Any wet roofing materials need to be removed and if those wet materials are next to the deck then the deck needs to be inspected for damage and repaired, overlaid with new decking or replaced. With that stated the issue is whether just the wet materials be replaced or does the entire system(s) need to be removed. While there are some other considerations that will require the complete roof removal (addressed later), it really boils to down cost. Figure out which type of new roofing system you want to use then obtain a price to install this new roofing system as a “go over” by replacing all wet areas with insulation of a thickness that matches the height of the surrounding area with a new system installed on top of the old. Then obtain a price to remove the entire roofing system(s) and install a new roof system. If the difference between the “go over” and complete replacement to you is substantial, just replace the wet areas and install the new roofing system over the old. If the difference to you is not substantial – go with the complete removal of the existing system(s).

Maybe. There is a difference of opinion in the roofing industry about the efficiency of vents drying flat roofs. Roofing systems for flat roofs are designed to stop water from coming into a building and further to provide energy savings through insulation. Minor consideration is given to what happens within the roofing system if the waterproofing layer fails. To keep the roofing system in place during high winds the layers of roofing are bound tightly together with no air flow. The lack of air flow inhibits the performance of a vent. Vents installed in the field of the roof will provide a path for some moisture to escape. Will this percentage of moisture escaping be meaningful? Depends on how many vents, how many roofing layers and if the leaks causing the moisture were stopped. If the roofing system is a single-ply membrane and the attachment method of the membrane is with bars/plates instead of adhesive, vents become much more effective, and in some cases two way vents will assist in securing the membrane to the roof.

1. If there is phenolic foam insulation next to a metal deck. This rusts the deck out. There may be money from lawsuits available. Contact a national company called Roof Options (877)460-7663 www.roofoptions.com if you find yourself in this situation.
2. If the interior of the building generates moisture (pool, sauna, etc.) and there is no vapor barrier.
3. If the height of adding a new roofing system on top of an existing one causes problems at the roof edge, with roof top penetrations or units and it would cost more to raise them than to rip the existing roofing.
4. If there are structural issues within the roofing system such as bad blistering or buckling insulation, or ponding water. In some cases buckling insulation can be mitigated by fastening the buckling areas (as long as no moisture is present).
5. If the additional weight of another roofing system will cause excess load on the structure (a licensed engineer should determine this).
6. If there is expanded polystyrene insulation (like foam coolers from a grocery store) installed next to the roof deck (which during a fire melts at low temperatures and can cause the flame to spread).
7. If the manufacturer of the roofing system being installed requires the existing roof to be removed. This is a requirement of some manufacturers to obtain a long term warranty (25-30 years).

If there is no moisture in the roofing systems and none of the previously stated conditions exist that would dictate a complete removal of both systems, the next step is to check with the town building inspector. Does the building code limit the number of roofing systems? Typically no more than 2 roofing systems can be used. There are times when the building inspector may not consider the addition of another waterproofing layer (single ply membrane or coating) to be a 3rd roofing system. Check and make sure. The top roofing system can be removed in its entirety and a new system can be installed on top of the bottom roofing system. If it is possible to remove just the waterproofing layer of the top roofing system and leave the top roofing system’s insulation in place then attempt to do that as it saves on removal and disposal costs as well as future energy costs. Most of the time removing the waterproofing layer will cause some damage to the insulation underneath and something should be placed on top of the old insulation to provide a sound substrate for the waterproofing layer. This usually is another thin layer of insulation or a manufactured board like DensDeck. Check with the manufacturer of the roofing system to be installed as to their requirements (which may change depending on the length of the warranty being requested).

If there is no moisture in the roofing system and none of the previously stated conditions exist that would dictate a complete removal of the system then just check the waterproofing layer. Can the waterproofing layer be left in place? If it is a single-ply membrane then most manufacturers allow the membrane to be left in place providing it has no contaminates (oil/grease) and is sliced into a 10′ x 10′ grid. The two explanations we have heard for slicing the membrane are that it either relieves the tension in the system or it prevents a vapor barrier from being in the middle of the roofing system. (Side note: we have seen a few instances where a polyethylene vapor barrier was actually placed on top of an existing roof and new insulation/membrane installed above. This invariably kept the bottom roofing system drier and even prevented water leaking on the tenants, but spread the moisture throughout the top roofing system – likely causing it to fail sooner.)

If the single-ply waterproofing layer cannot be left in place or sliced and left in place then it should be removed. If the system is asphalt based without gravel (modified bitumen or bald asphalt) then usually no extra measures need be taken – another roof can be placed on top. If the system is asphalt based with gravel (asphalt/tar/pitch and gravel) then the gravel should probably be removed. We have seen many roofing systems where the gravel was left in place, insulation mechanically fastened and EPDM installed, with no adverse effects. We have seen a few instances where the same scenario produced tenting fasteners – but it is hard to say if the fasteners tented due to a poor roof deck, water in the bottom roofing system or the gravel shifting a little as time went on. If a second roofing system is being put in adhesive over an existing roof with gravel then the gravel and dirt must be wet vacuumed off to obtain a good bond.

All roofs need slope to move the water to drains, gutters or scuppers. Most of the time the slope is built into the roof deck, which allows flat insulation/roofing to be installed. In some cases the deck is flat and the slope is built with roofing materials. In our experience if the roof is not sloped by the roof deck, then tapered board insulation or sloped lightweight concrete or gypsum was used. If those materials are removed then whatever new roofing materials installed will have to generate the proper slope, which is a considerably more expensive proposition. Checking the slope can be done by visually inspecting the roof deck from underneath. If a visual inspection is not possible or does not produce conclusive results then take 2 core samples, 1 near the peak of the slope and 1 at the bottom of the slope (where the roof drains) and measure the insulation – if the thickness is the same then the slope is in the deck. When the deck has lightweight concrete or gypsum the slope could also be created by that material – again 2 test cuts should determine this. Creating proper slope in a low-slope roof is essential and usually required by code to be minimally 1/8″ per foot. The more slope there is the better the roof will perform.

Ideally there should be no standing water on the roof. Most manufacturers require standing water to be gone in 48 hours. Standing water increases the weight on the structure (8.35 lbs. per gallon) potentially to a collapse. Standing water also lowers the lifespan of the roofing system by constantly attacking seams and flashings. An attempt should be made to determine why the water is ponding. Is there something stopping the flow of the water to its proper drain point or is the roof slope insufficient in that area? It may be easy to fix if something is blocking the water by placing tapered insulation “crickets” to divert water around a roof top unit, raising a pipe or conduit off the roof, lowering the lip of the drain/gutter/scupper (all of which should have been done during the installation of the roof). If the ponding water is due to the lack of slope, and it is a small amount, an inexpensive solution is to strip in the seams and double flash the penetrations that stand in the water. For larger amounts of water there are typically two approaches – install a roof drain to remove the water directly from that spot or install tapered insulation to force the water to an existing drain, gutter or scupper. From a roofing perspective installing a roof drain is fairly simple and inexpensive. Installing a new roof drain in an existing building can be more challenging from a plumbing perspective.

Installing tapered insulation is slightly more difficult and has some issues. Sometimes installing tapered insulation can just result in the water ponding in a different place. Consideration has to be given to how high the insulation will slope up to if it is near the perimeter of an area. The two biggest ponding problems we encounter are created by multiple roof drains next to each other at the same level (tapered insulation in a diamond shape should be installed between them) and scuppers along a roof edge (tapered insulation in a half-diamond shape should be installed between them along the edge). Again the roof deck should be inspected to make sure the structure itself is not failing. We have seen the sides of a structure sink into the ground and change the slope of the roof.