# underlayment for metal roof panels



## Rick18071 (Nov 3, 2021)

Plan review for deck roof and a car port.
IRC wants underlayment per manufacturers instructions under metal panels. What do you usually see for this.


----------



## classicT (Nov 3, 2021)

With conventional truss and OSB roofs, still seeing the typical underlayment. Fastening is typically done at the seam via nails.

For all metal framing (much less typical) using metal purlins and standing seam roofs, then no underlayment at all. Fastening is done through the panel sheet with a neoprene tek-screw. Had the conventional SimpleSaver roof insulation. Note that this was seen on a PEMB, a barndominium type house.


----------



## Rick18071 (Nov 5, 2021)

Thanks. What about if there is no sheeting?


----------



## ADAguy (Nov 5, 2021)

as in spaced sheathing?


----------



## Rick18071 (Nov 8, 2021)

2x4's spaced on top and 90 degrees of the rafters.


----------



## Beniah Naylor (Nov 8, 2021)

Never see underlayment on metal panels installed on 2x4 purlins as you describe. Not that that is the correct method, I just have never seen anyone install underlayment. There is really no point to install underlayment, IMO. If the screws that attach the metal panels are installed properly, the only place a metal roof can leak is the ridge cap if it isn't properly installed.

I guess there could be issues with flashing around the edges, but none of the above should be a problem if installed properly.


----------



## Paul Sweet (Nov 8, 2021)

A structural metal roof can be installed over spaced purlins without underlayment.  It can be installed on a 1/2" per foot or less slope.  It has corrugations to strengthen it aand raised seams that are closed by a special machine to make it watertight, and is typically used on industrial or commercial buildings.

A typical residential standing seam metal roof has to be installed over a deck and underlayment.  The seams aren't watertight, and it generally needs a 4" per foot or steeper slope.  

The old hand-seamed tin (terne metal) roofing required a rosin paper underlayment because the tar in typical roofing felt would attack the metal from underneath.  I don't think that is a problem with galvanized or galvalume metal, and most underlayments are synthetic nowadays.


----------



## Beniah Naylor (Nov 8, 2021)

I guess standing seam vs screwed down panels is an important detail... I assumed that it was the screwed down variety...


----------



## my250r11 (Nov 8, 2021)

If over existing shingles IMHO furred or not the old roof system would be your barrier.  If over unconditioned space it wouldn't need anything. If over conditioned space some sort barrier should be provided. Preferably on the the decking.


----------



## bill1952 (Nov 8, 2021)

So all those conditioned post frame buildings with just purlins across top chord of trusses and insulation piled on liner panels attached to lower chord are not permitted?


----------



## mtlogcabin (Nov 8, 2021)

1507 is for roof coverings applied to a roof deck. The metal roof installed on purlins serves as both the roof covering and the roof deck and an underlayment is not needed. 

[BS] ROOF COVERING. The covering applied to the roof deck for weather resistance, fire classification or appearance

[BS] ROOF ASSEMBLY (For application to Chapter 15 only). A system designed to provide weather protection and resistance to design loads. The system consists of a roof covering and roof deck *or a single component serving as both the roof covering and the roof deck*. A roof assembly can include an underlayment, a thermal barrier, insulation or a vapor retarder.

1507.1.1 Underlayment.
Underlayment for asphalt shingles, clay and concrete tile, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles, wood shakes, metal roof panels and photovoltaic shingles shall conform to the applicable standards listed in this chapter. Underlayment materials required to comply with ASTM D226, D1970, D4869 and D6757 shall bear a label indicating compliance with the standard designation and, if applicable, type classification indicated in Table 1507.1.1(1). Underlayment shall be applied in accordance with Table 1507.1.1(2). Underlayment shall be attached in accordance with Table 1507.1.1(3).

Exceptions:

4.   * Structural metal panels that do not require a substrate or underlayment.*


----------



## Rick18071 (Nov 9, 2021)

mtlogcabin said:


> 1507 is for roof coverings applied to a roof deck. The metal roof installed on purlins serves as both the roof covering and the roof deck and an underlayment is not needed.
> 
> [BS] ROOF COVERING. The covering applied to the roof deck for weather resistance, fire classification or appearance
> 
> ...


My 2015 IRC does not have that exception. But even if it did it would still  come down to the manufacturers installation instructions to say if underlayment is required. Would you ask for the instructions on a plan review?


----------



## Robert (Nov 16, 2021)

Metal roof panels get hotter than other roofing and some companies require (or offer) a membrane that withstands higher temps. Grace is one.


----------

