Putting a metal roof over asphalt shingles

[Randy88]

We've been using a cordless impact driver made by rigid bought at home depot, slickest thing I've ever found to install screws of any configuration into anything.   It'll torque things down gently and can stop in an instant as its torquing down, turns fairly slow which is a plus compared to any drill.   

If these screws are meant for valley installation, what's the best for ridge installing screws?   Is there any major difference other than the length?

[Farmerjw]

I was taught to put the screws on the ridges on the roof and on the flats on the walls.  More water runs on the flats on a roof and putting the screw on the ridge reduces leaks. 

[POSTON WIDEHEAD]

If your screws have the rubber seal on them, they should be drilled on the flat of the roof.

Screws with out rubber seals go on the ridge.

99% of the roofing today is installed with the screws with rubber washers. Just make sure your screw makes a flat fit.

Note: The reason 2x4's were recommended was that the screws will not back out over time. Using 1x4's will not give the screw enough meat to hold on to over time. So if your running your screws into a shingles roof and on into the plywood under the shingles....watch for your screws to back out over time.

[thecfarm]

The screws that I have shown can be used on the ridges or the flat part,The links shows it.

[Randy88]

Postonlt40hd, why would one put screws on the roof in the ridge without any washers on them, what keeps them from leaking then or keeping the tin from wiggling, I was always told the washers held out water and also kept a snug fit on the metal, I have no idea if its true or not.   

[POSTON WIDEHEAD]

Sorry Randy, I honestly did not mean it like it sounds. My Dad built a barn back in the '60's. Screws and nails for roofing did not have washers on them then.
That is why they were installed on the ridge so the water would shed away from where the screw/nail went in.

I did not mean to be confusing.

I think most of the nails and screws now have the rubber washers like the ones in the pic that Thecfarm showed.

[elk42]

     
  Back in the 50's they used lead head nails, then went to nails with rubber
washers for metal roofing

[CRThomas]

I'm about to put a metal roof over our existing shingle roof and have been told it is wise to lay 2x4s between them. That's a lot of 2x4s as the roof will take about 150 square of metal. I'm assuming you would do this to prevent moisture accumulation between the metal and shingles.  First, is it necessary and second, if so can it be thinner than a 2x4.

I just put a metal roof on my home and layed it right on the shingles. no problem thats the way its done here in my area southern illinois later

[drobertson]

More than one way to skin a cat, they say,  1x4's, chalk line and square the ends, makes for easy installation, they way it's done here, yes its allot of lumber, but then again its a roof.   david

[john_bud]

A
  How much and how fast does the metal roofing move? I think it would take a very long time for the shingles to wear through the metal.

Doing the math, a 100c temp change on a 30ft long steel piece will change by 0.612".

The speed of change is proportional to the change in temperature.

Typical roof won't see quite that temp raise, unless you live in the SE desert.

[jwilly3879]

When you install metal roofing over shingles it is hard to snug up the screws without deforming the tin.If the screw happens to fall by the bottom edge of the shingle and is tighteneddown it will wrinkle the tin.

Putting screws on the ridges in the tin can cause the sheets to grow in width if the ridge gets compressed. A 36" wide piece of roofing starts out as a 42" flat sheet before it is formed. A trick used by some roofers to make the tin fit an out of square roof is to step on the ridges causing the sheet to become just a little wider. It is amazing how much you can gain over the width of the roof. 

[Larry]

Note: The reason 2x4's were recommended was that the screws will not back out over time. Using 1x4's will not give the screw enough meat to hold on to over time. So if your running your screws into a shingles roof and on into the plywood under the shingles....watch for your screws to back out over time.

Exploring this topic further, screws come in different sizes.  Typical is the #9 or 10 used 90% of the time.  Great for new construction into 2 X lumber.   Bigger screws can be had in #12 size to replace screws that failed to hold and really big #14's are made to use in plywood and OSB.  Of course the bigger the screw the larger the expense.  And when that little bitty screw backs out of an existing OSB roof or 1 X 4 stripping it really gets expensive.

[Peter Drouin]

I have time to do it right, I don;t have time to do it twice comes to mind. strip the roof lay 90lb felt, , put the tin down. like putting new shingles over old ,my dad said do it right or not at all.
best of luck your way