Working on the roof
It may not be possible to replace a slate immediately. In order to minimise any water damage, weather proof the slate temporarily. Prop up the slates that overlap the cracked one with wooden wedges, and use a wire brush to clean the surface round the crack.
Using a piece of flashing strip
Use flashing strip primer to coat the cracked area. The primer will ensure a good bond once the flashing strip is in position. Make sure you use enough to cover the width of the strip to be used. Use a section of flashing strip long enough to cover the crack completely. Push the strip into place and check it is well seated. Use a small roller over the strip to firmly push the strip down into position.
Using a bituminous sealant
Use an applicator gun to inject the bituminous sealant along the crack.
Working in the loft space
Use an applicator gun to inject adhesive expanding foam along cracks or gaps from inside the loft.