Metal Roof over composition aspalt shingles

[Amir Faghih]

Hello, Wondering if metal roof over old composition shingles are allowed. is there a building code against it?
I live in Austin , Texas area.
Thanks

[Larry D. Kage]

Standing seam type?...often done on one layer with an adequate underlayment and base.

IRC R907.3 Recovering versus replacement.
New roof coverings shall not be installed without first removing existing roof coverings where any of the following conditions occur:

1. Existing roof or roof covering is water-soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing.
2. Where the existing roof covering is wood shake, slate, clay or asbestos-cement tile.
3. Where the existing roof has two or more applications of any type of roof covering.

Exceptions:

1. Complete and separate roofing systems, such as standing-seam metal roof systems, that are designed to transmit the roof loads directly to the building's structural system and that do not rely on existing roofs and roof coverings for support shall not require the removal of existing roof coverings.
2. Metal panel, metal shingle, and concrete and clay tile roof coverings shall be permitted to be installed over existing wood shake roofs when applied in accordance with Section R907.4.

[Marcel R. Cyr]

No code that I am aware of to prevent this type of installation, it is all around me up here left and right.
I think it is controlled mostly by a standard of practice for execution and by Manufacturers recommended installation procedures.

Here in Maine, North and Central, this type of product is being utilized in lieu of re-shingling. Why, it is cheaper and lighter.


Installing these products on existing roof composite shingles could and will eventually damage the underside of the panels caused by the thermal movement of expansion and contraction over time.

It is a standard for some installing these panels on existing roofs to install a slip sheet that breaks the bond between the two materials and eliminate damage to the panel underside.
Another procedure is to fur the existing roof with 1x2 strapping and this elevates the panel and allow the under side to stay cooler and dry.
Some condensation might occur under metal roofing products as such, that is why a proper underlayment is required on new build.

As usual, most contractors or homeowner will take the cheapest route to get the job done.
All I know is I would strip the existing shingles and do it right to begin with, but that is just me.

[Greg Keene, CPI]

Most of the counties in the Denver area (and foothills) no longer allow 2 layers of roofing - all layers must be removed first - had hail damage and will remove the existing 2 layers and will install a Pro-Panel metal roof - A lot of AHJ decide this whether it is in the code or not -

[dquigley]

Originally Posted by mcyr

Another procedure is to fur the existing roof with 1x2 strapping and this elevates the panel and allow the under side to stay cooler and dry.
Some condensation might occur under metal roofing products as such, that is why a proper underlayment is required on new build.

As usual, most contractors or homeowner will take the cheapest route to get the job done.
All I know is I would strip the existing shingles and do it right to begin with, but that is just me.

This is the Florida way, however i always recommend that the existing roof be striped so that water damaged ply/fascia can be inspected and repaired. In Florida the humidity and rainfall work in mysterious ways as there is often damage even when none would be suspected.

[Peter C. Russell]

I agree with Marcel. I won't install a metal roof unless I strip the existing roof, make sure the sheathing is secure, then we add either 30# felt or ice and water shield, install 1X3 strapping attached to the rafters using 8 penny ring shank nails, then install the metal panels to the strapping using the recommended screw from the MFG.

[Sean Fogarty]

One manufacturer in NC will not warranty their product unless rosen paper is placed over the shingles first. Another that I know of will not warranty them period unless the asphalt shingles are removed and rosen paper is put down. The reasoning is they claim the petroleum and other chemicals in the shingles and tar papaer will corrode the metal quicker and therefore no guarentee.

[dquigley]

Originally Posted by sfogarty

One manufacturer in NC will not warranty their product unless rosen paper is placed over the shingles first. Another that I know of will not warranty them period unless the asphalt shingles are removed and rosen paper is put down. The reasoning is they claim the petroleum and other chemicals in the shingles and tar papaer will corrode the metal quicker and therefore no guarentee.

Doesn't rosin paper dry out and decompose rather quickly? Probably a cold weather climate thing.

[Marcel R. Cyr]

Originally Posted by dquigley

Doesn't rosin paper dry out and decompose rather quickly? Probably a cold weather climate thing.

NRCA also recommends a slip sheet be installed over the underlayment for metal panel roof systems. A slip sheet is a layer of smooth building paper, such as rosin-sized or unsaturated building paper. Its purpose is to protect the underlayment from damage, as the panels can adhere to and tear the underlayment.

[Amir Faghih]

Thank you all for your responses. Was very helpfull.