Sheathing under a metal roof.

littledog · June 20, 2007, 05:22:28 AM

Hey there. I have been planning a 20X30 1 1/2 Storey. When I figure out how to post the floor plans I will. Meanwhile I have met with a contractor who is basically going to assist me in building the house. But I am a little wary of a couple of the techniques that he uses. First of all I am planning on using a standard corrugated metal roof. It comes in 36" sheets and you specify how long you want it. Typically metal roofs are mounted to purlins. This is where I get nervous. My contractor plans only to use tyvec house wrap over the outside of the rafters and no sheathing. So we would be placing the purlins accross the rafters over the house wrap. What makes me nervous is the fact that we plan on using blown in Cellulose in the rafters. It seams to me I would want the added protection of some sheathing. If so How would I ensure proper ventilation? I have seen a description of a "Cold Roof" in the do it yourself homebuilding book. Any Thoughts? I know that was a long post but I could use any help you all can give me. I also have questions regarding insulation in the basement but that can wait. Aaron. P.S. I am building in western Massachusetts.

MarkAndDebbie · June 20, 2007, 06:41:57 AM

I would be worried about puncturing the tyvek. It would also seem really difficult to install purlins/metal over tyvek with no decking. Several people on the board have said not to put tyvek on a non-vertical surface - way too slick. I also think the vapor retarder would go under the rafters in your climate - you said outside - maybe I misunderstood.

I'm sure we'll get a more professional opinion here soon.

June 20, 2007, 06:42:13 AM

I don't like Tyvek for anything. Too much money spent on advertising and not enough on a product that works in all situations.

30 lb felt has a much better record, Purlins or sheathing have both been used with success.

If you want the sheathing, then that is what the contractor should use. If he won't I'd find a different contractor. Problems before the start of a job are indicators of bigger problems once you are committed.

littledog · June 20, 2007, 06:52:56 AM

Thanks for the prompt replies. The contractor is definitely open to doing things to my specs. He has just been letting me know how he Usually does things. As far as the vapor barrier, he did say he installs the tyvek on the outside. Also what do you think about using cellulose in the rafters? There is also a question as to whether to insulate the basement on the inside or the outside. I know this is a touchy subject with some. I am planning to face any exposed concrete with brick eventually. I like the idea of using foam on the outside of the concrete, but I have no experience to back that up. Again, any thoughts. Thanks.

glenn kangiser · June 20, 2007, 07:01:00 AM

http://www.countryplans.com/search.html
SEARCH

Try that link for Tyvek. Check out our discussions. I have to split soon.

I like the idea of insulation on the outside in cold areas.

If Tyvek gets a leak it causes mold and rot problems. If it doesn't get a leak, condensation behind it can cause mold and rot problems. Houses and apartments have been torn down in a few years due to Tyvek problems. PNW is where I heard that. Felt - 15 lb on sides per PEG, won't.

June 20, 2007, 07:17:35 AM

Thanks again for the reply and I am checking out the past posts. I am sure I will have more things to ask. But for now thanks a bunch to everyone for such awesome support. Aaron.

JRR · June 20, 2007, 09:24:59 AM

I agree with Glenn on Tyvek. I would not install any membrane over purlins.

I don't want to talk you out of sheathing it certainly is the safe way to go, but having installed a little corrugated roofing ... I would "almost" be comfortable without sheathing ... depends on your expected wind conditions and (of course) code. The correct gauge roofing metal will provide a lot of anti-racking once it is properly attached. Make sure you use the heavier gauge, deeper profile corrugation that is recommended for roofing. Use at least 26 gauge ... 24 gauge is usually available on special order. Per instructions, double the overlap on connecting edges, and use plenty of poly roof adhesive (I like the PL brand roof sealer) on the overlapping metal surfaces.

Make sure you use the hold-down system screws that include the sealing washers. I never install screws in the "valleys", only on the "crests" (terminology??). To avoid overtightening the screws, I always make those last snug-up turns by hand.

OldDog · June 20, 2007, 10:11:42 AM

I have metal roofs about any way you can do it.

Metal over shingles, metal over purlin on shingles, metal over insulated purlins

and plain old metal over purlins.

I plan on using a metal roof on my cabin with "Double Bubble" (i don't remember the correct name) over the purlins and under metal.

I don't see any difference in the performance of the roofs, just in how much they are insulated.

My metal supplier now suggests the screws on the flat and not on the ridges??

Good Luck!

littledog · June 20, 2007, 10:31:07 AM

Just to be clear, the proposed tyvek would go over the rafters and under the purlins. But it would be the only barrier, other than the metal, between moisture and the insulation. If I blow in the insulation into the 12" rafter cavity will I be risking wet insulation.? Or would I be better off using a bat if this ends up being the way the roof is finished?

June 20, 2007, 01:19:21 PM

Metal roofs can and often are done on open purlins - it is the most common way to do a barn or utility building roof, but this technique is not as good for an insulated roof. Here is what I would suggest for the longest lasting roof. Do the cellulose with a 1" vent channel and 1/2" roof sheathing as shown on the Wall & Eave Detail (sht. 4) of the 1 - 1/2 story plans. That will keep the roof cooler in summer and vent any moisture that might make it through. Over the sheathing layout 30# felt and then the metal roofing (w/ skip sheathing if needed). This system protects the insulation, provides a roof diaphragm for wind rigidity and has a full backup waterproof roof under the metal.

This roof will also be quieter and less subject to critter problems such as bats. It will cost more initially but will likely be a 60 to 100 year roof.

Micky · June 20, 2007, 03:43:43 PM

I used a product called titanium UDL instead of the tar paper. I worked great. Much easher to use than paper because it didn't tear. http://www.interwrap.com/dev/udl_30_main.html

I also used sheating on my 20x32 1-1/2 to avoid critter issues.

Ernest T. Bass · June 20, 2007, 11:09:24 PM

We ended up with a pretty similar roof to the one the Aaron's contractor is proposing on our home. Basically, we have a durable screen type of material over our rafters, 2x4 purlins over the screen, and an industrial-grade 24 gauge standing-seam metal roof over that. We have 10'' of cellulose between the rafters, and our vapor barrier is on the inside. The purpose of the screen is to let any moisture that may have made it's way up into the insulation to escape.

It's actually a bit more complicated than that, as we used round rafters, but you get the idea... We actually stapled the screen fabric (a waste product from a local paper mill) Between the rafters from the inside, once the metal was already in place. That worked out nice because it wasn't in the way when we installed the metal. Also, our metal roofing system used 1'' thick styrofoam strips under the panels to raise the roof up off the purlins. That gave us a total of a 3'' air gap from the screen to the metal.

littledog · June 21, 2007, 07:00:30 AM

I spoke with a builder friend of mine yesterday. She Was of the same opinion as John. She used sheathing with the felt over, then purlins and the metal roof. That roof sounds really good to me. She also reminded me of the styrofoam spacers under the sheathing. I am using 2X12s for the rafters. So with the gap I am looking at around 10"or 11" of cellulose. Not bad. I am also debating using a double wall method. Several of my friends around here have used it. Even with the added challenges of vetilation they are really satisfied with the results. Anyway, I will post my plans and stuff in the project section once I can. Thanks everyone for your help. Aaron.

Ernest T. Bass · June 21, 2007, 08:06:12 AM

I agree that the addition of the decking would definitely make for a more durable roof, especially if you use a lighter metal. It would be quite a bit more expensive, though. And in our case, it wasn't an easy option considering our round pole rafters. The pitch of our roof and the gauge of our metal allowed us to space the purlins up to 5' apart, which was really great when we had to try and get them all straight...

martyv · June 21, 2007, 02:23:17 PM

I have sheathing, felt and metal as John suggested. We occasionally get wind gusts in excess of 100mpg and so far no problem. It's been 10 years now. In the last big wind storm I saw a house with purlins and metal come completely apart. The 1X4 perlins did not have enough holding power to the rafters to withstand the wind once it got under them.

We also have blown in cellulose and with the 12 inch shoulder (I don't think shoulder is the right term), in the trusses there is enough room to have eave vents along with a ridge vent to ensure a cold attic. We have those cardboard vents that hold the cellulose down around the eaves.

I think it's more expensive but works good.