Figuring out re-roofing needs for low slope roofs starts with knowing what materials make-up your roofing.
One or more test cuts can determine the materials and deck type. Most low slope roofs have the insulation immediately above the deck and the waterproofing layer on top of the insulation. Sometimes there are multiple layers of roofing systems. The deck type usually can be determined from underneath (you may have to pop a few ceiling panels). See our test cut page for examples of what different material looks like. Anyone can cut into a roof but most roofs need to be patched by a professional. For example – asphalt based patching products are chemically incompatible with most single ply membranes.
Next step is to determine the condition of the materials. This determines whether or how much roofing material needs to be removed or replaced.
The no risk, high dollar approach is to rip off everything, inspect and replace damaged decking, then go back with whatever roofing system you choose. This option is favored by many roof consultants and roofers as they are guaranteed there is no moisture remaining in the roofing or damage to the deck. Under certain conditions this is the proper approach. Under other conditions this is wasteful of the owner’s resources and un-friendly to the environment. Removing the roofing to the deck incurs larger costs for the labor and disposal of the old roofing material as well as the usual installation of thicker insulation to meet your state’s building code.
How to determine the condition of the deck:
If the deck is visible from underneath that is the first place to check. Rust, flakes, soft spots, sagging, discoloration all point to deck problems. The other way to check is through test cuts. Take a 2′ x 2′ test cut and examine the deck. Some deck issues like rust will not show up from the bottom until too late. Examining the deck from above takes some experience. Metal decks should be inspected for rust and poked with some gusto to make sure the metal is solid. Wood decks should be poked also to make sure they are solid. Concrete decks are trickier. Some concrete decks utilize lightweight concrete or gypsum to provide the slope in roof. Some roof decks are completely lightweight concrete or gypsum. There are some very strange combinations of materials on some older buildings. Once in a while ponding on a roof can point to deck problems (usually the problem lies within the roofing system).
If there are major deck problems the entire roof should be removed and the deck issues addressed. If there is a small area with problems it may be cost effective to only remove the roofing in that area, replace or repair the decking, then install new roofing material to the height of the adjacent roof area. If the condition of the deck is suspect and the new roofing system you choose utilizes mechanical fasteners (or adhesive directly on the deck) to secure either the insulation or waterproofing layer a pull test should be done. This involves screwing a fastener into the deck and utilizing equipment that measures the force (lbs.) required as it pulls the fastener straight out of the deck. The roofing manufacturer of your intended system should be able to perform the test and tell you what length warranty is available based on the test. The same principal applies to a pull test with adhesive. A pull test is especially important for metal roof overlays. The pull test also may help decide the fastening pattern required to meet certain wind uplift scenarios.
Why does it matter if there moisture in the roof?
Moisture in the roofing system is bad. It attacks a roofing system in many ways. It works its way down to rust or rot the deck. It turns into gas when heated and attacks the seams, penetrations and other weak spots trying to escape and causes earlier failure. It freezes in the cold and lowers the thermal resistance, increasing heating costs. The freeze/thaw can cause excessive shifting of the waterproofing layer causing earlier failure. If moisture is in a roofing system it should be removed. However, a cost saving approach can be taken to remove just the layer or area that contains the moisture.
How to tell if there is moisture in the roof?
The answer to this depends on how many and what type of roofing system you have. Test cuts can be done at different spots on the roof. This is the only certain way to tell if there is moisture in the roofing system(s). Test cuts should be done anyways to determine the make-up of the roofing system(s). Since water runs downhill, test cuts should be taken near drains and gutters, as these areas are the most likely to contain moisture. Test cuts by themselves don’t pinpoint the exact wet areas. For that a moisture scan is needed. We are familiar with two main types – the nuclear scan and infra-red scan. Infra-red scans are done in the early evening as the air temperature cools down. Any wet areas in the roof will retain heat longer than the surrounding areas and an infra-red camera can pick up the thermal differences. A nuclear scan can be done at any time of day. To do the nuclear scan the roof is sectioned off into an imaginary grid, usually 10′ x 10′, and at each intersection point the device is placed on the roof and a reading is taken. The higher the reading the more likely moisture exists. Both moisture scans should be confirmed with test cuts.
Test cuts need to be patched properly to last. Materials similar to the to the roof’s waterproof layer need to be used (asphalt cannot be used on rubber, etc.) and any asphalt patches should use embedded fiberglass mesh. Plastic based membranes (TPO, PVC) should have the seams of the patch heat welded. If the roofing system is under warranty it is usually a requirement to have certain qualified contractors do any work (check the warranty language). The infra-red and nuclear tests have their limitations. The infra-red, which is the least expensive and easiest, does not work well on ballasted single ply systems or show moisture well in anything but the top roofing system if there is more than one. The nuclear is more labor intensive hence costs more. If there are multiple roofing systems then utilizing both provides the most accurate reading.
If there is moisture in the roof what has to happen?
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 an “overlay” by replacing all wet areas with insulation of a thickness that matches the height of the surrounding area and 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 “overlay” 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).
Can vents be installed to dry the roofing material?
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.
What conditions require the complete removal of the roofing to the roof deck?
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).
A building has two roofing systems-what is the most cost effective way to prepare for a new roofing system?
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).
Update about exposing the insulation…
If any insulation is exposed the current building code may require the new roof assembly to be increased to the current energy code. This R value could be 30 in MA or 25 in RI.
A building has one roofing system-what is the most cost effective way to prepare for a new roofing system?
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.
Before removing the roofing system(s) figure out what structural element generates the slope in the roof.
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 1/4″ per foot. The more slope there is the better the roof will perform.
Ponding water considerations.
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.