Best Practices


  1. Wetting agents have the best chance of protecting structures in WUI fires if used in conjunction with traditional preventive measures including defensible space, the use of difficult to ignite building materials and other “firewise” measures.
  2. Gels are the most effective when pre-applied at a time shorter than one hour prior to the arrival of the fire front.  Therefore, homeowners who wish to protect their property, and then evacuate, should use gels exclusively.   Firefighters when wearing protective gear, and are not expected to be exposed to high radiation exposures, might be able to successfully use water and foam if applied just before the fire front arrival.  CAUTION: since the exposures required to ignite pre-wetted plants and structures are higher than what their protection is designed for, firefighters might get trapped by WUI fires, and should avoid attempting saving houses when uncertain about the exposures.
  3. The effectiveness of gel pre-applied at times longer than one hour prior to the fire front arrival has not been studied.  Also, only one type of gel was investigated in this project.  However, the experiments indicated that gel applied to plants tend to dry faster than in one hour in dry and windy conditions.  That can lead to reduced protection from ignition.
  4. Plants with leaves similar to Hollywood juniper and Leyland cypress cannot be protected from ignition with gels or other wetting agents because the gel cannot penetrate the thick outer layer of the leaves.  The inside of the plants thus remains unprotected, and typically ignite immediately if exposed to a small flame. 
  5. If mixed landscaping vegetation is used, it is likely that the plants that ignite at relatively low radiation exposures can produce relatively large flames that might ignite other plants, which in turn can ignite structures . 
  6. Completely dry plants cannot be protected with gels as effectively as relatively dry but still alive plants.
  7. The full-scale tests demonstrated that the gel can protect structures from ignition and fire growth when exposed to direct flames for short periods of times (up to 10 seconds).
  8. No pre-applied wetting agents can protect structures from long duration exposures caused by nearby burning structures.
  9. The exposures from wildland fires cannot be quantified at this time.  As guidance, Cohen’s experimental results and SIAM model calculations [1] can be used to estimate exposures.  The further away from a structure the wildland vegetation ends, the lower the exposures will be.  In addition the thinned vegetation, and lower height vegetation will produce lower radiation exposures on the landscaping plants and structures..  Cohen found in [1, 2] that the flame size 20 m (height) x 50 m (width), will produce the radiant heat flux of about 46 kW/m2 when about 10 m from a structure.  Higher distances and lower flames will produce lower exposures.  Also, flames from scattered or thinned trees or bushes will produce lower fluxes than flames from thick forest or densely grown bushes.
  10. It is likely that any pre-wetting will be ineffective in protecting structures located very close to a forest or tall and dense bushes (less than 10 m) where a possibility exists for very high radiation exposures and direct flame impingement on structures.     
  11. Gels should be applied uniformly and at a highest possible thickness that does not cause flowing.  Attention should be paid to edges and cavities. 
  12. Any other combustibles around structures (trash, wood piles, fences,  etc.) must also be protected by gel to avoid large ignition sources that can be ignited by ambers or flying brands. 
  13. The gel applied should have sufficient viscosity to produce a layer of at least a few mm thick.  Necessary adjustments must be made, on the spraying equipment to achieve this, if necessary.  Practicing is recommended before a fire.
  14. An ASTM standard test method is being developed, which (if successfully developed) will allow testing of other wetting agent/plant/building material combinations in the future.


[1]           Jack D. Cohen. 1995. Structure Ignition Assessment Model (SIAM). General Technical Report PSW-158. Albany, CA: USDA Forest Service, Pacific Southwest Research Station: 85-92.