At subduction zones, the level of friction on the deep part of the plate boundary fault controls stress accumulation and release, which govern the transfer of stress to the shallower, locked part of the fault that can slip in a megathrust earthquake. In some subduction zones, the deep fault slips slowly, at speeds far less than shallower, regular earthquakes, and is often accompanied by weak seismic waves called tremor. Tremor and slow slip can be triggered by small stress changes induced by ocean or solid Earth tides. Here I use seismic data combined with calculations of tidal stress to determine the influence of tides on 31,000 tremors generated by six large slow-slip events in Cascadia between 2007 and 2012. I find that the sensitivity of tremor to tidal stresses rises during each slip event, as slip at each spot on the fault accumulates. Specifically, tremor rate is an exponential function of tidal stress, and this exponential sensitivity grows for several days, implying that the fault weakens during slip. I use the relationship between tidal stress and tremor to calculate a coefficient of intrinsic friction for the fault and find values of 0 to 0.1, which indicate that the deep fault is inherently weak.