Replacement metal roof....best method

[LD1]

Gonna be helping a friend of mine put a new roof on his house. My house will get a new roof at some point as well so the question becomes....when going from shingles to metal, what is the BEST way to do it. Cause it seems there is a handful of different ways, and just like building a pole barn....each contractor has their own way they want to do it, and reasons why.

No contractor will be used in this case. That was just for an example.

1. Roll out some kind of barrier like felt and put metal directly down. I have seen some contractors do this, and say it's just fine.

2. Put down 1x4 slats for "purlins". Leaving the shingles there. Attach metal.

3. Remove shingles and put metal directly to sheeting.

4. Remove shingles...but barrier, install metal to sheeting with barrier between

5. Remove shingles, slat with 1x4, install metal

These 5 methods I have all seen used. And all have pros and cons. Leaving shingles on is less mess, less labor, and less likely to damage anything inside the house IF something on the metal roof leaks....since it's essentially double roofed.

But the concern the. Is trapping moisture between the metal and shingles.

Slatting/purlins adds height...either with shingles on or off. Makes facia and trim pieces all need made taller and probably replacing facia boards to accommodate, and readjusting gutters, etc.

No slatting and the concern is thermal expansion of metal rubbing on whatever is under it. Either shingles or the sheeting. Ultimately wearing off the paint/coating and rusting due to condensation.

So....when building a bran new house from the get go and planning/designing for a metal roof.....what's the "approved" method?

For now, assuming just ribbed metal siding like a pole barn. But may not rule out some of the newer products made of metal....like the ones that look like shingles. Don't know if the method for installing them changes....but obviously those probably wouldn't work just purlins or slats. And I don't think standing seam would either. They need the support of sheathing.

So I know there are a lot of builders and remodelers on here....so just curious what method you prefer and why? Or any real life horror stories with any of these methods....or something that cause significant issues long-term

 

[ponytug]

I think roofing is about being obsessive about the details the first time, and being a perfectionist in the execution.

I would always recommend removing the prior roofing material, for a bunch of reasons;

• You don't know what the substructure of the roof is like, e.g. it could be failing/rotten.
• it is an unnecessary fire hazard
• it makes fastening the metal roof on top of it harder, and less secure
• you run the risk that the prior roof fasteners will work themselves loose and wear a hole in the new roof

I would always, always use a membrane, and not just asphalt roofing material. You get a much better seal against the water that will get under there. Make sure that you use a membrane that is rated for use under metal roofing material. Some of the cheaper ones will melt, and run, in heat, which requires a whole new roof.

Using purlins gets your metal roof up and away from the roof material and allows ventilation, adding to the longevity of the roof. Finally, when you do use purlins, I prefer to have water gaps in them at regular, and short distances if the purlins are not running up and down the roof slope. Again, there is always going to be water under there, why let it pool any more or any longer than it has to? If you have the money, butyl tape on top adds another layer of seal, and will help keep the fasteners from working loose, and keep the fastener from rotting out a hole in the purlin.

As a former (part time) roofer, my view is that stuff always, always happens, like that freak rainstorm from the southeast with 100mph gusts that manages to drive water in under your roof, or the ice dam that backs things up under the roof, or the one caulk joint that had some unseen metal forming oil on the metal and the caulk slowly peeled away.

My other $0.02 is never, ever, make a roof penetration if you don't have to. Vent to walls if possible, or internally if allowed locally, or if you have to, try to combine as many as possible into a single vent. I would bear in mind that the fasteners that you use on a metal roof are usually the weak point and the seal will leak over time and need replacing.

I like standing seam roofs, as they shed water and snow well, and the fastening systems tend to be less prone to leaking, if installed well. However, plan the roof for where it is going to shed the snow or ice, e.g. where are the shrubs and entryways relative to the roof slope, and how will you prevent ice from falling on someone?

I do have a roof that is the faux tile material. It looks great, but you have to be under a 100lbs to walk on it, or make up custom roof boards. (rug on the underside and foam or wood to spread the weight. They give me the heebie jeebies personally, because I worry about them slipping, but everyone is different.)

All the best,

Peter

 

[wawajake]

I agree with Ponytug, as it is by far the most thorough , safe from moisture way. Unfortunately some around my area use the Amish to do their steel roofs, and they just wood strap over the shingles and screw metal on strapping . Also with no venting to accommodate space between shingles and steel to mitigate moisture somewhat in our extreme temperature swings here. Asking for moisture problems is what a retired Building Inspector tells me .

 

[Roadworthy]

The method of installation you select may be determined by the type roofing you select. The metal shingles may be the best choice but the cost can be prohibitive. Standing seam would probably be your best value and give the least trouble in the long run.

 

[CliffordK]

I would choose metal roofing with hidden fasteners. I know there is some debate about it, but that would be my choice.

I'm still trying to figure out the best way to keep moisture off the back side of the metal. If one has open purlins in the attic, then one can spray the bottom side with foam closed cell foam, and it would keep the back side dry.

 

[2manyrocks]

Adding pine purlins can be a good idea if the roof sheating is OSB because pine will hold screws better than OSB. The purlins will also tend to even out any waves in the roof decking if there are any.

Synthetic underlayment is lighter to carry and more slip resistant than asphalt underlayment. Synthetic can also be left exposed to the weather longer if you can't get the metal on as fast as you'd like.

Standing seam would be the best choice if you can afford it. The seals on metal screws seem to have about a 10 year life span before heat and UV exposure makes them start leaking. If you use screws, they need to be set securely, but not so tight that they crush or damage the seals.

If you need to cut metal, buy yourself a set of DeWalt cutters like these. You can zip through metal roof like butter with these things. Great timesaver. Amazon.com

You will also want to get a magnet pickup tool because you're going to drop screws if you go with that kind of roof.

The disadvantage of metal is that it's slick as ice when it's wet.

 

[BlueKnife]

I'll just generally back up what @ponytug said. Only thing I would add is that, in most cases, metal roof fasteners of all types are supposed to have at least 5/8" of purchase in wood. If the roof was sheeted with 7/16" or 1/2" material, additional purlins or sheeting will be needed. Personally, I'm not a fan a screwing metal to OSB, even if it technically meets the thickness specs. Definitely need a moisture barrier between wood and metal roof.

 

[2manyrocks]

It's also worth mentioning that I didn't have to change any of my gutters or facia when we added 3/4 x 4" purlins. The metal roof just came down to the gutter with no issues, and the sidewalls were covered with a gabel rake.

 

[drizler]

metal screws seem to have about a 10 year life span before heat and UV exposure makes them start leaking.

I beg to differ on this one. I’ve got some screws I put on a purloin metal roof from 1990 ish . I pulled that old garage down in 2018 and aside from some minor surface rust they were fine. The seals had no cracking and were still soft.
i wouldn’t use them on a house but they are still there waiting for the next shed I build. These were plain galvanized with a plain unpainted galvanized roof.

 

[jyoutz]

Gonna be helping a friend of mine put a new roof on his house. My house will get a new roof at some point as well so the question becomes....when going from shingles to metal, what is the BEST way to do it. Cause it seems there is a handful of different ways, and just like building a pole barn....each contractor has their own way they want to do it, and reasons why.

No contractor will be used in this case. That was just for an example.

1. Roll out some kind of barrier like felt and put metal directly down. I have seen some contractors do this, and say it's just fine.

2. Put down 1x4 slats for "purlins". Leaving the shingles there. Attach metal.

3. Remove shingles and put metal directly to sheeting.

4. Remove shingles...but barrier, install metal to sheeting with barrier between

5. Remove shingles, slat with 1x4, install metal

These 5 methods I have all seen used. And all have pros and cons. Leaving shingles on is less mess, less labor, and less likely to damage anything inside the house IF something on the metal roof leaks....since it's essentially double roofed.

But the concern the. Is trapping moisture between the metal and shingles.

Slatting/purlins adds height...either with shingles on or off. Makes facia and trim pieces all need made taller and probably replacing facia boards to accommodate, and readjusting gutters, etc.

No slatting and the concern is thermal expansion of metal rubbing on whatever is under it. Either shingles or the sheeting. Ultimately wearing off the paint/coating and rusting due to condensation.

So....when building a bran new house from the get go and planning/designing for a metal roof.....what's the "approved" method?

For now, assuming just ribbed metal siding like a pole barn. But may not rule out some of the newer products made of metal....like the ones that look like shingles. Don't know if the method for installing them changes....but obviously those probably wouldn't work just purlins or slats. And I don't think standing seam would either. They need the support of sheathing.

So I know there are a lot of builders and remodelers on here....so just curious what method you prefer and why? Or any real life horror stories with any of these methods....or something that cause significant issues long-term

I don’t know this is the best method, but this is what was done on my house 6 years ago when I went from shingles to metal. The roof only had one layer of shingles. They removed the cap shingles and edging, installed new edging, then rolled out a layer of silver material (with ripstop squares) and nailed it down with nails that have large plastic fender washers. Then they installed the metal over top this. They said that this can only be done with one layer of shingles and by using thicker steel panels, so the shingle lines wouldn’t imprint the metal over time. I’ve had no roof related issues since.