Lecture for Tele-Conference Discussion on “Asian Architecture and Environment 2009” How to keep buildings dry from rainwater 2009.7.3 by Hirozo Ishikawa Professor Emeritus, Tokai University
The most common building defects in newly built buildings in Japan Rain penetration
Annual report of Japan Advisory Center for Maintenance and Dispute Settlement for Housing (2008) Rain penetration Number of consultation received from house owners by building failure types
Annual report of Japan Organization for Housing Warranty (2007) Rain penetration in roofs Number of payment Sum of Money Rain penetration in external walls Proportion of insurance payments by building failure types
Effects of rain penetration in timber framed buildings Decayed wall frames
1.Waterproofing How to prevent rain penetration History of using waterproofing materials in buildings Roof membrane : 150 years Joint sealants : 60 years Before then? 2. Rain excluding design of building shapes and details
Definition example：Waterproofing roof membranes, Sealant filled joints. Waterproofing “Forming continuous impervious skin over building envelopes utilizing waterproofing materials. “ example：Large eaves overhangs. example：Adequate slope and enough lap length for roofing tiles. Rain excluding design and details “Choosingbuilding shapes and combination and layout of building components so as to be effective to prevent adverse movement of rainwater on and into the building fabric.”
Traditional Japanese technical term for “Rain excluding design and details” Ama - Jimai Putting in order Rain + Designing building shapes and the details to control the behavior of rainwater in order to avoid inconvenience.
Typical Ama-jimai design Rainwater is smoothly shed away from the structures below. Traditional timber framed gate building Board overlapping keeps run-off rainwater out. Suiten-gu Temple Gate, formerly built in Nihonbashi, Tokyo, now in Shonan Campus of Tokai University
Upper part of walls or openings sheltered by eaves or hoods from wetting. Moisture-vulnerable materials are protected from wetting.
Ama-Jimai • Used to be the only technique for craftsmen to keep buildings dry from rainwater before the introduction of modern waterproofing materials. • Successfully protected buildings from rain for thousands of years. • Principles still useful. • Insufficient scientific explanation bars its effective application in modern building design.
Advantages and disadvantages • Waterproofing • Can work against any rainstorm condition unless being damaged. • Liberates architectural design from gravity. • Finishing materials must be water-resistant. • Performance very much depends on craftsmanship. • Relatively short service life. Needs for frequent maintenances. • Blocks moisture permeation. • Ama-Jimai (Rain excluding design and details) • Performance depends on severity of rain and wind condition. • Results in rather traditional architectural design. • Finishing materials need not to be water-resistant. • Performance less depends on craftsmanship. • Maintenances can be minimized by using durable materials. • Allows free moisture permeation.
An example of architecture with design concept opposite to Ama-Jimai Rain freely fall and run over the wall Materials are fully exposed to rainwater Walt Disney concert hall, Los Angels (2004) Architect: Frank Gehry
Source:World Weather Information Service (WMO-sponsored web site) http://www.worldweather.org/pilot.htm Facts behind the hall design Rainy days once a week at max. in L. A. L. A. rainfall only one-fifth of Tokyo. Little possibility of snowfall in L. A..
Built 17 years ago. Total cost for completion:160 billion Yen The roof and the external wall are now under urgent remedial works due to severe rain penetration spending six years and sum of 1 billion Yen. We should be aware of the maintenance costs after the completion. The Tokyo metropolitan government office, Tokyo (1991) Architect: Kenzo Tange
What is the essence of Ama-Jimai(rain excluding design and details)? Three conditions needed for occurrence of rain penetration Method for preventing rain penetration • Presence of holes (or gaps) through which rainwater can enter. • 2. Presence of rainwater over the holes. • 3. Action of forces that make rainwater pass through the holes. 1. Blocking holes Waterproofing 2. Prevent wetting 3. Control the forces Prevention of penetration can be achieved by eliminating any of the three conditions. Ama-Jimai(rain excluding design and details)
The first principle of Ama-Jimai Wetness of building façade is caused by; • Direct hit of wind driven rain. • Splashes of rainwater fallen on the ground/roofs. • Run-off rainwater from above. • Wrongly designed /broken gutters.
Studies needed for preventing/reducing wetness of building façade include; • Knowing the behavior of wind driven raindrops around buildings. • Estimating amount of rainfall on external walls. • Evaluating the extent of shelter provided by projections over the wall against wind driven rain.
Knowing the distance, height and amount of rainwater splashes on the ground/roofs that reach the foot of walls. • Knowing run-off rainwater shedding performance of drips over the walls. • Developing a method for evaluating overall severity of exposure of external walls against rainfall.
The second principle of Ama-Jimai Forces causing movement of rainwater on and through the building skin. Kinetic energy Gravity Surface adhering force Capillary suction Pressure difference
Forces causing movement of rainwater on and through the building skin. Capillary suction Kinetic energy Pressure difference (gap filled) Gravity Pressure difference (gap unfilled) Surface adhering force
Considerations for designing building details against adverse movements of rainwater include ; • Guiding rainwater to harmless directions. • Weakening the forces that cause the movement of rainwater. • Limiting the extent of penetration to a tolerable level.
Some design vocabularies for rainwater movement controlling details Labyrinth joints Sloping Drips Capillary break Overlapping Pressure equalizing
Design vocabularies for rainwater movement controlling details Capillary break Labyrinth joints (capillary action) (kinetic energy) Overlapping Sloping (pressure difference, gap filled) (gravity) Pressure equalizing Drips (surface adhering force) (pressure difference, gap unfilled)
Conclusions • Preventing rain penetration is the most essential function of buildings, but many buildings still fail to fulfill it. • The traditional rain excluding design concept of Ama-Jimai is still an important tool to keep buildings dry from rainwater in spite of development and wide availability of modern waterproofing materials.
Conclusions • The essence of Ama-Jimai is; • Sheltering building façade from the effects of rain and wind. 2. Controlling the movement of rainwater both over the building surfaces and within the building envelopes.
Conclusions • Scientific approach based on meteorology and hydraulics /fluid mechanics is necessary for the effective application of Ama-Jimai design concept to modern architectural design. Thank you for your attention! Any question is welcome. Write to: firstname.lastname@example.org