Write as though you were speaking to them in a conversation. I would also 'possibly' consider a bit of humour so you stand out from the crowd especially being in drainage. BE CAREFUL WITH THAT THOUGH AS YOU DO NOT WANT TO OFFEND.

I would still use the tried and tested formula of AIDA (Attention Interest Desire Action) try to get the call to 'Action' to be to get their email address eg by going to your website and leaving it in return for something of value.

In your particular case as you are not asking for immediate call back (unless you are lucky enough that they are currently sourcing) you want to stay on their 'radar'. I would consider some kind of promotional gift to keep on their desk (especially of you are thinking of doing this to a few architects) something like desk calendar, pen, coaster etc.

I would also consider rather than sending out just a letter send out a folder with your letter so it is more likely to be kept on file rather the other 'kept on file/bin'. You don't need to spend a lot on the folder (they do decent ones in Stationery Box etc) and include customer testimonials, any press coverage you have add and copies of qualifications, awards or certificates.

If you can find some way of getting their email address would be best and keeping in regular touch with them by sending some industry news that may be of value to them or something else they value such as a whitepaper or guide (don't try and push your business on them in this way just stay on their 'radar' until the time is right).

In summary don't just rely on sending them one letter (or even 1 method of marketing) and expect them to call. Look to be doing at least 5 methods of marketing and trying (and testing) new methods all the time. STAY ON THEIR RADAR ANYWAY YOU CAN