Housing and You…Use of Guyanese hardwood in construction

Mar 09, 2014 News

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By Leonard Gildarie
We have been examining the National Building Codes as published by the Guyana National Bureau of Standards over the last few weeks. After taking a break last week to talk about a new kind of lumber product being offered to the market by Bulkan Timber Works, we turn our attention once again to the codes. This week we will be examining some of the main points of the “Use of Guyanese Hardwood in Construction”.
Now, for the layman, in simple language, the use of lumber does not just mean buying any pieces from a sawmill or a lumber yard and saying that it will be used to build a home. There are specific uses and the codes set out what is acceptable. In other words, you would not be using a quarter-inch plywood to build a roof or as rafters.

The standard was developed in 1999 by Guyana National Bureau of Standards (GNBS), after being finalised by a Technical Committee – Civil engineering, and approved by the National Standards Council.
It was supposed to provide guidance on the use of Guyanese hardwood for construction purposes.
According to GNBS, the standard is intended to be made mandatory. It includes recommendations on quality, engineering properties and the various design considerations and principles for simple members, build-up components, composite structures and sub-structures incorporating other materials. Requirements and recommendations for sound construction and typical details for residential construction are also included. Further, recommendations for the design of heavy engineered structures, nailed, screwed, and bolted joints are also presented.
The code speaks of dry stress: The stress applicable to solid timber exposed in conditions which would result in it having specified maximum moisture content. For the purpose of the code, the moisture content for dry stress shall not exceed 18% in service.
It also talks about grade stress: Stress which can safely be permanently sustained by timber of a specified specie, grade, strength class and section size. The code made it clear that the heartwood of many local timber species is naturally durable. However, the sapwood of all species is susceptible to bio deterioration.
Importantly enough, timber that has high natural durability may be used for structural members without preservative treatment, provided that sapwood content is excluded or minimised.
Now I don’t have the space to include what size of wood has to be placed, whereas the code contains several tables. It would be instructional for contractors and others to familiarize themselves. I rather suspect that many of them are aware that there is an acceptable standard.
The codes made it clear that lumber itself has a load-bearing limit – meaning simply that with too much weight you will end up with problems. I remember a neighbour who was building and the workers fell from the roof after the rafters came crashing down. They were not properly attached.
The code said that there is specific spacing between the rafters and ceiling joists that should be used: rafters where close-boarded ceiling is used – 900 mm; rafters where purlins are used – 1200mm;
ceiling joists supporting plastered ceiling – 400 mm centres; and ceiling joists supporting other forms of lightweight ceiling, such as fibreboard, plywood or tempered hardboard – 600 mm.
Roof members shall be effectively anchored to satisfy uplift requirements.
According to the code, in areas where relatively high wind loads resulting in uplift pressures are not normally expected, each individual rafter may be notched over and nailed to tie beams, plates etc, on which it beams. At a ridge, the rafters shall also be effectively nailed.
Where relatively high wind loads and uplift pressures are normally expected, each rafter shall be anchored to the tie beam, plate or truss on which it beams with steel straps or equivalent metal fastener. Ceiling joists shall be nailed to the bearing plates, to each other where they lap.
Ridge boards, hip and valley rafters shall have sizes not less than the largest rafter framing thereto.
This is all technical language that drives home the point that construction of a home is not just nailing two pieces of wood together and placing a few blocks and windows.
I know that many, including myself, may have to ask the contractor or someone with technical knowledge to explain.
What is good about the codes is that it tells you what size of lumber can carry what kind of load.
I stop there and ask, like before, that we read the codes as there may be many, including contractors and homeowners, who will want to cut corners. One can never know when an accident will happen. That costs money to fix, in addition to keeping in focus the importance of safety considerations.
In addition to that, one day the Government and its regulatory arm will wake up and start enforcing some of these codes in a big way. Let us not wait on them.
It would be better to familiarize ourselves with what is acceptable in terms of standards, globally, than continue to experiment. That is the only way we can move from the so-called third world classification to become a developed country. Rules are made to be followed.
Enjoy the weekend and please continue to send those emails to [email protected] or call me weekdays on 225-8491 for comments or suggestions.

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