• Welcome to the new and improved Building Code Forum. We appreciate you being here and hope that you are getting the information that you need concerning all codes of the building trades. This is a free forum to the public due to the generosity of the Sawhorses, Corporate Supporters and Supporters who have upgraded their accounts. If you would like to have improved access to the forum please upgrade to Sawhorse by first logging in then clicking here: Upgrades

Shingles over shingles

I

Inspector Gadget

Registered User
Joined
Mar 5, 2020
Messages
795
Location
New Brunswick
This is one of those threads where I think I (sadly) know the answer, but....

I've seen some roofing contractors do new shingle jobs by simply slapping up the new stuff over the old (to save removal/refuse disposal costs). It irks the heck out of me, but I don't see anything in Code to prevent it, as long as nails penetrate the sheathing 12mm as required.

Any disagreements? NBC references, please.
 
Shingles have to be installed to the manufacturers instructions. 9.26.2.2

I would assume the manufacturer does not list old shitty shingles as an acceptable substrate.

Also, 9.26.6.6 underlay beneath shingles.
 
California Residential Code
Canadian Code is different.
I noticed this after I posted .... I really should pay attention .... Oh well it might be of value to the American audience.

R908.3 Roof replacement. Roof replacement shall include the removal of existing layers of roof coverings down to the roof deck.
Exception: Where the existing roof assembly includes an ice barrier membrane that is adhered to the roof deck, the existing ice barrier membrane shall be permitted to remain in place and covered with an additional layer of ice barriermembrane in accordance with Section R905.

R908.3.1 Roof recover. The installation of a new roof covering over an existing roof covering shall be permitted where any of the following conditions occur:
  1. Where the new roof covering is installed in accordance with the roof covering manufacturer’s approved instructions.
  2. Complete and separate roofing systems, such as standing-seam metal roof systems, that are designed to transmit the roof loads directly to the building’sstructural system and do not rely on existing roofs and roof coverings for support, shall not require theremoval of existing roof coverings.
  3. Metal panel, metal shingle and concrete and clay tile roof coverings shall be permitted to be installed over existing wood shake roofs where applied in accordance with Section R908.4.
  4. The application of a new protective roof coating over an existing protective roof coating, metal roof panel, metal roof shingle, mineral surfaced roll roofing, built-up roof, modified bitumen roofing, thermoset and thermoplastic single-ply roofing and spray polyurethane foam roofing system shall be permitted without tear-off of existing roof coverings.

It has generally been accepted that two layers of shingles are permitted. The existing roof covering shall be free of mold, moss and rotten shingles. The existing shingles can't be curled and several spots must be opened to demonstrate that there is just one existing layer.

Now here is a common rub. Contractors want to install just shingles and no underlayment. (Not all contractors....not even many....just the real slugs) They show me the definition of a roof assembly and point out that roof covering and underlayment are separate components. The contention is that R908.3.1 only refers to roof covering so they are not required to install underlayment. They also point out that there is a layer of underlay under the existing roof covering.

I counter with the premise that the existing underlayment is old and brittle, will be damaged with new nails and being dried out, will not seal around the new nails. Then I ask them to install two layers to thwart telescoping the old roof covering through the new shingles. They go away mad....but at least they go away.

ROOF ASSEMBLY. 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 includes the roof deck, underlayment and roof covering, and can also include a thermal barrier, ignition barrier, insulation or a vapor retard

This was all strictly residential. Nonresidential is one layer unless an engineer approves a second layer.
 
Last edited:
ICE said:
R908.3 Roof replacement.
Click to expand...
Canuck codes in play, good Sir.

Plumb-bob said:
Shingles have to be installed to the manufacturers instructions. 9.26.2.2
Click to expand...

Thought of that, but the Canadian Asphalt Shingle Manufacturer's Association has a bulletin that essentially says "Meh."

Plumb-bob said:
Also, 9.26.6.6 underlay beneath shingles.
Click to expand...

You may be in BCBC. NBC 9.26.6 has two clauses, none of which seem to give much help.
 
ICE said:
I should avoid the Canadian threads when a code is involved. I can do better.
Click to expand...

Eh, sometimes things cross over. NFPA standards being one, for example. I've picked up a couple of nuggets surfing US-based Code discussions on this forum that delve into NFPA.

tmurray said:
To my knowledge, there is nothing to prevent multiple layers of shingles on a roof.
Click to expand...

This is the conclusion I've come to. I was watching a ... contractor who is not known for quality, shall we say ... re-roof a commercial structure in a neighbouring jurisdiction by slamming new shingles over old. It looks like total crap from my view, but absenting a climb on a roof that I'm not authorized to climb on, I couldn't see anything wrong with it Code-wise.

That got me to thinking.

The *only* angle I can really entertain would be asking for a certification from the truss manufacturer that the assembly can handle the extra weight. That's quite the stretch though, and not one I'm willing to make.

So I guess I am going to have to file this in the "We can only enforce Code, not best practice" bin and move on.
 
Inspector Gadget said:
The *only* angle I can really entertain would be asking for a certification from the truss manufacturer that the assembly can handle the extra weight. That's quite the stretch though, and not one I'm willing to make.
Click to expand...

Not sure you are out of place for asking for verification that the roof truss system will support, 2 layers of roofing material.

I have a final inspection on a indoor archery range tomorrow that I had to take over from another person, with the price of metal ceiling material I looked at possibly having it dry walled. When I had the truss manufacture pull the specs, the dry wall would have exceeded the specs.

But if your thought is that the roof design is over kill and would meet the loads, then no need to wander down that path.

On a second note, it will be 2 years next month that I had the roof on the townhouse re-done. Old unit from 1875, it had 5 layers of asphalt shingles on it.

Took forever for them to pull it off, 2/3 of the day.
 
For what it's worth we are now looking at whether this is something we need to look at.
A square (100ft2) of shingles is 180-240 lbs.
Typical snow loads in our area are 42 psf.
Hence a layer of shingles adds 18-24lbs/ft2 to the roof. Not insignificant.
 
Inspector Gadget said:
For what it's worth we are now looking at whether this is something we need to look at.
A square (100ft2) of shingles is 180-240 lbs.
Typical snow loads in our area are 42 psf.
Hence a layer of shingles adds 18-24lbs/ft2 to the roof. Not insignificant.
Click to expand...
May want to check your math...

240lbs per 100 sf = 2.4 lbs per sf = 2.4PSF
 
tmurray said:
If the worst thing you do in a day is look a little stupid, that's a pretty good day!
Click to expand...
One of the most salient pieces of advice I have gained in this life was from a functioning alcoholic working as a carpet installer.

"Professionals only make mistakes that they can fix."
 
You must log in or register to reply here.
Top