Virtually any caulk will stick to a leaky gutter once the gutter is dry and clean. The best caulks, however, will provide years of service, won't harden over time and will remain flexible enough to move with the gutter's expansion and contraction due to temperature changes. For these reasons, the following caulking compounds are some of the best options to fix a leaky gutter.
Butyl Rubber Caulks
Butyl rubber caulks, marketed by DAP as their Butyl-Flex line, is an excellent choice for repairing leaky gutters. The Butyl-Flex caulk applies well in cold and hot temperatures. The caulk adheres quickly and as it cures, it remains flexible. These features mean that the caulking compound will move with the gutter as it expands and contracts due to changing temperatures. It will also maintain a watertight seal.
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Polymer Based Caulks
In the category of professional caulking compounds, the polymer and tri-polymer caulks are excellent choices for repairing leaky gutters and sealing new gutter installations. One brand of tri-polymer caulks, the CT1 Sealant, is a hybrid polymer formulation that contains no solvents. This means the caulk will not shrink as it cures. Polymer caulks work well on aluminum and PVC composition gutter systems. Polymer caulks are great choices for contractors because of their multiple uses. They cost more than the butyl rubber caulks.
Urethane Based Caulks
Another contractor grade caulk, urethane grade OSI caulk is an excellent choice for repairing gutter systems. The OSI products are based on urethanes and polyurethane composites rather than silicone. This gives these products many of the best qualities of silicone caulk, such as UV resistance and long lasting flexibility. They are low odor when compared to silicone caulks. They also cure quickly and are easy to paint. Moisture curing polyurethane can be applied when a gutter still has a bit of moisture present because the presence of water causes this caulk to cure into a flexible, durable bond.
Avoid Silicone Caulk
Silicone caulk should be avoided when repairing gutter systems. Silicone caulk is an excellent product, as it sticks well to dry surfaces and cures to a water tight bond. However, silicone caulk cannot be painted. Also, nothing sticks to silicone caulk after it cures, including more silicone caulk, so if a joint must be repaired a second time the entire seam must be torn apart and all existing silicone caulk removed from the surfaces before new caulk is applied. Thus, silicone caulk is a poor choice for repairing leaky gutters.