Metal Roof on Purlins v.s. Decking

Guest User
9/20/2014
Hello! I am installing a new 24 ga. standing seam metal roof on a house built in 1907. It currently has decking over the rafters, with the old metal shingles (yes, metal shingles) applied to that. I will be removing the old roof and installing the new one, along with a radiant barrier under that. I am getting two different stories from contractors regarding purlins v.s. no purlins. I have the option of installing the new roof directly to the existing deck (or installing new decking if need be), OR installing 1x4 purlins 2 ft. o.c. onto the current decking, and then installing the new roof to those. Again, in both cases, I intend to use a radiant barrier. The following questions/concern that I have arose and, in all my googling, I can't find a definite answer. -Energy efficiency - One contractor said that purlins help by adding an air-gap that provides insulation. The other says that gap provides more space for condensation to accumulate on the underside of the roof and cause inside-out rusting. -The old decking is composed of 1x8's and I'm concerned that a 100 yr-old house with that style of decking won't have a flat enough decking to install the new roof to and not have "wrinkling." -On last question, what do you think of these purlins I found, with vents built into them, the Cor-A-Vent Purlin Vent? http://www.cor-a-vent.com/purlin-vent.cfm I know you can't make an educated guess if you haven't used it previously, but I just wanted to see what you thought at first glance. Thanks for your help! -Mat
Info @windowsonwashington.net
An informed customer is our best customer.
9/20/2014
Above deck venting will help with convection. Based on where you are, you don't need insulation, you need air movement and cooling. The Cor-a-vent stuff is fine but you can do that with wood going in two different directions as well and cheaper.
Guest User
10/10/2014
I am building a small office building (12x24) and have decided to go with metal roofing. The roof will be 2x6's attached to a 2x8 ridge board...all 16 oc. My question is...can I attach the metal directly to purlins which have been nailed to the rafters or do I need to put down 4x8 sheeting first then then 1x4 purlins? I would like to skip the plywood step if possible but also want a dry and sound roof. Thanks Greg
Todd Miller
Classic Products, Inc.
10/10/2014
There are metal roofing products that can be installed over purlins without decking. However, my concern is that doing so might lead to condensation problems. I would encourage you to use decking and underlayment and also to ventilate the attic space.
Info @windowsonwashington.net
An informed customer is our best customer.
10/11/2014
+1 If the space is going to be conditioned (and even in cases when not conditioned), going over purlins will usually create a condensation issue. Plywood isn't that expensive. Spend the money and do it once and do it right.
Erik Muller
10/28/2014
I am building a barn the roof will be constructed of 2x10 rafters decked with 3/4 T&G and then the purlins, then reflective insulation/vapor barrier, then the 26 guage meal roofing. My question is whether or not to istall felt on the decking before putting on the perlins?
Info @windowsonwashington.net
An informed customer is our best customer.
10/29/2014
Yes. You will want an underlayment. Synthetic is easier and probably preferred for vapor permeance at this point.
Guest User
10/31/2014
Hi, I have a small cabin that has a metal roof screwed directly into the perlins. No decking. The insulation was put in between the joyces with a drop ceiling. I ripped the ceiling down to open up the room. I now need to insulate between rafters. My idea: 1. Put foam proper vents from soffits to peak between rafters. Staple directly to perlins. (i can daylight thru the peaks so should vent. 2. Then put batt insulation in between rafters 3. Close up with sheetrock or pine boards I won't have decking or anything esle. This is New England climate.
Info @windowsonwashington.net
An informed customer is our best customer.
11/1/2014
That should work but I would make my own venting system onsite as compared to just stapling up the foam vents. http://www.greenbuildingadvisor.com/sites/default/files/Attic%20venting.jpg
Guest User
11/3/2014
Thanks for the info. Nice pic in the link. Very straightforward. I just got back from the cabin and realized it didn't even have soffit vents. I drilled (3) 2" holes in each soffit and put up 10" screens. That should help vent. Also every single roof nail (no grommets either) was sticking up 1/4 to 1/2 inch.They were nailed in the rises. I bought 2" screws with the rubber grommets and will replace. Thinking about squirting a little sealant in every hole too before putting in the new screw. here's another question: Many proper vent instructions say to not make the vent continuous from roof peak to soffit. They call for spacing in between to allow ventilation from the interior to escape. If I build my own vent would I do the same? From the pic link you sent it looks like it might be calling for a complete continuous barrier. thanks again Rob
Guest User
11/3/2014
Sorry 1 more question. Since the tin roof is corrugated the ribs do not form a seal against the soffits. I don't have decking either (tin roof nailed to perlins). Should I spray a little foam into the ribs to keep bugs outs? The cabin had about 50 old wasp nests inside the soffits. I attached a pic of the soffits and ribbed roof.
Info @windowsonwashington.net
An informed customer is our best customer.
11/3/2014
Needs to be be continuous. You want zero communication with interior/conditioned air. Caulking or some mesh wedged in there should keep out the bugs.
Guest User
11/4/2014
In regards to "build your own venting system".. Would I use something like this? Squared Edge R4 Insulation Board 3/4 in. x 2 ft. x 8 ft thanks, Rob
Info @windowsonwashington.net
An informed customer is our best customer.
11/5/2014
Anything will work and installation is the pivotal part. Use something in the XPS of foil faced iso family. Foil faced with the foil facing up will work as a radiant barrier as well.
Guest User
9/3/2017
We have a pergola in El Paso which we want to cover from sun, for grilling, etc., while enjoying back yard. The pergola is flat and we would enjoy installing tin roofing for rain effects, etc. Any issues with using tin or steel roofing on flat pergola, and also screwing directly to pergola (no plywood). The pergola beams are 16 or so inches apart and we will not walk on roof and it is simply for blocking sun, etc. Thank you, Alex
Todd Miller
Classic Products, Inc.
9/3/2017
It's important to always use products per manufacturer specifications. There are products that can be installed over open purlins, provided the purlins are spaced closely enough together and the right fasteners are used. You do run a risk of wind getting under the panels and trying to push them off. Additionally, if the panels are installed at a lower pitch than what is required by their manufacturer, you run a large risk of water getting into the seams when it rains.

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