Can you re-roof over existing shingles if the attic was built with 2x4 rafters?

Re-roof over another layer is never a good idea. After 30 yrs in construction I can tell you not to. And the warranty from the factory will not cover it if something goes wrong. With it being extra lumpy there is a better chance of being blown off, Not being fastened down right( because you can't tell with so much material under it) If you do get a leak you will have to try and through two roofs not one. If you need a repair they will have to charge extra because of two layers, when it comes time to replace what already may be a weakened roof you will have to have extra heavy tools to rip of twice as much material and try to rip out twice as many nails. When you have to tear it off it will now cost you more because of inflation and all the material to haul away. NOT A GOOD IDEA. SORRY Good Luck

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Tearing down will take so little time and be such a small part of the price of getting a new roof that I don't know why you wouldn't get it done. It's better for the new shingles to go on a flat surface, and it gives the roofers a chance to make sure the sheathing and other components are in good shape, etc etc. Sure, you could probably go right over the existing shingles, but taking them off gives a better result.

There are too many unknown variables to give you a definite answer. Factors such as the pitch of the roof, the span and the 50-year snow load average all go into determining the load capacity of the rafters. That said, 2 x 4s do not have a great deal of margin for error and asphalt shingles are heavy -- I seem to remember 90 lbs to 120 lbs per 100 sq.ft.

I have to tell you, if it were my house, I would not add any more weight on those 2 x 4 rafters. BTW, although its done all the time to save labor, there is no other benefit to installing new shingles over old.

Every roof has a designed weight limit. The limit is affected by truss height, truss lumber dimensions, distance between trusses, sheathing type, roof angle and a bunch of other stuff. Snow weight is assigned based upon where you live. Almost, but not all, roof surfaces are designed to carry 2 layers. If a roofer adds a second layer and the thing starts to sag, he's liable. It sounds like your design might be on the cheaper side and half of the roofers are afraid of causing damage. To get an educated answer, you'd have to hire an architect. The architect would cost more than the tear-off. Personally, I'd go for the tear-off. Safety first. Another resource might be your homeowners' insurance agent.

Even though they are 2x4 trusses they should still be able to support a 2nd layer of shingles. If the house was built only 18 years ago it was most likely built to construction codes. Im no structural engineer but I don't think its going to be a problem. Anyway, you should consider tearing off the old layer, it really is the way to go. Any damaged wood gets replaced, pipes get re flashed, ice/water shield is applied(if necessary) and it is just a much better job. Also, if you shingle over existing shingles the lifespan of the new shingle may be greatly reduced.

The roof rafters were sized to carry the weight of one layer of shingles only. If you were to add another layer on top, it may not show for a year ot two, but after that you may find the weight on the roof is starting to bow the rafters slightly and this means more weight on the supporting walls which I have seen bow from the extra weight. Don't take the gamble, tear off and have one layer to not have these future problems.

if you only have one layer now,yes. 3 layers is getting a little much but 2 isnt a problem. if its done by an experienced roofer itll look like only one layer is on it.all this hype about weight is a good concern but stems mostly from ins companys and banks. ive seen homes with 3 layers on them and no problem but there is a weight concern when you have TOO much weight. the steeper the pitch of your roof the less worry about snow weight. pitch is the angle of the rafter and sheeting attached to it.any thing less than a 4-12 pitch should be tore off,any roof 4-12 or more can handle 2 layers. 35 yrs roofing exp.

Not exactly my area of expertise, but I'll give it a shot anyway (based on what I have heard talking to roofers).

Usually, you can go over 1 layer. But, some roofers will not give you a guarantee if you choose this option. It may also depend on your locality. Some local codes may not allow this.

You did not post your location. Snow load may be a valid concern. I would advise getting estimates/advice from several reputable roofers in your area. Go with the majority of advice. If they advise a tear-off, ask why.

You most likely have a truss system which can with stand the 2nd layer. If you get a qualified licensed contractor to look at it for you he should be able to confirm it.Good Luck