So when do you stop patching and start planning to replace the roof? Obviously one good hint is when, no matter how many times you call the roofer out, the roof still leaks. An aging roof can only be repaired for so long before it has to be replaced. Once the roof becomes brittle and starts to split, there is no patch that will permanently cure it. In that case, a better strategy is to perform emergency repairs as they are needed instead of wasting whatever roofing money is available trying to save a failed roof.
Waiting too long can result in an unsafe condition in the building and an extremely high cost to fix the roof. One warehouse had repairs done to the roof for about five years because the building was unoccupied and the owner did not want to spend the money to replace the roof. Unfortunately, the leaks were so pervasive that the steel deck rusted through underneath the roof. So, while at one time a relatively inexpensive re-cover would have been an option, now the owner needed a complete tear off and, worse, a total replacement of the steel deck to make the building safe again. Correcting this mistake will cost about four times what the re-cover would have been five years ago. To make matters worse, the building is now occupied and replacing the deck may lead to terrible complications with the tenant.
If you have a consulting architect or engineer survey the roof to begin with, you can take their opinions of probable construction costs and determine if it is time to replace or re-cover. Take the probable cost to replace the roof and divide it by the number of years you would like the new roof to last (10 for a basic roof with standard grade materials, 20 for a well-designed, well-installed roof using good materials). If this number is close to or less than the cost of the repairs, don't waste your money. It's time to replace the roof. Your consultant can help you plan the best option for re-covering, replacing, applying a coating or continuing to repair and hope for the best.
Long-term repairs can stretch the life of the roof if they are done promptly and according to good roofing practice. Ignoring the roof will not make your problems go away — they will just get bigger and more expensive to fix. Repairs should be done in conjunction with a well-managed plan for roof inspection and maintenance to be sure the repairs catch little problems that if left alone will cause the roof to fail sooner. Remember, all roofs will need replacement eventually. Knowing when that will be is the last step in the repair sequence. If you do not feel comfortable predicting the demise yourself, a qualified consulting architect or engineer can help steer you in the proper direction by providing regular inspections, moisture surveys, and plans and specifications for repairs, re-covers, maintenance coatings and replacements. 
Karen Warseck, AIA, LEED AP, is president of Building Diagnostics Associates, a Hollywood, Fla., architecture firm. She is a contributing editor for Building Operating Management.
Assessing and Repairing Roof Leaks and Damage
When Repairs Aren't Enough and the Roof Needs Replacing or Recovering
