clarification about the usage of infinitive

Hi,

I read about the following usage of infinitives:
1. Simple infinitives (acts as nouns)
2. Gerundial infinitives (does the work of adjective or adverbs)

I found the following sentence in my grammar text book:
This house is to let.

Is the infinitive used to qualify the noun (house) or is it used as the complement of the verb (is)?

Thanks,
Richards Peter.