felt or synthetic underlayment for metal roof

[2manyrocks]

Looking to replace my worn out asphalt shingle roof with metal. The current roof is sheated with 3/4 pine boards. One roofer gave a bid including synthetic underlayment instead of felt. After some reading, there seems to be a difference of opinion on the ability of synthetics to seal up holes from fasteners versus traditional 30# felt. There also seem to be some issues that the synthetics particularly require adequate attic ventilation.

There also seems to be a difference of opinion about using purlins to get the metal up off the old roof deck to help with condensation issues.

Any input welcome.

 

[brandoro]

I replaced my shop roof with metal last fall and used the synthetic underlay. I was really pleased with it for several reasons, it rained before I had all the metal on and the underlay shed water really well and it has a non skid finish so it wasn't the least bit slippery when wet or dry, it is also very tough so no tears when handling or when the wind got under an edge.
The fellow I dealt with when purchasing the material suggested strapping the roof with 1X4 wood but it was to reduce sound transmission during rainstorms, since this was for my shop roof I don't care very much about that. If I am awakened by rain on the shop roof it is time to get up off the creeper and go back to the house.:laughing:
I did go with the full length ridge vent to improve attic ventilation along with the ventilated soffits already in place. Ventilated ridge is easier than trying to seal around more traditional roof vents.

 

[JEH412]

We had our shingled roof replaced with standing seam metal a couple years ago. The three contractors who gave us bids all quoted for synthetic underlayment, and one told us that traditional felt paper doesn't work well under metal. One side of the old roof still had the original slate, which was also being replaced. The slate came off and the synthetic went down and we then had two very heavy storms that kept the crew off the roof for close to a week (December ... don't ask). We had no leaks.

I'm sold on it.

John

 

[2manyrocks]

Do either of you happen to remember which synthetic was used on your roof?

How was it fastened down? (staples, roofing nails, plastic capped nails) Some of what I read said staples didn't seal well or tended to tear the material.

 

[PILOON]

Most of the synthetics are self adhesive as well as self healing (nails)
Generally you simply unroll and it sticks to the roof decking, especially on a hot day.
IMHO WR Grace makes the best grade of shield (AKO Ice Guard)

I would consider mandatory under metal roofing as the metal sweats and will rust underneath as well as cause the wood deck to rot eventually.

 

[ray66v]

I have used Titanium UDL synthetic underlayment since it came on the market, and I really like it.

You can't tear it, it won't blow off, it's 12'' wider than felt, and a 10 sq. roll only weighs 50 lbs.

I have never seen a self adhesive underlayment.

I have seen self adhesive ice and water shield. Grace is the best.

 

[PILOON]

I have used Titanium UDL synthetic underlayment since it came on the market, and I really like it.

You can't tear it, it won't blow off, it's 12'' wider than felt, and a 10 sq. roll only weighs 50 lbs.

I have never seen a self adhesive underlayment.

I have seen self adhesive ice and water shield. Grace is the best.

U are correct, however many use the self adhesive ice and water shield as the ultimate underlayment.
The trend for 'best roofing' is first class underlayment (Grace or?) followed by an 'economy' grade shingle.
The shingle then provides the element resistance while the 'ice guard' takes care of the leakage aspect.
With this method you can allow all sorts of ice dam build ups and not have leaking roofs.
It is one method of cureing what was bad construction without re building the whole roof structure.
I have used this technique where a cathedral ceiling cottages were converted over to full time usage as otherwise a second roof would have been needed to create an attic space for decent ventilation and isulation.
The differance was like a $3000. roofing job vs $30,000.
The best way would have needed 2ft sizzor trusses and whole new decking plus regular roofing and all the facia, flashing that goes with it and often extending chimineys vents etc. not to mention electrical entry masts etc.and you ended up with a thick roof that looked kinda goofy.
Also to note that laying a total coverage of 'ice shield' is not labor intensive, simply unroll and walk on it. DONE!
Also (on smaller dwellings) the differance between ice guarding eaves, valleys and overhangs is rarely more than a roll or two of ice guard. VERY CHEAP insurance!

 

[JEH412]

Do either of you happen to remember which synthetic was used on your roof?

How was it fastened down? (staples, roofing nails, plastic capped nails) Some of what I read said staples didn't seal well or tended to tear the material.

Our contractor used Titanium UDL, but I don't recall which particular product in that line. Fastened with plastic capped nails.

John

 

[2manyrocks]

I'm in middle Tennessee so we don't get ice dams that I can think of.

What about reroofing down the road if I use a stick down type of underlayment? I figure the nail down underlayments will come off like nailed down felt, but I figure a self stick type would only come up with the roof deck.

My local Lowes has the nail down Grace tri-flex 4x250 roll for about $108. It's not the triflex 30 which I have read is a thicker product than the tri-flex.

 

[ray66v]

Our contractor used Titanium UDL, but I don't recall which particular product in that line. Fastened with plastic capped nails.

John

They call for plastic capped nails. But, but I have had it sit for months with only staples. And it holds fine.