Quote:
Originally Posted by al76slc
In this town we are permitted to lay 2 courses of shingles over the existing shingles.
Or am I over-thinking this and should I just put up new shingles over the old?
I want to do this once – we have lived here 20 years and I want to stay here
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I ran a roofing/contracting business for a while (back when I was at university).
My thoughts -
Strip all the old shingles off, never lay new shingles over old. If you do:
- they won't seal properly to the old shingles, creating air gaps and moisture zones
- they will look unsightly as the roof's appearance will become uneven as the new shingles settle over the old, uneven surface
- the extra weight can cause sags between joists, depending on how your roof was built
Furthermore, removing the old shingles will allow you to inspect your deck. This is critical and I would never do a roof without this step. Customers would always be amazed at the (often) extensive damage to the sheeting resulting from waiting too long to replace shingles. Often, by the time you have an actual leak inside the house, the roofing sheeting will have rotted in many areas, particularly along eaves, valleys, chimney and wall flashing areas (where the step flashing has been compromised).
Not sure what your building code is there, but here builders will often use thin OSB, just minimum code. If you want to strengthen your roof, don't layer over a sagging/aged layer. It adds extra weight and isn't really effective. Your joists may have a maximum weight allowance depending on span/beam type. Remove the old sheeting and re-sheet with 5/8 spruce. This is a big job though.
As shingles age, they begin to dry out, and will shrink in size and curl at the tips. This is when you should replace them, IMO. As they continue to dry out, the gravel surface of the shingle (which gives it protection and color) will also wear off and the tar paper portion of the shingle is exposed to the elements. It will wear very quickly at this point as the sun and UV break this layer down.
I would definitely remove all the old shingles, sweep the roof clean and then thoroughly inspect the roof deck. Nail (or screw) down any loose boards, resheet where necessary. I always used spruce plywood, not OSB.
Then apply an ice/water shield membrane to the first 3' of the eaves, the rake and along any valleys and around chimneys. Apply regular roofing tar paper to the rest of the roof, using a stapler (hammer stapler with 12mm staples for most roofs). Used painted (color match to shingles) and galvanized drip edge along eaves. Apply the drip edge first over bare wood, then cover with ice and water shield. Some people apply ice and water shield over the entire roof, but this is unnecessary IMO and a waste.
I would buy a premium shingle if you can, the 3D architectural ones are popular and look nice. Since they are often dual-layered shingles, they will last a long time.
Don't skimp on roof vents. The best vent last time I checked is a multi-ridged maxflo roof vent. Make sure all bathroom exhaust vents are properly connected to the appropriate roof vent, and not venting into the attic (which will destroy and rot the sheeting).
On the shingle warranty/lifetime numbers - they aren't very accurate.
Here in Ontario, a 20-yr shingle will last about 14 years. A 25 yr shingle maybe 18ish. If you are in an area with wide seasonal and temperature variations, the shingles will deteriorate faster.
Hope this helps.
P.S. If you are doing the roof yourself, rent a quality pneumatic roofing gun (I swear by Stanley Bostich) and wear a harness...it's a pain but so is landing on the ground after falling off (don't ask my how I know).