Straight-grained timber should be chosen. Twisting, which may occur to the extent of 45°, is not so apparent in young trees. The beam is thereby weakened as regards tension and compression.
Large knots should be avoided. They weaken the beam for tension and cross strains, but serve good purpose when in compression.
Sap in the wood is denoted by a blue stain, and betokens inferiority as to strength and lasting qualities. This blueness is more noticeable when the wood is wet. It must not be confounded with dark weather stains.
The Decay of Timber.—Timber exposed to the constant changes of weather tends to decay early. It has been noticed that wood when dried after being exposed to dampness not only lost the moisture it had absorbed, but also part of its substance. This loss occurs in a greater degree when these changes take place a second time. It is therefore apparent that scaffolding timber, exposed as it is to the vicissitudes of the weather, without any protection such as would be gained by painting, will soon from this cause show signs of decay.
Quicklime, when wet, has also a most destructive effect upon timber; the lime, by abstracting carbon, helping the decomposition materially. Scaffold boards suffer most in this respect from contact with fresh mortar, the lime in which has not had time to become mild.
From a similar cause, the ends of putlogs which are inserted in a newly built wall, as when used in a bricklayer’s scaffold, tend to decay rapidly.
Scaffolding poles, when used as standards, have an increased tendency to decay at their butt ends, owing to their being imbedded in the ground to a distance of two or three feet.
The presence of sap in improperly seasoned wood is also conducive to early decay.
Preservation of Timber.—Scaffolding timber being comparatively cheap has little attention paid to it in regard to preservation.
When out of use the poles should be stacked, and a free current of air around each ensured, not laid carelessly on the ground, which is too frequently the case. The same remarks apply to the care of boards. Nails which have been driven into the timber should, after use, be carefully drawn. When left in the wood they rust, and set up a new source of decay.