When to replace asphalt shingles?

This 17 year old roof is 12:12 and located in the Pacific Northwest. Moss growth seems to be causing some curling. No leaks so far. Should we have the moss removed again or consider replacing shingles? If we replace, can we add a layer on top of the dimensional shingles or will the moss prevent proper application of another layer?

Thanks for any input,


Is there moss on both sides of the roof, or just the northern side? Are there any cracks in the asphalt shingle that you can see? The cracks in the shingle are usually pretty good indication that you may need a new roof. If there are no cracks than you may be able to squeeze a few more years out of this roof.

When you are ready to re roof, you will want to check how many layers of asphalt shingle you have. If it is just one than you may put another layer over it, if it is more then one, then you will probably have to strip the roof before applying the next layer.

Stripping the roof will also allow you to check for and replace any rotten wood, or plywood that you may have even if the roof was not leaking. It may be possible that moisture has passed thru the shingles rotted some of the wood or plywood on your roof. If that is the case, then you may want to have the roof stripped, and have the rotten wood replaced.

And by the way, 17 years for the asphalt shingle is not bad at all!

Yes it is time for a new roof.
While it may be allowed (by code) to go over the existing roof it is not recommended.
From the pics you posted there appears to be a significant amount of moss on the roof, you don’t want to trap that under your new roof…
Tear it off down to a clean deck so that you can see if there is any deck damage.

Fortunately almost all of the major brands of shingles are algae resistant nowadays, that is no guarantee that you will never get moss on your roof but it should cut it down significantly.

by the pics…yes you are ready. and no you can not put anything over a dimensional shingle.

new roof!

I’d let it go. i don’t see any missing granules.
And like metalguy sez, no cracks.

your moss is caused by very close trees. algea resistant shingles prevent “blue green” algea (black streaks) not moss. my theory is replace it before it leaks…otherwise you will be fixing drywall and insulation if you let it get that far.

Yes, it`s time for a new roof.The moss make it out of order.Install in near the ridge a strip of coper and paint the gutter. The coper kill moss.


I would get some more time out of this roof. Shingles look good.

You can get chemicals to kill the moss.

You roof will function fine. Looks right now would be the only concern.

If you have the cash laying around to do a new roof then go for it. If you do not then check into ‘American Roof-Brite’.



don’t even touch it!!
you will have to rip it anyway. its too wet to go over with way too much moss. you can probably get 3 to 5 more years out of it if you stay off it. plan to trim the trees before the next roof it will keep it nicer longer.
algea resistant shingles are a must and you may want to add zinc strip if that is a real wooded area.
if you want peace of mind do it now its ready.

Looks like at least a 30 year laminate shingle and at 17 years it’s time for a new roof? The moss more than likely took life out of the shingles and yes it could last another 3 year years pushing it to 20 years but if you got high wind and rain you could be in trouble.

Moss can and will eat right through your shingles and also eat right through your decking.

Find myself telling home owners in older homes 30+ years that they need to keep all the trees back away from growing over the roof deck. Usually the husband says, “I told you so” while the wife says, “I love those trees”.

The shade the trees give off is good but when the trees are up and over the roof and touching your asking for trouble.

Most shingle manufacturers put a 10 year warranty against staining or algea. At year 11 if your roof has staining the manufacturer is no longer on the hook.

Zinc strips work too and to my knowleage will work much longer than year 10!

Thanks to everyone for all the input. I can’t see any cracked shingles so I think we will probably get the roof done soon, but not worry about another year. We will definitely add zinc strips for the next roof.

The trees are not touching the roof but the moss is present on north and south parts of the roof near / just under the trees. The trees are not on our property but looks like we will have to get them trimmed anyway.

Its time for a roof. Apparently you have water and moisture problems, by the downspouts really bad, they should be extended down to the lower gutter farther to keep this problem from happening again, apparently you have large upper roof water run-off. The moss problem at the shingles is due to water sitting/moisture, also in my opinion the bonds are the shingles are wrong allowing the moisture to sit there, also is roofing vented properly? , the shingles are beyond their useful life,
the curling, granual loss, discoloring, mildew, etc.

I would not recommend roof over, The new roof will only lay as the existing,which will cost you, also my opinion
you could have roof decking issues.

Typically depending on different areas shingles usually only have a life of 15-18 years.

I have never done a shingling job over exsisting shingles not I ever intend to. I just do not like the idea or the picture I get in my mind of what it would look like. I have stripped roofs with several layers of shingles and they all were pains in azzz. In my opinion it will be cheaper in the end to strip it and start over.