Steel roofing input needed

[T1320T]

Hey guys.. I'm new to this forum (but have been watchin it for a while). Due to a recent tornado, I'm going to be adding a 14'x26' bay to my garage & will be installing a steel roof on the entire structure, 50'x26'. I've done some looking, but still have a few unanswered questions...:
1. It seems like I can install wood furring strips to the trusses over the existing shingles then attach the steel roofing to the furring strips... BUT everyone I've seen install the furring strips installs them perpendicular to the trusses which basically blocks the airflow. What's the deal? What's the proper/most efficient way to go?
2. Should I insulate between the exsting shingles & the new steel roof?

Thanks for any input

 

[Wingnut65]

Welcome to GJ.

The strength in a metal roof panel is in the length of the corrugations or ribs. It is very weak widthwise, allowing some panels to curl if not fully supported. The method of installing furring strips across the trusses is to spread the support across the span of the roof and the width of the weak part of the roof panels. Running the furring vertical would give roof that would be very uneven, sagging and weak to walk on. Not Advised. Not sure if the insulation is really necessary if the garage is alreay insulated.

Also, check with your local building department to see what they will require for a re-roof project.

 

[T1320T]

Thanks for the input Wingnut. I understand how the strength of the steel panels work. It just seems that running the furring strips perp to the ribs eliminates the majority of the airflow... except the airflow that travels through the v shaped rib itself, which isn't much.

 

[tdkkart]

So why use furring strips at all?? I had steel installed on one of my sheds, they attached the steel directly on top of the old shingles. Seems to work just fine, and I really don't see any reason why it shouldn't.

 

[1969]

Why are you concerned about airflow? Do you have a ceiling in your garage at this time ? If you have an enclosed ceiling, then the air flow needs to be thru the attic. Being concerned about airflow between the metal roof and the existing shingles is moot IMO.

 

[csp]

I was wondering the same thing about needing airflow.

 

[LIVELY]

CHECK WITH YOU INSURANCE COMPANY
ALOTof them will fight you on a claim [if you have one later on]--if the shingles are left on

they will say things like--TOO MUCH OF AN UNEVEN SURFACE UNDER THE METAL/SHINGLES ARE TOO MUCH OF A FIRE HAZARD UNDER THE METAL/TOO MUCH WIEGHT WITH THE SHINGLES UNDER THE METAL/ NOT SCREWED DOWN INTO THE WOOD BECAUSE OF THE SHINGLES UNDER THE METAL

ALOT OF LAME EXUSES FOR REFUSING TO PAY BUT-I HAVE DEALT WITH ADJUSTERS FOR OVER 35 YEARS AND HAVE HEARD ALOT OF THEM

 

[W-Cummins]

I can't imagine why you would ever consider putting a new metal roof over shingles. I don't even really like to put 2 layers of shingles on, let alone putting a nice metal roof over some crummy worn out shingles! Tear off the **** and start clean.....

William....

 

[Justanoldguy]

Tear off the **** and start clean.....

ditto to that..

 

[MScott]

So why use furring strips at all?? I had steel installed on one of my sheds, they attached the steel directly on top of the old shingles. Seems to work just fine, and I really don't see any reason why it shouldn't.

I am planning a steel roof for my new house and garage. The steel people told me that they do not warrantee the roof if you attach it directly to the plywood roofing; you must use the horizontal strapping. Since this will be new construction there was no mention of applying it over old shingles.

 

[Steve in Mi]

A couple of thoughts, can't really back it up with facts/numbers.

A guy I know put a new steel roof directly on top of asphalt shingles, no furring strips between them at all. Steel will last well as long as the exterior coatings are in tack. Long term expansion and contraction of the steel with temperature change, I think, will cause a sanding ('shingle grit' movement against coating at the interface), rusting of the less protected steel and premature failure. The difference in thermal coefficient of expansion for steel vs. asphalt shingles will establish the relative motion between the two materials. I'm sure that a NUMBER could be established as a best estimate and conjure that to/with rust on the underside to arrive at some estimated reduced service life for the steel roof. I think of it as a real coarse sandpaper, maybe 36 grit, rubbing/grinding on the steel roof coating (whatever it may be, paint galvanize, etc.) and with enough time wearing through the coating. Longer still may result in wearing away the steel itself. Who knows?

Rather than get into a bunch of numbers mumbo jumbo, I personally would not put the two in contact based on my gut feel.

As for the leaving the existing shingles and adding furring strips between I'm of the opinion expressed by W-Cummins, the shingles are unnecessary weighty baggage that would better be shed for a new, clean, crisp start on re-roofing.

 

[portcity]

i would not add to the top of the shingles. first off, your roof probably isnt designed for that load. Thats the main reason im against double shingles also. Secondly, as the shingles get older, they tend to curl on the end, possibly pushing on the metal. Steves theory makes good sense also, ive never thought about it that way. As for the ventilation, I always leave about a 4-6" gap between the stripping where they **** together, for air to travel through.

 

[tkiranch]

I would think that the old shingles would have and keep some moisture. which could rot the deck below it. I am a pretty cheap guy and try to cut corners alot. I would put the metal over the shingles if the building was fairly old and not going to be up for more than 5-10 years.
In all practicality, I would remove the shingles and start fresh if the building was worth it. My delema right now is after a barn fire, do I replace the metal on a building that is no good to begin with or just fix it, and add another 4 grand for a whole new building.

good luck

 

[T1320T]

Thanks for the feedback guys.. good points. My first plan was to tear off the existing shingles, but, that would incur dumpster fees along w a lot of labor. So, I opted to install over the existing shingles, also, the shingles are not shot, heck, they're only 8 yrs old, but some of them were snapped or ripped off during the tornado.
From the research I've done, the reason you install furring strips on top of the shingles is to give you a good flat surface to attach the steel to. If you attach the steel to the shingles, you'd end up w a wavy, uneven surface. Also, like Steve posted, there will be abrasion between the 2 surfaces.
The reason I asked about the airflow is bc it just seems like there should be airflow between the 2 surfaces. Also, my ceiling is insulated.