In a word - nothing!
Both are self-certification bodies used by contractors to ensure that their work is covered by Building Regulations. In essence, the contractor pays an annual membership fee, as well as a 'per job fee' and the customer receives Building Regulation certificates directly from the appropriate body.
"I've heard of FENSA but not Certass; why is that?"
What an excellent question! Well done, you!
FENSA was set up in 1992 and Certass in 2006. Because of this, FENSA have Top-Of-Mind-Awareness (TOMA) in the marketplace that consumers (that's you with a tie on) associate with windows. As FENSA has been around for longer, their name has become a 'Known Brand'; in the same way that 'Hoover' is a brand name that one associates with vacuum cleaners (this is know as a 'metonym') I hope you're enjoying the marketing speak and taking notes; there'll be questions afterwards.
"So what does FENSA stand for?"
FENSA stands for the 'FENestration Self Assessment' scheme.
"And Certass? What does that stand for?"
Certass stands for 'CERTification and ASSessment'. (I hope that Google doesn't flag the last up as a rude search term)
Is FENSA better than Certass or the other way around?
Pay attention; they're both the same. FENSA simply has greater 'Market Saturation' than Certass.
"How do I get my certificate?"
When you've paid your bill (in full, please); I register your work on the Certass website and they send you a certificate about 4-8 weeks later. You need to keep this safe as it belongs to the house, should you ever sell it. The other chap's solicitor will use the absence of the certificate as a reason to knock around £10,000 off the asking price of your house, because that is what solicitors do.
"£10,000?!?!?!?! You're having me on!"
Well, maybe a little. But you really do need to hang onto the certificate and not line the cat basket with it or anything.
"I've lost my certificate, what should I do?"
Oh, for goodness sake! I've just told you to store it safely! I don't know why I bother, I really don't; might as well be talking to a brick wall (NB: Brick walls are NOT covered by Certass. Or FENSA). Well, have another look for it.
"I still can't find it, have you got a spare copy?"
No, the window company doesn't get a copy. In fact, since the certificates are sent direct to the customer, the window company probably doesn't even know what they look like. You need to call Certass directly, give them your name and address and approximately when the windows were installed and they'll send you a duplicate.
"Is the duplicate free?"
No, it's £10. That'll teach you not to look after it. Anymore questions?
Right, you're free to go. Next time we'll be talking about customer warranties and why they're not transferable.