If my neighbor blocked my sewer drainage ditch with debris from home repairs, is he at fault if the health board deems my home uninhabitable?

Your neighbor is liable for nuisance whether or not the health board deems your home uninhabitable.  Nuisance is a serious and unreasonable interference with your use or enjoyment of your property.

Damages (monetary compensation you are seeking in your lawsuit) may be an inadequate remedy in a case of nuisance because blocking the drainage ditch is an ongoing problem which would result in multiple lawsuits.  Another reason that damages would be an inadequate remedy is that land is unique.

When damages are an inadequate remedy, you can pursue an equitable remedy which would be an injunction to prohibit your neighbor from blocking the drainage ditch.  A court will weigh the benefits and burdens to both parties in determining whether or not to issue an injunction.  The benefit to you of being able to use the drainage ditch and septic system would appear to outweigh the cost to your neighbor of  removing the debris and not blocking the drainage ditch with its impact on your lot resulting in problems with the health board in addition to causing similar problems for other neighbors.  Procedurally, the court issues a temporary restraining order until there is a preliminary hearing at which time a preliminary injunction may be issued and would be in effect until trial when the court may issue a permanent injunction.  The injunction's language will be framed in the negative to avoid enforcement problems; for example, your neighbor shall not block the drainage ditch.