Painted sheet metal roofing over asphalt shingle?

[Bibbyman]

The asphalt shingles on the roof of our house have been there too long.  We've decided to cover them with painted sheet metal roofing (a.k.a. tin).  We've went so far as to pick up enough metal to cover two areas.

We were told at the place that we bough the metal that we could screw the metal down right over the shingles.  We only have one layer of shingles.

I've been told other times that we should put down nailers so that it's not in contact with the old shingles.  That would sure be a lot more work.

Anyone out there with experience with painted metal roofing over shingles?

Sorry that this post is a bit off topic for the Timber Framing/Log construction section but I figured I'd get better answers here.

[pappy]

Bibby,

I'd strap over the old roofing...

Two reasons,,,

one ,,  it'll make it easier climbing and fastening of the metal ,,

second with the vented air space it'll help keep the house cooler in the summer...  8)

[pineywoods]

I've done it both ways.  Best bet--if you bought the thinner gauge metal (usually 29 gauge), go with the nailing strips, fasten them down with lots of screws. The thicker metal (26 gauge) will work right on the shingles without wrinkling if you don't pull the screws down too tight. BTW, don't attach the metal with nails, use the screws with neophrene washers. Most suppliers will tell you to use short screws and put them down on the flat part of the metal.  I use 2 inch screw and put them on top of the ridges. My take on this is-yes the rubber washers will prevent leaks even on the flat part of the tin, but about 10 years down the road the rubber will start to come apart and leak and I'll be too old and feeble to get on the roof to fix it. Pay attention to the laps between sheets. The 2 edges of the tin are different for a reason.
They lap right backwards from what you'd think. I've done my house, shop, 2 equipment sheds and a barn. It's the only way to go..

[Jeff]

Two schools of thought on what I have been told.  Nailing flat eliminates the condensation that will occur underneath if there is an airspace.  On the other hand, if there is an airspace, condensation will be evaporated by the air flow.   I do know one thing from the cabin. You definitely will get condensation on the underside of a metal roof. It can rain inside under the right conditions.

[Bibbyman]

Bibby,

I'd strap over the old roofing...

Two reasons,,,

one ,,  it'll make it easier climbing and fastening of the metal ,,

second with the vented air space it'll help keep the house cooler in the summer...  8)

The roof is either 4/12 or 12/12.  Not much of a climbing issue.  One is easy the other can't be (or won't be) done.  We'll work for a ladder or in a box held up with the Terex on the 12/12 pitches. Fortuatly,  most of the roof is 4/12 and the 12/12 pitch roofs can be worked from the ground on the bottom edge and ridge top on the top edge.  

I didn't think about the screws pulling dents in the metal because it's on top of the shingles.  I've got some scrap pieces. I'll give it a try. I'll check the gauge of metal – I suspect it's the thinner gauge.  

Yes,  we've lapped both ways - right way and wrong way.  

We've also screwed to the ridges and screwed to the flats.  Only a few recommend screwing to the ridges.  The tin company says only on the flats.  I know I've got leaks in the section where I've screwed to the ridges and none where I've screwed to the flats.

Yes,  our metal sheds "rain" about every cool morning.  I'm thinking this should not be the same condition when it's put over a shingled roof with an attic under it.

[ely]

i have put the tin on both ways you have described. the best way i have done it yet is put your runners across the roof on 4ft between the boards. if you use 3/4 boards  then we put 3/4 inh foam sheets  between the runners. that takes care of the condensation problem and also makes for better insulation too. downside is it does add to the cost.

[clif]

 Is it corrugated metal or does it have a profile?  If it is a profile then I would not screw through the ridges except for stitch screws at the laps,  very bad deal.  It is harder to make a good seal. Corrugated metal I do not use any more.  More expensive, but we use the seamless .  it sheds snow and ice better, probably not a problen in your area.  Only problem with metal is in heavy snow areas you may have to put on snow tabs if snow falling off the roof could hurt some one as they walk beneath it . clif

[Qweaver]

I'd put down strips,  but if you don't have the strips to work off of; here's how I work on my steep roof.  I screw the toe rail from underneath and then I can slide the lightweight ladder frame along as I put each new sheet of metal on and fasten the first few rows of screws. (as you can see in the picture) Then I unscrew the toe rail and work the remaining rows of screws from angle brackets and boards attached to the posts.  I know that REAL roofers walk these types of slopes, but I'm no roofer and I'm not into falling off roofs.  I'm sure that there may be a better method...but not knowing any better, this is what I came up with.  Also, it's a lot cooler on the knees working against that 1/4" plywood that the hot tarpaper.
Quinton

[Bibbyman]

 

It's got ribs just like we used on our sawshed.  I also looked it up and it's 29 gauge.

We put up two sheets and if you don't go crazy with the cordless drill,  you can put it on with out denting.  But Mary knows no other way then ALL THE WAY.  It's white in color so the dents are hardly noticeable.  I'm pre-marking where to screw so we don't have the meandering all over the place like on some of our other project.  She insists on doing the screwing down part.

It was misting when we stated and then accelerated to a down right drizzle when by the time we got the second sheet on.  She wanted to come down.  We'll try again when the sun is making it blearing hot and it's a 100 degrees in the shade.

[TexasTimbers]

I too have dome it both ways. Mueller will tell you to lay it right on top of good old fashined 30# felt as long as you have a rep there like I have had for 8 years and isn't trying to sell you the radiant products. With the amount of cellulose going in my house couple with the proper attic ventilation  the radiant is overkill anyway.

I don't like the idea of the metal on top of the comp though. At least lay some felt.

[scsmith42]

Bibby - consider pre-drilling the holes rather than marking every one.  You can predrill a stack of 20 sheets or so all at once, and it's quick and relatively easy - all you got to do is to measure and mark the top sheet and then use it as a template to drill the rest.

[Ironwood]

Bib,

Well, noone else said it. If there is decking below the shingles (there should be) get rid of the shingles. I did it on both my shop roof (Grand Rib 3, Fabral) and house roof (Fabral, standing seam architectural grade). If you can put the "pan" right down on the deck then should you ever need to go up there, there is no denting, etc..... Wood is your insulator , and there is no fire hazard (singles LOVE to burn). It would be one thing if you had purlins below, but if it's "decked" do it right, "can" the shingles.

              .02  ;)

            Reid

[Woodcarver]

I raised the question of installing a metal roof over shingles a while back. Here's the link: https://forestryforum.com/board/index.php?topic=9899.0

The consensus then was to remove the shingles as Reid has recommended.

[TexasTimbers]

Well, noone else said it.

I don't like the idea of the metal on top of the comp though.

???

Definately pre-drill the holes as Scott says. I guess I take for granted alot of things this is a given. Be sure to use clamps on the stack when you do it so they all stay together well.

[PineNut]

I would consider using some of the newer screws to fasten the metal down. The new screws have a cupped head (lower side) with the neoprene washer inside the cup. Then you can pull the screw down as far as it will go and the neoprene washer will be compressed the correct amount and also it will be totally inside the cup and protected from the elements.

[TW]

I have been warned to never put sheet metal directly on asphalt felt.

When the sheetmetal expands and contracts with temperature it moves some tenth s of a millimeter back and forth. Over time the sand on the asphalt felt will wear through paint and zink to bara metal. Then the sheetmetal rusts from the underside.

So they say.

[pappy]

Noticed a place in town yesterday where the metal roofing was placed right over the asphalt shingles and the metal was wavy looking ...  :

[Furby]

House across from my parents did steel right on top of asphalt shingles.
Based on what I see after a couple of years there, I'll never do it myself.

[Bibbyman]

House across from my parents did steel right on top of asphalt shingles.
Based on what I see after a couple of years there, I'll never do it myself.

Why you say that?

[Left Coast Chris]

Bibby.... this is somewhat off subject but Im throwing this in because it happened to me:

when you work on the roof you typically kneel down and put your knees onto the surface and face the roof with your heels up and toes bent into the roof.    If you stay in this knelt position for very long (several hours) with your feet under you and the achilles tendon stretched it can slowly tear without you knowing it.

I faced the roof knelt down with the achilles stretched for maybe 4 hours moving only every 10 min or so on a 8/12 barn roof renailing loose nails one day and the following day I could hardly walk.  It got worse the second day.  Went to the doctor and he said the achilles was partially torn and put it in a cast.   I was on crutches for three weeks.

I am 47 and an ex college athlete so the body has high miles  ........that could have been it but suggest not to tax the body too long on something you are not used to doing.    The doctor chewed me out asking what I was doing on the roof to start with because he said he sees some very serious injurys from roofers and wana-be-roofers.  I always do everything myself and work safely (tie myself off for fall protection since the roof was so big and high off the ground) so would have done it anyway.  Its just good to think about safety and don't tax the old bod too much too fast. 

[Bibbyman]

 

One section 10'x33' 4/12 slope over a porch and anther section 11'x30' of 12/12 over the south slope are done.

Thanks for all the input.  

For a bunch of reason – mostly to "Get-r-done" - we went ahead and screwed over the old shingles.  I must say,  it looks DanG good right now.  We'll let you know how bad we screwed up.

Mary worked on the roof of the 4/12 section while I pushed tin up from the ground and screwed down the eve edge from a ladder.   

We had our son helping on the 12/12 pitch section.  Mary worked from the top where it was 4/12 on the other side.  She could screw down the top row.  Gabe used the ladder to reach all the middle rows and I push up the metal and screwed the bottom row.  The gutter height on this section is only about 4-1/2' off the ground.
 
We did do the trick of pre-drilling the sheet metal before putting it up.  It sure made the screws line up nicely and make the job on the roof go faster.  

We've got quite a lot more to do and on one big section we'll have to take the shingles off to fix a couple of problems and I want to keep the weight to a minimum.

[Furby]

Simply put Bibby, most of the ones I've seen over shingles look like dog doo!
This one is no different and actually one of the worse I've seen.
Some look good for a while, and some look better longer then others.
Getting the snow we do up here don't help things too much either.

Having said that, I've seen some done on new construction that look real bad as well and don't impress me none either.