EVERY TIME I CLOSE MY LOFT BITS OF CEMENT SEEMS TO DROP OFF THE ROOF LEAVING A COATING OF DUST EVERYWHERE WHAT DO I NEED TO DO.

hoover it up

Sounds like your roof has not been felted, you could line the loft with something like hardboard....... its a cheaper option than stripping the roof to felt

DAMP TECH PRESERVATIONS i take it your propery is around 1920 roofing in them days use to lime point the in side on the battons this in time get lose and powdery if you look in the loft area you will see this run along the state battons i whould not worry hope this help THANKS MR WOODS

I agree with Damp-tech, sounds like you have what is known in the trade as torching. This is a mix of sand/lime with horse hair added to the mix for strength. If the roof covering is in good condition theres no need to strip the roof and refelt etc. Any tradesman that has been trained in torching and working with lime can make good repairs to last many years.

Iam presuming there is no underfelt on the roof and it is a slate roof in its original condition.when it was put to-gether along the lathes it was cemented this is what is falling of into your attic it just old age,if you go into your loft without any lights and see if you can see daylight comming through anywhere if not you dont need to do anything at the moment, the big problem with having no underfelt is if a slate falls of there is then no secondary protection and water will come straight into the house this may not show up until it has created damage internally if you are planning to stay in the house for a while you may want to think about having the roof underfelted know as a strip and relay if going for this option you do need to upgrade your loft insulation at the same time and require building regs this is done on a builders notice the cost to re-felt will depend on the size of the roof expect to pay a few thousand hope this is of help

you could try giveing a coat of clear varnish should seal it so no bits fall off

give it a coat of polyurathain