Rendering the wall can be an option if the brickwork has begun to weather and fail. If the bricks on the parapet wall are still in good condition and the mortar joints are soft and can be raked out, these can then be repointed using a 3:1 cement mix. Check the coping stones are still in good condition, the lip is over both sides of the walls so water is not running down the walls, if water is reaching the walls, the coping stones may need replacing or re-cementing in place. The lead flashing on the parapet walls can also crack and need to be re-sealed of replaced, a thorough roof inspection will pick up any small areas where water can get into the loft space or the room below.