The world is built on the efforts of private enterprise where the natural desire is to maximise profits by charging the highest price the market will bear and by minimising costs.
An important regulatory factor as that aspect of activity described as quality.
Good quality products earn a better reputation than poorer quality products. Yet, there are those who will put a fine exterior on a product and use low quality materials and methods where things are hidden from view.
A second regulatory factor is that of safety. For many products safety is of minor consideration whereas for others it is a very important consideration. In housing safety of the structure and its occupants in the long-term is important yet it is an aspect that may be neglected through expediency or genuine lack of knowledge.
The level of available knowledge throughout the world varies from country to country. In the building industry those countries where the climate is windy or prone to regular hurricanes, and have a developed economy, the level of knowledge in methods of safe construction are highest. In other countries certain areas of knowledge are not relevant. It is incumbent upon those who have a statutory responsibility for regulation and its administration to ascertain the relevant knowledge and to embody it in the relevant regulatory codes.
Codes in themselves are an important basis for the establishment of required knowledge but they also need accompanying methods of education and inspection.
Building Plans and Permits
A fee schedule needs to be established which will generate sufficient income to cover the costs necessary to pay for all aspects of
- Town Planning
- Building Code Development and Publication
- A Plan Approval Process
- A Building Inspection Service
- Provision of Advisory Services
This is a very important procedure. Plans need to be inspected to see that they comply with 2 main things
Once a plan is approved a permit for the construction should be immediately issued.
- Town planning and legal requirements in relation to Land Use
- The Building Code and applicable Standards and Engineering Requirements
A Building Inspection Regime
This has 10 main components
In addition to what is detailed here general compliance with the Code may be checked at each inspection.
What is detailed here are the most critical items which must be inspected at the correct point in the construction programme.
- Wall Structures
- Roofing Structures
- Ceilings and Internal Linings
- Completion Inspection
| They are generally aspects of construction which will be covered up at the next step. || |
For instance, reinforcing will be enveloped in concrete and become generally invisible when the concrete is poured.
Utilising this concept it is possible to design a building inspection system which revolves around a series of inspections at critical points in a building's construction.
A series of inspections along the following lines is generally adequate for residential construction. Commercial and Public Building construction can follow similar lines but may require a custom designed inspection regime depending on the particular project.
Make an inspection when the foundation trenches have been dug, boxed and the steel placed ready to pour the foundation concrete.
- Check that the bottoms of the trenches are flat as curved bottoms can result in the foundations becoming concrete wedges which gradually drive themselves into the ground.
- Check that the steel is correct in dimensions, properly tied and lapped as required at corners and that it is clean.
- Check that the boxing is sufficiently strongly braced that it will not top, split open and will enable a foundation to be created in accordance with the plan with a horizontal top surface. (Slopes in the foundation result in slopes in the house!)
- Check that the required reinforcing, which is to tie into the floor and or walls, is in place.
- Check that the concrete strength matches or exceeds that specified in the plan using the appropriate instrument following the relevant time-lapse. Slightly lower levels may be tolerated. Substantially deficient strengths will necessitate the destruction of the concrete and re-commencement.
Concrete Floors at Ground Level
Prior to the pouring of Concrete
- Check that the necessary hard-fill is in place.
- Check that there is a properly taped and sealed damp-course material in place.
- Check that the required rein-forcing material is in place supported in such a manner that it will be centered in the concrete floor slab.
- Check that the boxing for the floor is such to enable the floor to be poured to the required thickness.
Window and Door Lintels
- Check that the framing is properly connected to the floor with bolts for external walls and any dividing walls which will be relied upon in the design in relation to wind up-lift forces. Check that the other internal walls are connected to the concrete with concrete nails, dynabolts or the equivalent.
- Check that metal bracing has been inset into the framing with each piece of bracing connecting to a top plate at one end and a bottom plate at the other end with the orientation of the bracing being at the angle closest to 45owhich is within the range of angles 30 - 60o. Where walls are long the bracing should be such as to zig-zag along its length. Where angles cannot be achieved with the 30 - 60o.range such as in narrow spaces between wide windows and the end of a wall then plywood bracing should be used for that part of the bracing.
- In wind and hurricane prone areas check that top and bottom plates are strapped to each stud with metal strips which must be galvanized and may be pre-perforated and nailed or of the gang-nail variety. In the case of multi-level constructions the straps should reach from the studs on one level across the intervening floor to the studs on the floor below either directly or indirectly.
- Check that there is an 8" x 4" (200 x 100mm) gang-nail plate connecting the top plate of each wall to that of the wall to which it connects. This applies to both internal and external walls.
- Check that there is adequate nailing down the length of the studs where walls join at corners and intersections.
- Check that walls are plumb.
- Check that studs are plumb and not bent or twisted in any way.
Timber framed with block or brick veneer
- Check that the beam size used for the lintels is correct for the window width (this should be checked at the plan approval stage.) in accordance with the plan and the code.
- As for Timber Framed with the following additional checks.
- Check that there is a 1 inch (20 mm) cement plinth, at the floor-foundation intersection, to shed water towards the outside of the foundation so that it cannot seep under the floor.
- Check that a bitumenous damp-course is painted onto the foundation prior to commencement of the laying of bricks or blocks.
- Check that at regular intervals there are gaps in the mortar of the bottom lay of the bricks or blocks so that water can drain to the exterior should any get behind the bricks or blocks.
- Check that the required cavity space is being allowed.
- Check that the bricks or blocks overhang the edge of the foundation but not more than the allowable amount.
- Check that the required connecting straps between the framing and the blocks or bricks are being inserted at the required intervals.
- Check that galvanized steel angle irons 4" on side are being used as lintels above doorways and windows and that they protrude 4" beyond the window space for every 3' of doorway or window width at each end.
- Prior to the commencement of interior or exterior cladding (this does not include brick or block veneers) check that the water content of the timber is below the maximum allowable level using a conductivity meter designed for that purpose.
- Prior to the commencement of exterior cladding check that the damp-course required is properly fixed. It is generally a layer of building paper with malthoid around the top and sides of window and door openings designed to shed water away from timber.
- Check that the paper has been laid from the bottom of the wall upwards so that the higher layers would shed water to the exterior. (The paper is best generally run sideways as that reduces the quantity of overlaps very significantly.)
Where these are structural then they will need to be filled with concrete with appropriate steel in those cells designated as columns and in the specified horizontal bond beams.
Pre-cast Concrete (Sections of walls)
- When the first few courses of blocks are being placed check that the required steel is being placed and being lapped and tied to the steel from the foundation or floor
- Prior to the pouring of the concrete to fill the blocks check that the steel for bond beams (or equivalent) is in place. There must be a bond beam at the top.
- Where steel is to tie the wall to a concrete roof ensure that it is in place and properly tied.
- If timber beams are to be set into the block-work or a cast bond beam at the top ensure that steel rods are in place passing though those beams as an integral part of the reinforcing system.
These may come in a range of sizes. Some may interlock to give larger panels which fill spaces between columns. Others may come as units which fill spaces between columns. Some are designed to interlock without the incorporation of columns.
It must be determined at the approval stage whether the assembly systems proposed will comply with the code, particularly in relation to storm conditions which may be encountered during the life-time of the structure.
An Engineer's certificate should be required at the approval stage certifying that they comply with the Building Standards and Code.
However that does not mean that checks should not be carried out as the Engineer's certificate will be relying on the correct method of assembly and inter-connection being used.
In complex situations it may be necessary to require the engineer or other person with sufficient specialised knowledge to make inspections and to certify that the assembly method being used is in accordance with the requirements.
An engineer or other specialist may be required to provide calculation demonstrating that at all levels of interconnection the structure will withstand normal loadings and storm loadings which may include downward forces and uplift forces.
It may be necessary to require additions to the design such as the use of slotting techniques for fixing at the floor and ceiling levels, columns to enable steel to connect between floor and roofing structure in those cases where the structure itself does not have an integrated set of columns to enable that.
Where pre-cast panels are to be used it is important that the reinforcing be adequate and properly placed. This may necessitate spot-checks being made at the fabrication plant.
Assembly - Prior to plastering
- Check that the panels are assembled correctly with the spacing at interlocking connecting joins being no more than the specified tolerance.
- Check that all auxilliary reinforcing is in place according to the plan.
Tilt-Slab Pre-cast Concrete (Entire walls)
When the precast walls have been lifted into placed and connected
- At the plan approval stage obtain from the design engineer a certificate stating the lift conditions and the reinforcing requirements.
- Check that the boxing will enable the correct thickness of wall to be poured.
- That the builder is aware of and has obtained supplies of the appropriate parting agent.
- That the steel is placed, supported, tied and welded in compliance with the engineer's design.
- That the steel which is required to connect into floors and others structures is protruding through the boxing in an appropriate manner.
- That the necessary connecting steel components have been set into the floor ready to be welded to those which are being set into the wall
- That the necessary connecting steel components are placed correctly prior to pouring the concrete wall.
- Check that the connecting bolts are all in properly in place.
- Check that the welding of the floor to wall slab connectors has been carried out correctly so that the joins will last with the required level of strength.
- That all metallic components are appropriately painted.
Nowadays the modern method for the construction of roofing structures, using timber, utilizes pre-fabricated timber trusses. Pre-fabrication may take place on-site or at a truss making factory. The strength of trusses is generally more satisfactory than using rafters - not that the timber is any stronger but the connection system is quite different and generally much stronger.
The timber truss system uses gang-nail plates which are nailed into the sides of timber members and chords which are butted together. As the nailing is horizontal it is inherently much stronger than skewed or vertical nailing.
Timber trusses can be designed to span any of the common dimensions normally found in residential construction with strengths able to withstand heavy snow-loadings or the strong uplift forces experienced in high wind prone areas.
A design certificate from a truss manufacturer or suitably qualified architect or civil engineer (where trusses are to be built on-site) or other appropriate specialist needs to be supplied at the approval stage. It must certify the upward and downward forces which the truss will withstand per unit length. From that can be deduced the spacing required to accommodate the risks of storms of particular intensities.
The architectural plans with a certificate, from an architect, engineer, or other consultant with specialist knowledge and expertise in the physics of roofing design for storm conditions, must give a truss and purlin plan with clear requirements as to spacings, fixings one to another and to the wall structure and the roofing to purlin nailing frequency.
On site - general and bracing
Connections to Walls
- Check that the truss construction agrees in all aspects with the truss design plans supplied.
- Check that no truss has had a cut made in it for any reason. Any cut in a truss will necessitate its replacement or rein-forcing with gang-nail plates.
- Check that the trusses are braced with metallic angle bracing per the plan across the top.
- Where gable ends are a feature of the design check that there is bracing on the top side of the bottom member of the trusses or nailing to an intermediate wall to prevent lateral movement during construction.
- Where there is a hip roof design check that any part trusses which butt to others are well nailed to them.
The force with which a roof structure is lifted upwards in a storm is proportional to its area and to the energy in the wind (which is proportional to the square of the wind's maximum speed). For a particular maximum wind speed the force with which a roof structure must be held down is then proportional to its area.
There are 2 excellent connecting components for connecting trusses to a timber plate at the top of timber framing or a concrete wall.
The following holding powers can be obtained wherever a truss connects to a timber bearer such as a top plate - anything else requires a special design.
- A Hurricane or Cyclone Strap - same thing - different manufacturers
- Z Nails
These 4 combinations are enough to cover what is needed for the normal range of residential dwellings
- 3500 N - 1 pair of Z nails
- 7000 N - 2 pairs of Z nails
- 10500 N - 1 Strap
- 14000 N - 1 Strap and 1 pair of Z nails
The specialist providing the roof structure design must specify the method of connection to the walls.
- Check that the components specified are being used in the correct numbers.
- With straps ensure that all nail holes have nails in them of the correct size and type.
- Check that Z nails are properly hammered home.
Unless there is a structural requirement in terms of rigidity plywood is not necessary when iron is being used with truss based structure. Plywood is also too thin a material to which to nail roofing iron.
3" x 2" purlins on the flat need to be fixed to the trusses as the structure to which the roofing iron will be attached.
Connections of Purlins to Trusses
Firstly a brief note on Purlin and Truss spacing. If the distance which a purlin spans between trusses is too much then it is much easier to snap than if it is shorter. The way to cope with this is to specify a maximum spacing between trusses. Other factors may further reduce this.
Similarly when the spacing between rows of nails along a length of roofing material become too large so that a sudden tug in the middle can cause it to tear at the nailing and rip off. The way to cope with this is to specify a maximum spacing between purlins. Other factors may further reduce this.
A roofing nail, nailed into a 2" thick purlin is optimal of it is 3" in length. (There is 1" in air between the top of the iron corrugation (or similar) and 2" into the timber.) It it is galvanized, twisted shank in nature then its holding power is 400 N. If there are 3 or 4 such nails into a purlin between 2 trusses they can hold the iron with a force of 1200 or 1600 N. It is not much sense to use a single 4" nail through a purlin as it will only hold with a force of around 500 N. Something with more holding power is needed. A Z nail will hold with 1750 N which is more than the 1600 N required. A 4" nail is needed also but for a different reason. It is needed to stop lateral movement of the purlin on the truss.
Are so many nails needed between trusses ? - The answer in a windy or hurricane prone area is Yes!
The following needs to be done at approval and construction stages
- Check that the design gives calculations that demonstrate, for the specified wind conditions, that the holding power of the iron to the roof is greater than the minimum required for the area in which the house is located.
- Check that the Z nails are being correctly used at each place where a purlin passes over a truss or rafter
- Check that 4" nails are being used through the face of the purlin into the truss or rafter
- Check that at gable ends of roofs there is 2 x 2 timber nailed down the length of the last truss between the purlins so that the iron can be additionally nailed lengthwise at the gable end. (The uplift forces are greater than normal at the edges of buildings.) This also need U nails into the outer-side (One, 2" in from the end of each segment.)
Connections of Metallic Roofing components to Purlins
This may done by one of a limited number of methods. Corrugated Iron (So called Galvanized.) needs to be nailed. The plan must specify the nailing frequency which. Some of the pre-painted baked enamel roofing materials are nailed whereas others are fitted onto clips fixed onto the purlins. Those which have a chip coating and simulate the appearance of tiles are nailed through the front of the downward facing lip.
Whichever material is used the manufacturer's instructions must be followed unless the Code or specialist's instructions over-rides them. Before the truss plan is designed the exact type of material to be used must be specified as it will affect the truss design and layout plan. The item which it must be careful to specify correctly is the exact spacing for nails of clips, to the nearest 1/16th of an inch (or nearest mm).
- Check that the nailing distance defined by the actual roofing material is identical to that specified on the plan. If there is a discrepancy obtain a new layout calculation and determine whether there is a need to make any changes to the purlin or truss spacings. (In those cases where there the truss span is on the small to medium size there is some tolerance for variations without changes to the layout plans.)
- Check that there is a layer of building paper supported by 2" gauge (mesh) netting being placed under the iron as it is being laid. The purpose of the building paper (which is waterproof) is to provide a method to drain condensation of water from the underside of the roof to the exterior of the building.
- Check that the nailing detailed in the roofing plan is being used - in particular that every second nail is not being omitted!
Prior to fixing of the ceiling lining
In relation to ceiling battens
- Check that there are 3" x 1.5" battens at 1' intervals across each room fixed to the underside of the trusses. (These serve 2 functions - bracing of the trusses and a medium to fix the ceiling to.)
- Check that the builder will use 1.5" (40 mm) clouts to fix the ceiling to the battens
Prior to fixing the wall Linings
Steel Beams, Box Steel and other Such-like
- Check that the water content of the timber is below the maximum allowable level using a conductivity meter designed for that purpose.
- Where insulation is required to be in the wall cavity check that is there with the required rating.
- Check that the electrical wiring is in place with switch holders properly fixed to studs at appropriate heights.
- Check that the plumbing required to be in the wall cavity is in place.
- Check that these are in places and correctly fixed to their support structures in accordance with the plan.
- An electrical inspector should check the electrical resistivities of the circuits and that each functions correctly when the electricity is connected.
- The fuses and or circuit breakers need to be checked that they are appropriate to the anticipated loadings.
- The plumber needs to carry out a pressure check once all fittings are in place to determine that there are no leaks in the piping.
- Each water circuit needs to be checked for functionality.
Prior to covering up sewage pipes in the trenches dug for them they need to be checked to determine that
- the slope is correct
- the pipes are properly bedded
- the pipes are properly connected.
Make a final inspection when the house is completed and check the following.
- That it appears to be water-tight.
- That the finishing is to the level specified in the plans.
- That all interior and exterior painting has been carried out to a proper and workmanlike manner.
- That any special requirements have been complied with.
- That everything required to be built, according to the plans, has been built.
- That there is overall compliance with the Building Code, Standards and that the house is in a generally safe and habitable state.
A building built with the code and the inspector in-mind will not only easily pass the inspections but will also stand the test of time, come rain, hail, hurricane or snow.
David L Evans Ph D
22nd November 2004
| For Creativity and Quality engage Evans Construction.... || |