Above are two different ways to form a loft trap for access into the attic space. One is a ply flush door cut down and hinged, better if you are installing a ladder. The other simply has an 25mm MDF panel that sits down on top of the lining material. If you are hinging a hatch door you can either use a touch latch or a barrel bolt to hold the door closed.
Whilst it might not look so good, I personally prefer using something like a barrel bolt because with a touch latch you need to leave a gap between the door and stops which can let a draft through.
First, you need to trim out the ceiling joists to create an opening. You'll need some timber the same size as the joists for trimming and some lining material, 90 x 18mm planed timber is fine.
After deciding on the best position for the trap to be, mark out the ceiling joists. Use a framing square like in the picture below to mark lines perfectly square on all three joists. If the trap will be in a hallway/corridor try to center it as best as possible and avoid positioning it too close to the top of a staircase or landing.
Don't forget to allow for the thickness of the frame material when setting out.
Once marked out, fix some scrap timber across the joists to hold the one that's going to be cut like in the picture above. This is especially important if the ceiling is already fixed. Cut the joist out that is in the way using a hand or circular saw, and fix two pieces of timber across. Nail them to the outer joists and to the trimmed joist to stop it dropping down now the middle section is cut out.
If necessary, a trimmer in the opposite direction can be used to make the opening the desired size or in the right position.
Unless you are installing a replacement loft trap or one in an existing property (rather than a new build) It's likely that a frame will be installed before the plasterboard and then after the flushing or plastering has been done the trap door, stops, catch and architraves will be fitted.
It's a bit like installing a normal door frame. You might find it easier to build the frame on the floor and then slide it up into the trimmed area, otherwise you can install 'in-situ'.
For strength the frame can be housed out like the picture below, this is more important if you are hinging a door not so much if the trap will simply sit down on top of the frame.
It will need to be perfectly square so the door works and the architraves are easy to install which means cutting opposing sections to equal lengths. Once glued and screwed together check the frame is square by using a framing square or measuring from corner to corner.
Once squared, if possible a door can be cut to fit the trap and hung. It will be easier to do it now before the frame is above your head. If you don't have the door yet but can chop the hinges in that will at least help. The door should be set out and hung the same as if hanging a normal internal door, but the hinges should only be about 50mm in from each end.
If you are using a touch latch the door stops will need to be fixed with a gap before the door so the door can be pushed up for the latch to engage.
To fix the trap, remove the door if you've hung it already and pre-drill some clearance holes into the frame. Ideally they'll be in a position where the door stops will cover them later on. Screw the frame in position remembering to let it protrude below the joists the thickness of the plasterboard and plaster that will be going on.
The architraves should be fitted exactly as if installing them on a normal door, click here for help installing the architraves.