See: http://techbeats.deluan.com/how-to-use-an-external-log4jproperties-in-you-0

resources.groovy

beans = {
    log4jConfigurer(org.springframework.beans.factory.config.MethodInvokingFactoryBean) {
        targetClass = "org.springframework.util.Log4jConfigurer"
        targetMethod = "initLogging"
        arguments = ["classpath:myapp/log4j.properties"]
    }

    ...

}

resources.xml

    <bean id="log4jConfigurer" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
        <property name="targetClass"><value>org.springframework.util.Log4jConfigurer</value></property>
        <property name="targetMethod"><value>initLogging</value></property>
        <property name="arguments">
            <list>
                <value>classpath:myapp/log4j.properties</value>
                <!-- or you could use an absolute path: <value>file:D:\\etc\\myapp\\log4j.properties</value> -->
            </list>
        </property>
    </bean>

web.xml

    <filter-mapping>
        <filter-name>sitemesh</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>


    <!-- Comment this out!
    <listener>
        <listener-class>org.codehaus.groovy.grails.web.util.Log4jConfigListener</listener-class>
    </listener>
    -->

    <listener>
        <listener-class>org.codehaus.groovy.grails.web.context.GrailsContextLoaderListener</listener-class>
    </listener>