Proper Method to apply 2nd Layer of Shingles

First and foremost I am against installing multiple layers of shingles.



Why?........well in the first place your roof is most likely at the end of its life cycle. Signs to look for are blistering, brittleness, breaking, cracking and curling of the shingle.

Why would anyone want to put a new shingle on an inferior substrate baffles me.



Due to the above mentioned, adding multiple layers increase the amount of radiant heat of the roof which in turns reduces the effectiveness of your attic ventilation which is often under vented in the first place.



Since the 70’s the use of trusses are more common. Trusses are often spaced at 2 ft. intervals with a layer of 7/16” – 1/2” plywood or OSB. Ever wonder why you see dips in new roofs……well now you know. Let’s just add another layer of shingles to the problem and see what happens. Anytime trusses are used I advise clients to check with the manufacture to insure the truss is designed to carry the additional loads. Many trusses use 2x4’s for their top & bottom chords as well as their webbing.



And while I am on trusses……..they should NOT be nailed to interior walls as most framers do, nor should they be cut without consulting with the manufacture first.



For stick built roof system I suggest the rafters be inspected by a professional prior to adding additional weight to it. Some states have little to no code enforcement when constructing or repairing a home…….as was the case with my brother in KENTUCKY. ( I fixed his leaky roof, now I have to fix the sag in it……and yes I am slamming Kentucky inspectors or lack thereof)



Warranties are only as good as the installation. Manufactures and claim adjusters like to point out such things when Harriet Homeowner decides to file a claim for a new roof due to hail and wind damage.

Which brings me back to the multiple layer argument…….multiple layers to not do as well during storms. Usually because of them not being sealed as needed….and the original substrate (shingle) should have been replaced in the first placed instead of overlaid.



The main advantage I like about tearing off is that it allows one to observe any damage to the decking material. On older homes the decking is probably 1x6 fir or knotty pine. Knotty boards have a tendency to be weaker…….an engineer thing that I don’t want to get into now but it is something to pay attention to.



These boards often have spaces between them now which can attribute to their deterioration as well as nails pushing their way back out, often into the shingle. Any lose or bad boards should be fixed or replaced.



For those that are after my heart and do tear down to the decking, pay attention to the soffit and eaves area. Inspect for moisture damage and repair same as needed.

Chimneys are probably one of the most common areas where leaks occur on a roof. Make sure the structure is sound in this area and seal around same with an approved sealant.



Inspect pipes coming out roof and replace the boots around pipes when re-roofing. Boots are very cheap and worth the time to replace when re-roofing.



If you don’t have soffit and ridge vents then now is the time you want to do such.

With all said, if you are going to put on a second layer then the proper method is as follows.



The first layer should be a 3 tab style shingle. Adding any additional layer over a dimensional or architectural shingle is a total waste. The method I am about to go over is for 3 tab shingles only.



The nailing pattern, and I did say NAILS and not staples……should be 4 along the tab line and 2 at the butt edge (top) of the shingle. When using air guns, set the air compressor at an appropriate p.s.i. rate so as not to over penetrate the shingle.



The second layer of shingles should be the same vertical length. Placing the butt (top) edge of the new shingle against the tab (bottom) edge of the old to insure a more evenly and flat course for each shingle. This will require cutting the shingles into half strips for the first / starter course. By using this method your roof is likely to do better when severe weather comes along. While the best method is to tear off and start over…….if one were to put a gun to my head then this is the proper way to do it.



Again, this method is based upon the assumption that the roof has not deteriorated to the point where shingles are blistering, curling, cracking etc.