A design for small, low-cost demountable homes with full facilities (cooking, washing, shower and toilet).
© Plateau Group
This H4H design is available to any volunteer groups for non-commercial use, but please make a formal request.
H4H Fabrication Details
Roof and Ceiling Panel
We would recommend 3mm normal ply for the main 2.4 x 1.8 room area, but since ceilings tend to get damp by condensation, they ply would need to have paint protection or be moisture tolerant.
The 1.2 x 1.8 ceiling liner over the raised shower area obviously needs to be either a more wet-tolerant ply, or have a couple of layers of protective paint.
[Note: You should cut the 40 mm slots for the tie-down straps on the last rafter (see below) before you insulate and move to the roofing stage. We suggest that glass wool fibre would be an ideal insulation material here.]
The roof panel has been deliberately made 20mm narrower than the side-wall spacing. This is to allow 10mm on each side for glue-nailing of the ceiling ply. However, the rafters themselves don't need to be shortened: so a viable alternative to the above plan dimensions would be to cut the rafters to a length of 1.94 m rather than the 1.92 m, and increase the length of the notch at each end by 10mm. This makes the rafter notches 80x30, leaving 10mm of rafter extending outside the longitudinal timbers. The iron can be screwed down to these ends, to maintain an edge curve.
To help roof-edge rounding, it would be helpful if the end of the rafters were given a small diagonal 'rounding'. The ridges on the metal overhang could be flattened a little to allow easier bending.
You should leave some vertical space on the end of the rafters for a couple of small fixing screws to keep the bending sharp (it may flap in the wind). 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]
The edge rounding will extend also to the front 30 cm overhang of the roof, and this might become a problem with taller local residents. You probably need to be cut and remove some of the excess, or make a small transverse edge cut and wrap the excess around and under the longitudinal timbers.
Note on the main diagram there are four ×s; one either side of the front of the last rafter, and two in front of the second-from-front rafter (above the front panel). These ×s are points where we need the roof to be tied down to the main structure for it not to blow off in a wind.
We suggest the use of four short (100mm = 70+30mm) straps made from flat 30mm gal iron. They should be drilled top and bottom, say 15mm from each end. They are to be fixed to the face of the second-from-front rafter (outside the front panel) and to the inside face of the last rafter (before adding the roof). The first rafter is easily accessible during assembly, since there is no ceiling liner, however the last rafter will need slots through the ceiling ply at the positions indicated on the plan. These straps mean that the back portion of the roof panel can be tied-down using two screws only from the inside during erection.
Galvanised Iron Roofing:
This is the most obvious roofing material. But, be aware that it comes in different thicknesses, and with different galvanised/protective coatings. It also comes with different contour depths (called ridge heights/depths): and since the whole roof p is likely to be fairly flat (and allowing for wind to back up heavy rain during a storm) you will be safest with a deep contour. This possibly means that each sheet will have slightly less coverage width.
Corrugated roofing and walling materials:
Be aware that some of the light-weight metal sheets made for use on walls, is probably not suitable for roofing. There are special lightly ribbed walling materials, but these may not be suitable for the 2.1 m height of the side panels, because they generally need noggins to be added between each stud. We have avoided using noggins; they generally need to be cut accurately to measure, and we believe that unskilled people with electric circular saws are potentially a dangerous combination.
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.
Gal Iron Coverage:
The iron is usually sourced at a fairly standard width before the manufacturer rolls it to give the sheet its "profile" (the corrugations). So those types with a deeper rib depth, will also lose some coverage area (width).
The standard Custom Orb has a rib depth of 16mm and gives a coverage 762mm. [Note: "coverage" refers to the useful width of the sheet. The sheet will actually be wider because this measure allows for a 'one ridge' overlap of adjacent sheets.]
Since the rafter is 1.92 m or 1.94 m wide, and we need two side-overhangs of 100 mm each (70+30), the side-to-side coverage needed (in total) is just over 2.1 m (2140). The average sheet of corrugated iron has a coverage of 760, so 3 sheets are 2280 mm which is only 140 mm wider than we require. So three sheets, perhaps with an extra ridge of overlap, is probably correct (and saves cutting sheets).
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.
Cutting gal iron lengthwise:
If you must cut a sheet, the best way with a long length of gal iron is to use a metal cutting disk on an electric handsaw (rather than tin-snips) Do this before building the roof frame so you can use the longitudinal timbers both to lift the iron off the floor, and as a clamp-on guide to keep the saw-cut straight. You will need a metal file to run down the cut edges and remove sharp dags.
If you use a half-sheet to get the required width, add it in the middle with full sheets either side.
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.
When the roofing material is being screwed down, give the iron at the back of the frame 15mms of overhang so rain can be collected in dry locations by adding a gutter. More overhang on the front of the frame would also be useful in hot and wet conditions, so the roofing iron could well be 4.0 metres or more. (If you already have 3.6m gal, you might consider shifting the first rafter back a few centimetres.)