good framing
book on how to install 2X4 bracing inside the scissor trusses. That is where you really get your strength. Wish I could recall the name of
the book I got that from. It's easy to do and only takes about twice the length of 2X4's as the length of your roof.
Okie Bob

I've got 2 framing books and can't find anything on bracing a scissor truss. Please let me know if you remember the book. I got the metal up this week, I tried running the roofing felt across the roof but it didn't work. I ended up having to run it down the same direction as the tin, I lapped it 6 to 8", I hope that will work as that all I could think of doing.

glenn kangiser · July 24, 2008, 11:08:05 PM

I did it that way on my garage but it was a garage and I didn't care. There should never be a leak in the metal or it will be pretty easy to fix, so I don't see a major problem. I think it will give enough of a break and vapor barrier to stop condensation in most cases.

Okie_Bob · July 31, 2008, 09:32:31 AM

GW it was in a code book I found on the internet. No clue how to find it again. Maybe Peg can guide you on this? Or Glenn? I saw it once and decided to go ahead and do it. And it was after I already had all the metal up and insulation blown in.
Okie Bob

July 31, 2008, 10:15:20 AM

Seems the truss company should have included bracing information- I would call them.

With steel we run horizontal braces tying the bottoms together - sometimes top also. Once the sheathing and sheetrock is on that should do it. Or slip in a temp 2x4 brace and a nail at each truss.

teacher2 · July 31, 2008, 09:24:55 PM

Steal this thread for just a minute - My husband and I boated by your lake house a couple of weeks ago. Very, very nice. Sassy wants to know if you will post some more pictures.

Len

teacher2 · July 31, 2008, 09:26:31 PM

OOPS!! That last message was to Okie Bob. Sorry.

Len

glenn kangiser · August 01, 2008, 09:40:04 AM

Not a problem -- you can also remove or modify your own posts if you want.

Okie_Bob · August 07, 2008, 07:36:24 AM

Hey Teacher 2, sorry for the delay in responding, just didn't see this till now.
Thanks for the nice comments on my little piece of heaven. I'd be more than happy to give you a tour of the house
next time you are over my way. Just tie up at my dock and come on up to the house. (my dock has 20066 Marimac Rd sign on it)
It's now Thursday, April 7 at 7:30am. I'll be at the lake this weekend, hopefully by Friday noon.
Okie Bob
Or call anytime 817-729-7865 c