Fig. 16 shows a design for a small outdoor storage unit made from timber and manufactured boards. The unit is intended to store gardening tools and will be ...

(a) A suitable hardwood for the frame of this outdoor storage unit is oak. [1]

Two properties that explain this choice:

1. Oak is strong and hard-wearing, able to support the weight of the unit's structure, door, and stored gardening tools without flexing or failing. [1]
2. Oak is durable and resistant to outdoor conditions when treated with a suitable finish, withstanding rain, frost, and UV exposure over many years. [1]

(Teak is also an excellent choice, with natural oils providing inherent resistance to water, rot, and insect attack.)

(b) A suitable manufactured board for the back panel and internal shelf is exterior-grade plywood (marine plywood). [1]

Plywood is strong, lightweight, and available in exterior-grade versions that use waterproof adhesive (such as WBP - weather and boil proof) between the veneers, resisting moisture penetration in the outdoor environment. [1] It is also available in large flat sheets that are dimensionally stable.

(c) A suitable material for the roof panel is exterior-grade plywood covered with roofing felt. [1]

Attachment method: The plywood panel is cut to size with a slight overhang beyond the side walls to allow rainwater to drip clear of the unit. [1] It is screwed to the top of the timber frame. Roofing felt is then laid over the plywood surface and secured with felt nails or bituminous adhesive, creating a continuous waterproof membrane. [1] The felt overlaps at the edges and is folded under to prevent water ingress at the joins.

(d) Two methods of protecting the timber frame from weathering and decay:

Method 1: Apply an exterior wood preservative (such as Cuprinol or Sadolin). [1] The preservative soaks into the timber and contains chemicals (such as copper-based fungicides) that prevent fungal growth, rot, and insect attack. Some preservatives also include a coloured stain that provides additional UV protection, slowing the greying effect of sunlight. [1]

Method 2: Apply exterior varnish (such as yacht varnish or marine varnish). [1] The varnish forms a hard, waterproof, transparent coating on the surface of the timber that prevents moisture from penetrating the wood while allowing the natural grain to remain visible. Multiple coats build up a thicker, more durable barrier. [1]

(e) Tongue-and-groove is a type of edge joint where one edge of each board has a projecting strip (the tongue) machined along its length, and the adjacent edge has a matching slot (the groove) cut into it. [1] The tongue of one board slides into the groove of the next, creating a continuous interlocking joint along the full length of every pair of boards. [1]

One advantage of this joint for an outdoor door panel is that the interlocking connection allows the boards to expand and contract with changes in temperature and humidity without gaps appearing between them. [1] The tongue slides slightly within the groove, accommodating seasonal wood movement while maintaining a sealed, draught-free, and weather-resistant surface.

(f) Timber must be seasoned before use because freshly felled timber has a high moisture content (often 40-200% depending on species). [1] If unseasoned timber is used to construct the unit, it will shrink, warp, and distort as it dries in service, causing joints to loosen, panels to crack, gaps to appear, and the overall structure to become unstable and weak. [1] Seasoning brings the moisture content down to a stable level (typically 12-20% for outdoor use) before construction begins.

(g) Comparing manufactured board with solid softwood for the side panels:

Two advantages of manufactured board:

1. Available in large, flat sheets that do not warp or twist, providing dimensionally stable panels without needing to join multiple narrow planks. [1]
2. Consistent thickness and properties across the entire panel, with no knots, grain variations, or weak spots that could affect strength or appearance. [1]

Two disadvantages of manufactured board:

1. Cut edges need sealing or edge banding to prevent moisture absorption, which adds an extra manufacturing step not needed with solid timber. [1]
2. Manufactured board does not hold screws as well as solid timber, especially at the edges where the material can crumble or delaminate, requiring careful selection of fixings or the use of pilot holes and inserts. [1]

(h) The shelf could be supported using metal shelf brackets (L-shaped brackets). [1] Two brackets are screwed horizontally to the inside face of each side panel at the desired shelf height, using appropriate screws for the panel material. The shelf panel is then placed on top of the brackets and can be screwed down through the bracket flange into the underside of the shelf for a secure fixing. [1]

Alternatively, timber battens (bearers) could be screwed horizontally to the inside of each side panel, with the shelf resting on top.

(i) Cedar is a softwood. [1]

Two properties of cedar that make it suitable for outdoor use:

1. Cedar is naturally resistant to rot and insect attack because it contains natural oils and extractives that repel moisture and deter wood-boring insects. [1]
2. Cedar is lightweight and dimensionally stable, resisting warping and shrinking even when exposed to fluctuating outdoor conditions. [1]