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Ceiling Panels

[Lowest Skill Level]

Roof panel structure

The roof structure is very simple consisting only of two longitudinal 3.9m lengths of 70x30 on the flat, and a ladder of eight 70x30 rafters each on edge. Except for the first (which produces a roof overhang), the rafters are spaced out using 600mm centers. They therefore match the Studs on the side-walls all the way, except for the last-two wall studs which are modified in position to form the 705mm WC cavity.

The only feature which distinguishes construction of the roof panel from the other structural elements is rebating (cutting out) 95x30mm pieces from the two ends of each rafter. This allows the timbers to be screwed together with an extra 10mm of overhang on the rafters so that the plywood ceiling liner has a thin strip of solid timber on the inside for glue/tacking.

The intention is to clad the roof with three sheets of galvanised corrugated iron (Note: a number of different profiles match this definition), and these should be available in 3.9m lengths to avoid having shorter sheets with an overlap in the middle of the roof. Check this first. Ideally you'd like sheets that were 100mm longer than 3.9m for Australian conditions (extra shade overhang) but longer lengths are difficult to obtain.

Roof Fabrication

The only difficulty here is in rebating the ends of the rafters. These rafters need to project on both sides a little to extend the roofing past the wall cladding materials (which is a variable we can't control). This electric saw work should be only done by the supervisor or an experience handy-person.

The distance between the rebates must be the 1.8m width of the ceiling ply ... LESS ... 20mm ... to allow the 10mm glue-and-tack space for the frame on either side.
  > So 1780mm is the primary inside dimension needed between the bearers.
  + Your rafters will then need to span two roof bearers on the flat (each 70x30 = 140) = 1920
  + add an extra 20mm (because the roof is deliberately narrower than the frame) = 1940
  + plus twice the thickness of the wall cladding (say 15mm each side) = 1970.

Rebating
  • On the basis of these figures, you will have eight rafters, each of 1970mm length.
  • Lay them side-by-side and edge up on a sheet of square plywood (we use the ply as an accurate square)
  • Mark the center of the two outside rafters ... then also mark both sides at a distance of 890mm away from the center. These will be the inside edges of your rebate.
  • Measure the distance between the electric saw blade and its guide edge.
  • Nail two straight battens to span across all eight rafters. They should be the required guide-to-blade distance INSIDE your marked rebate points.
    - Note: You should always set your saw guides within the not-to-be-cut areas. And check measurements twice.
    - Now jam a wedge or knife between two of the timbers, so the nailed battens hold the whole eight together tightly.
  • Set your electric saw to the cutting depth of the rebate (ie. 30mm)
  • Check finally by laying a piece of spare batten against your guide, and make a preliminary cut on the rebate side.
    - This provides a guaranteed check for depth and the cut-from-guide distance, etc.
    - Only now should you make the first accurate cuts which set the sides of the rebates.
  • Make multiple rebate cuts across the projections of all eight rafters. A tap of a hammer will clear most of the excess wood out.
  • Smooth the rebates by hand with a chisel.

If you have decided to angle the ends of the rafters at the top, you'll need to do these one at a time. There isn't an easy way to do them all together. However, make sure you leave 10-15mm of vertical end-timber to allow a roofing screw driven in horizontally to hold the sides in.

When fitted to the beams there is a gap of 25mm between the Roof beam and the extended end of the rafters. If there is a possibility of possums intrusion, you may consider nailing on some ply-offcuts or batten timber here just in case. It will be under the curl of the roof edges.

The rest is pretty obvious. Begin at the back and space the rafters out on 600mm centres. Put two screws into each end of each rafter. We rely on the plywood liner to maintain roof squareness, but you could always add some diagonal straps if you have doubts.

Ceiling liner

We would recommend 3 or 4mm normal ply for the main 2.4 x 1.8 room area, and we suggest glue-tacking it down to resist flexing during transport.

Just before you add the ply, decide how you intend to tie the roof down to the walls. (see below). If you decide to use straps, you will need to cut short (30 mm) slots into the ply edge for the tie-down straps to descend; they will attach to the longitudinal bearers. This is not the strongest method because you are tying down the bearers, but not any of the rafters which have the roofing attached.

Since ceilings tend to get damp by condensation, they ply will probably need some paint protection, however most cheap ply is reasonably moisture-tolerant these days.

Note that there are no electrics in the roof; we rely on wall lighting for simplicity and safety. This makes it beneficial to have white-ish lime wash on the ceiling, which will improve the reflected light levels significantly. There is not much window area. Windows are destructive to wall strength in such a light-weight structure.

The 1.2 x 1.8 ceiling liner over the raised shower area probably needs to have a couple of layers of protective paint, and the limited light access here means that it would benefit from a gloss surfaces. You should do this before you insulate and move to the roofing stage.

Before moving on: add a batten (1 m) to the ceiling ply edge inside Rafter 2 (over the bed) and also diagonally on the other side of the roof to the inside of Rafter 8 (over the dry-floor area). These serve as useful attachment points for a cradle or bunk-bed, so they should be directly above the ply and glue-nailed to the rafters.

Tie downs

The roof needs to be tied down strongly to the side-frames, and the side-frames to the floor, and the floor to the ground if the cabins are being used outdoors.

One value of having the Raised-Floor Area at the back, is that stakes can be driven into the ground here, virtually at any time. Also they can be driven into the ground across the front.

The Roof plans indicate a number of tie-down point between roof and walls, but straps can be added virtually anywhere along the wall-roof junction. If the cabin is likely to experience extreme weather, the most important tie-downs are across the front-face of the cabin, under the overhang.

Heavy-weather tie down: If you expect heavy weather and have something solid embedded in the earth that can be used (ie. Lock-bolts drilled into concrete or rock) then the best way to tie down the most vulnerable front overhang will be an eye-bolt and chain or steel cable. You will probably still need to pin the roof and walls together against side-ways shift however.

Direct screw down: The most obvious way to lock roof and walls together is to use 60mm wood screws driven up thorough the top wall plate. But this can only be done easily in the back unlined sections of the walls and on the indented area on the Dry Floor area.

Straps: Originally we intended to only using flat gal iron strapping as tie-downs, but it is probably easier, and just as strong, to use wire.

Wire: Use stock-standard 3mm wire coat hangers. Most homes have an excess of these and some come with a permanent plastic rust-proof coating which is well worthwhile.

You could leave the twisted sections intact, and simply re-bend the body shape with pliers and a strong pair of hands. Make it into a single long loop. There is just over 80mm of wire in a hanger below the twists, so the use of a full hanger makes the loop 40mm in length which is probably a bit too long (mind you, the rest just stays in the ceiling cavity).

If you cut the hanger in half, there are a couple of different approaches to using wire tie-downs. You will have two "V" shapes, and you need the wire to project about 60mm below the ceiling level (30mm for penetrating the side-wall's top plate, and another 30 for working space to add the screw. Straighten the kinks in the hanger as much as possible, and narrow the present shoulder area into a two-sided "V". This will give you a total V-length of about 200mm.

You can use the wire as a single loop, or as a two-sided 'V'.
  • The 200mm length is enough to tie the top-plate and rafter together (70+30) and leave 60mm projecting (total 160mm) ... and still have enough extra to bend the wire-ends over the top of the rafter where they can be bent into rings or hooks and just nailed down with a couple of flat-headed clouts.
  • Alternately, you can anchor the wire ends in 3mm holes drilled horizontally into the side of the rafter, and lock the wires in place with a clout driven into the same hole.

These are the best options:

1. Rafter No. 2 - Front tie-down: Drill 6mm holes through the roof beams centres directly adjacent to the outside of this rafter. These holes will line-up with the front of the building outside the Side-Wall panels. During fabrication, you can push the two free-ends of the "V" through this hole in the beam leaving about 60mm projecting in a single loop. During erection, a single screw on the outside of Stud 1 will now lock the roof rafter and its beam down to the side wall (The wall will need to be locked down itself to the floor and the earth, obviously).

2. Rafter No. 8 - Back tie-down: This can be treated much the same way as the front except that the tie-down wire loop now must also pass through the side-wall's top plate. The erectors will need to do this (and they may be fighting wind at the time) so make it easy for them. You should drill larger holes (say 8mm) through both the Roof beams and the Side-wall plates.
   Remember: The Roof Beam projects inside the room by 10mms to provide ply attachment, so you should:
       • Drill through the center of the Side-wall top-plates directly adjacent to the last stud (Stud 7).
       • Drill the matching hole about 25mm from the outside edge of Roof beam, so that the two holes line up.

3. Other Rafters: The problem here comes from the fact that both roof rafters and wall studs are evenly stepped back in 600mm increments from the front. The rafter will therefore be directly above a stud in all the lined areas. If you are to add tie-downs here you can either use the strap technique, or choose a double-sided wire "V" with one wire passing either side and over the rafter.
       You'll need to drill two 3mm holes through the inside edge of the Roof Beam at the 7mm point (10 - 3mm - allowing for plywood thickness). These drill holes can go in at an angle, and each accommodates one side of the wire V. They should independently bend over the top of the rafter and be locked down by clouts in the same way.
       This leaves a wire "V" of, say 60mm, hanging below the ceiling with its apex directly over a wall stud.

Finishing touches:

For family disaster relief it would helpful to have a couple of attachment points behind the ply in the ceiling which can be used to embed a solid hook, or even a wood bolt, in order to hang one side of a small bunk bed, cradle, etc. Normally such attachments would be drilled and screwed directly into the rafters, but the insides of Rafters 2 and 8 are hidden behind the front and back walls of the cabin, and these are probably the most useful.

The table makes a useful emergency bunk bed for small children. So if the table's hinges are attached with gutter-bolts, the table can be removed and short lengths of chain can suspend the table as a bunk-bed above the Dry Floor area at the back -- alternately it can be attached above the main bed (resting on the upper shelf). It is a simple but maybe useful modification.

You could now pre-drill a couple of 3mm guide-holes into the ceiling rafters as well as these extra battens. Do this both at the front end over the bed (Rafters 3 and 4) and on the diagonally-opposite side over the Dry Area (Rafters 6 & 7). These should be about 500mm and 700mm out from the side walls.

Insulation and roofing

We suggest that glass wool fibre would be an ideal insulation material in the roof cavity.

Roof Shape at Rafter Ends:

The roof panel has been deliberately made 20mm narrower than the side-wall spacing. This is to allow 10mm on each side of the ceiling liner for glue-nailing of the ceiling ply. However, the rafters themselves weren't shortened.

We suggested that, to further help roof-edge rounding, it would probably be best if the rafter ends were angle-cut or given a small diagonal 'rounding' (depending on how it fits with the corrugations). The ridges on the metal overhang could be flattened a little to allow easier bending.

The roofing iron can now be curved over the ends and screwed down tight at these points with short flat-head screws, giving firm metal-to-rafter contact. This will help maintain an edge curve when the roof panels are being transported and make them easier for the roof panel to fit in some circumstances.

When erecting a number of units side-by-side, there may be difficult getting access to the sides or the backs of the units. So you want the gal-roofing to wrap as tight as possible on both sides without necessarily needing further screwing when being erected. [Note: the main roof tie-downs are done from inside the unit]

Free standing cabins may have other problems. We are concerned that the roofing edge may flap in the wind, but this is only likely outdoors probably when a cabin is standing alone. You should have left some vertical space on the end of the rafters for a couple of small fixing screws to keep the bending sharp, but you may consider adding some longer screws into Roof Bearer or even into the top Wall Plate.

The edge rounding problem may extend also to the front 30 cm overhang of the roof. You probably need to make a small transverse edge cut and wrap the excess around and under the longitudinal timbers.

Roofing

We suggest the use of three single 3.9m lengths of galvalised corrugated iron. There are a number of profiles, some with less metal weight, and some with less ridge height to the corrugations than others. It depends on your expectations of really heavy weather.

It is likely that many of these units will be used indoors and not have roofing at all.

If 3.9m lengths are not available, pay particular attention to sealing the overlap because it is likely that the cabins will not be given the normal roof runoff slope. You may need to paint on a bitumen compound and pop-rivet the sheets together.

When adding the roof, begin with the middle one, then do the sides. Try them out before you begin the screw fixing. You will probably have a couple of ridges of overlap between the sheets ... and don't forget that there are both two ends and two sides. If you flip one sheet end-to-end, you will finish up with symmetrical side overhangs. It is obviously best to have the last half-corrugation pointing in, rather than out ... but from the viewpoint of the erectors, it will be easier to handle with the opposite.

Miscellaneous Notes

Galvanised Iron Roofing:

This is the most obvious roofing material. But, be aware that it comes in different thicknesses, with both corrugated and square/ridged profiles, and with different galvanised/protective coatings. There are different contour depths (called ridge heights/depths): and since the whole roof is likely to be fairly flat (and allowing for the possibility of wind backing up heavy rain during a storm) you will be safest with a deep contour.

You may be able to buy it pre-cut to length, but the cheaper material is sold in standard lengths at a cost of about $13 per linear metre and a coverage of 0.75 m in width.

The advertised coverage width for most Colorbond corrugated and Trimdek roofing is 762mm. You need 1.95m for the flat roof's surface width, and at least 90mm on each side for the edge roll-over. So, with three sheets you will have coverage of 2286 while needing a minimum of 2080 - which means you have excessive width of only about 100mm on each side. You can best work out whether to use up this extra by another ridge overlap (on corrugated) on the flat surface, or extra width of the edge overhang. Think also about end-to-end flipping of one of the edge sheets.

In length you can make the 3.9m up with two sheets overlapped, but you will need to use a very good bitumized mastic between the sheets and pop-rivet them well, then seal the pop-rivets. The roofing angle specified as being needed for heavy rain run-off is not likely to be available in many installations without the floor noticeably on a slope.

Roof slope and rib depth: For construction simplicity by unskilled workers we settled on creating a flat-roofed box unit, which can be given a slope by chocking up the front floor of the unit, and allowing the back to be lower. The roofing industry calls this slope the "pitch" and it is generally in the region of 1-in-20 at a minimum.

These standards are set for residential and commercial properties, usually with a roof area which will collect large amounts of water in torrential rain. We doubt that this is amount of pitch is necessary on such a small construction.

Galvanised iron and similar materials also have different profiles. The roofing business talks about corrugated iron as having shallower (16mm) or deeper (21mm)'rib depth'. Obviously the water is more likely to penetrate under the overlaps if the rib is less.

However, you may be forced to use shorter over-lapping lengths of iron, if 3.6 m lengths aren't available. If so, seal the joints with bitumen or shower-seal materials, and pop-rivet across the top of the profiles.

Potentially heavy weather conditions.

The ideal if you can obtain it, is the Integrity profile which is specially made for almost-flat roofs. It has a particularly deep rib of 48mm and a coverage of 820mm. Two sheets is 1640 mm, which needs a wide flashing or ridge to make up the width. Three sheets gives a width-excess of 320mm, and since this extra is probably not useful it's probably best retained and used as extra overlap.

[Just for information:] BlueScope gal iron, for instance, comes in 14 different steel roofing profiles and a number of metal thicknesses (BMT = Base Metal Thickness) and also with different coatings. Usually the metal thickness is 0.42mm for lightweight domestic use.

CustomOrb (the most common type used today) officially requires a minimum pitch (roof slope) of 5 degrees (1 in 12) to carry the water off before it floods sideways under the ridge overlaps. However, this is almost certainly an excessive pitch for a small house of these dimensions, except in the tropics.

There is a heavier commercial version of CustomOrb which has a deeper profile which would be better in tropical conditions (with a coverage of 762 mm), and another type with an even greater rib depth which is OK to use with only 3 degrees (1 in 20) of pitch (almost flat).

We get the rain-shedding roof pitch simply by sloping the whole unit so this choice may be important outdoors.

Make sure that you have a 50mm overhang of the metal after the last rafter. We are concerned that rain flowing off this edge will get blown down behind any gullies on the back-wall cladding. If you have a length of the 20x20 gal angle you might use this to cap the top of the back wall panel.

Use of walling materials:

Be aware that some of the light-weight metal sheets made for use on walls are probably not suitable for roofing. There are also heavy-weight fencing materials with very substantial ridges, and they are also unsuitable mainly on the grounds of weight and cost.

Some special lightly-ribbed walling materials may not be suitable for the 2.1 m height of the side panels, because they generally require noggins to be added for mid-length attachments between each pair of studs. We have avoided using noggins, but you may differ. They generally need to be cut accurately to measure, and we believe that unskilled people with hand-held electric circular saws are potentially a dangerous combination.

Screwing down: You will need to screw the front edge of the overhang down well, with standard gal roofing screws through holes drilled in the sheeting. Since wind can get under the overhang, it would be wise to screw the iron down also to the next rafter, and probably every second rafter thereafter. Using a sealer (bitumen or shower sealer) and gal pop-rivets to tie sheets together if there is a risk of water penetration.

End treatment:

The wind can't easily drive rain under the front gal iron, or the sides. But it can under the back because there is only a 10mm overhang. Luckily this is only onto Marine Ply, but it is better if we keep the rain out anyway. Before you screw down the last line of screws holding the roof to the last rafter you could plug the space under that line of ridges.

We think the best material to use here is probably remnants of the Coolite fruit-boxes you probably used for back-wall insulation. Cut some wedge-shaped pieces that will fill most of the space, and perhaps put a touch of contact cement on each just to hold them in position, screw down the roof finally, then cut off the excess Coolite neatly.

Guttering:

You would only bother fitting gutters in dry areas where you need to harvest the rain for a water tank.

When the roofing material is being screwed down, consider giving the iron at the back of the roof a 10mm overhang, both so rain can be collected and to keep the drips from constantly wetting the back surface. Standard guttering is designed to take heavy rain falling onto large roof surfaces. The dimensions are therefore many times that necessary here, so you could use part of the guttering's 'wall' also as 'flashing'. More overhang on the front of the frame would also be advantageous in hot and wet conditions, so ideally, if available, the roofing sheets could well be up to 4.0 m in length without frame modification. If you've already bought 3.6m gal, you might consider:
  (a) shifting the first rafter back a few centimetres.
  (b) (then) adding a piece of flashing to the front overhang to extend this part of the roof surface.
  (c) Cut and bend the back wall of the guttering to use some of its height as flashing under the gal.
  (d) Adding a 1.8 length of conventional flashing.