Learn to Build – Constructing a Ceiling
First Floor Ceilings
Obviously, your ground ceiling is formed by the first floor joists, so we move on to the first floor ceiling. In the case of a ceiling in a flat roofed extension, the ceiling is formed, either by, the roof joists or you have the better choice of a lowered one.
I say, the better choice because, this gives more insulation options. I can’t emphasise the importance of ceiling/roof insulation too much.
Of all the money saving, eco projects that you may carry out on your home, this is the one that you will feel the benefit of the most.
It doesn’t cost a fortune but, saves you one.
A lowered ceiling can be constructed from sawn timber 100X50mm 4″X2″. Measure up from the floor to the desired height and make a mark.
Using a piece of the 100X50mm timber, get someone to hold up one end of the wood to the mark (better to start in a corner).
Rest your spirit level on the wood, and when perfectly level, make another, or several marks. Move the wood on and continue like this all way round the room.
If you don’t finish at the exact level as you started, you have made a ****up. Ensure your wood is perfectly straight. The other alternative is to use a laser level.
I keep being told how good they are but, unless you buy an expensive one, they are difficult to see.
And, when, or, how often will you use it? Once you have the correct marks around the walls, you can fasten a continuous run of the wood to the wall, with the bottom of the wood resting on the marks.
Drill and screw the wood using 100mm casement fixings.
These are strong screws with the plastic plugs already attached. Drill the hole straight through the wood and wall, then simply hammer the fixings in.
Don’t try and put the plug in first and screw the screw into it. The screw thread is for removing it. It doesn’t hurt to use a large screwdriver to nip it up. Drill the holes at a minimum of 600mm centres.
Now you can begin to fix the cross joists at 400mm centres. IMPORTANT…remember to take your first 400mm measurement from the wall, and not from the centre of the timber that’s fastened to the wall.
The measurement for these joists is taken from the face of one piece of wood across to the face of the opposite piece of wood, across the shortest side of the room.
In other words: if your room measures 3metres X 5metres, your measurement will be the short side, 3metres =3000mm less 100mm the thickness of the 2 pieces of wood attached to the walls, giving a length of 2900mm 2.9 metres. Always saw them 3mm too long and gently hammer them in place.
Cross pin them, 1 100mm nail at each side of the wood, hammered at a 45 degree angle into the wall wood. Start your nail about 50mm back from the end of the joist and just above or below the centre.
Hammer the first nail until it begins to bite into the wall wood, but, not all the way home, or you will most likely knock the joist out of line.
AS soon as the first nail begins to bite, start the nail from the other side. Pre-mark the 400mm centre clearly on the underside of the wall timbers,
before you begin, in order that you can continually check that your joists haven’t moved sideways whilst you are hammering.
Like most of the jobs, that you have completed on this extension, 2 people can do it far easier than one.
I have done this particular job on my own by screwing small metal strips onto the upper side, at each end of the joists, protruding beyond the length by 50mm.
I used these to rest onto the wall timbers at both ends until I’d hammered them in place. Laboriously slow but, effective.
After completion of this job, you must fit the noggins. These short 350mm or thereabouts, pieces fit between the joists, in the place that the plaster boards are joined.
This, depends upon which size of board you choose. Choose the thicker board, preferably 15mm.
Ceilings to roof spaces must be finished using minimum 12.5mm foil-backed plasterboard with a skim coat or proprietary finish.
In practice, as the NHBC require that where the timber support spacings exceed 450mm, all 12.5mm plasterboard joints are timber-nogged, it is often easier and quicker to use 15mm plasterboard.
Roof Insulation and Ventilation
Flat ceilings to a roof void should receive 100mm of mineral wool insulation laid between the joist with a further 200mm laid across the joists, continuous with the wall insulation but stopped back at the eaves or at junctions to maintain a 50mm air gap.
Cross-ventilation can be provided by proprietary eaves strips equivalent to a 25mm continuous gap at eaves level.
If this cross-ventilation is not possible, as with a mono pitch, then it can be provided by means of proprietary ridge-level ventilation equivalent to a 5mm continuous gap in the form of vent tiles spaced in accordance with the manufacturer’s instructions.
Sloping ceilings can be insulated using a combination of between and beneath rafter rigid foam insulation so as to provide an air gap immediately beneath the underlay of at least 50mm, namely:
Celotex to a thickness of 120mm between the rafters and 25mm beneath, Kingspan to a thickness of 100mm between and 25mm below and Xtratherm to a thickness of 105mm with 25mm below.
Ventilation to the roof space can be omitted altogether if a breathing underlay or felt is used with a minimum 25mm thick treated counter batten beneath the tile battens with proprietary eaves carrier systems.
In short, you should fit 300mm of mineral wool above the plaster boards before boarding. Plus, your electrician must run the cables for the lights.