Should you strip your roof completely, or is it OK to re shingle over top of the old shingles? It is faster to put new shingles over the old ones. This can save on the cost of labour and disposal fees at least for the time being. You will pay more when the new roof needs replacing because the next time your roof needs replacing it will definitely need to be stripped and the labour and disposal fees for two layers of shingles will be higher. So if you shingle over top of the old shingles you are only going to save if you move and don't have to pay for another roof.
It is the save some money today and pay a lot more down the road when labour rates and disposal fees are higher.
When can you overlay shingles?You can overlay roof shingles when there are no existing problems with the roof sheathing or structure and the existing roof shingles lay flat.
No matter what method you choose, roof ventilation must be maintained. For your roof to last you need cross flow from your soffit vents to the ridge vent. Failure to have proper cross flow will shorten the life span of your shingles.
When is it a bad idea to overlay shingles? If you live in a cold climate and the existing roof does not have an ice and water membrane. This membrane offers added protection against roof damage from ice dams. If the existing roof has curled shingles or any other problems that would prevent the new shingles from laying flat. If there is more than one layer on the roof now. If there are any areas that that you suspect may have fungi, dry rot or other problems with the roof sheathing. Any sagging of roof sheathing or rafters should be repaired now requiring a complete tear off. If the roof feels spongy when walking on it a complete tear off is required to repair this roof problem.