Roofing/strap vs no strap metal roof
QUESTION: I am getting estimates to have a metal roof put on over the shingles on my single story double wide.Two contractors said to put the metal on with no strapping.The other one said to use strapping.The shingles are ok but discolored so I want to put metal on.Which way is better?FYI I live in Northern NY near the Canadian border if that matters.Just for giggles one guy said there is 28 squares on the roof and the other one said 32 squares.I would like to see his tape measure.Thanks...Bob
Sorry for the delay in my response, hopefully it is still timely enough for you.
I am not entirely clear on what the contractors are referring to with regard to strapping. If this is form of 'Z' channel attached to the roof deck first to elevate the metal panels, then it is not a bad idea. This will serve a few functions that will improve the overall final appearance of the roof surface and will keep the metal panels separated from other surfaces which can hasten deterioration negatively affecting overall service life, particularly if they are planning on installing the metal panels over the shingles.
If this is not the 'strapping' concept, please get back to me.
It's probably worth measuring your square footage and make sure that they are not altering what ever roof ventilation system you currently have, unless they are improving it.
Bruce Ryan II, RRO
Professional Roof Consultants, Inc.
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QUESTION: The strapping that they are talking about is usually a 1 inch by 3 inch piece of wood screwed into the shingles then the metal is put on.How long will the metal usually last if it was put right on the shingles with no strapping?Thanks....Bob
Okay Bob, that is what I was thinking, just not the wood part. The wood (hopefully pressure treated) should be applied over the shingles over one or two layers of underlayment depending on the slope of your roof. The strapping should be fitted with shims under each fastener that anchors through the existing roofing that raises the strapping sufficiently to allow for drainage under the battens. Or maximum four foot long battens should be used with butt ends gapped 2" to allow for drainage; the former option is better, and any manner of strapping is better than not.
It is difficult to say how long the metal panels will last when applied without strapping as it is largely dependent on how much moisture is present between the substrate and the panels. Moisture should be anticipated frequently and will originate from condensation and general leakage (metal panels are not watertight, much like shingles). Depending on the frequency and duration of the moisture, corrosion will be faster or slower. If this is a direct application over the shingles (without underlayment - which is not recommended) then the granules of the shingles will accelerate the corrosion by abrading the protective coatings with thermal cycling. Depending on the coatings and their quality (protective paint and galvanizing) these rates will vary.