Decking is more popular than ever before - but poor design and construction can store up problems. Peter Lardi looks at the basic principles behind creating a solid, elegant and lasting structure.

Article used with kind permission of The Garden Design Journal. (www.landscape.co.uk)

Dealing A Good Deck

Decking evolved in the USA where timber frame houses are common. Often the house had a semi-basement which raised the ground floor above local ground level. Raised decks, or decks on sloping ground, are very economical compared with raised areas constructed in masonry.

However, when decking is used at ground level as a substitute for paving, visually and economically it is hardly ever successful. Much decking is constructed in the wrong context, without regard for essential design criteria - this is storing up problems for the future. Building disputes can take years to resolve, so it is important to get the deck right at the outset.

Translating often vague client aspirations into reality is always complex. A well designed deck looks simple, yet a simple design is always difficult to achieve. First, consider how the deck will be used - the relationship between house and deck, and the main routes across the deck. Often the client initially specifies a deck which is too big. Your 'use' study may show that the size can be reduced, and the corresponding cost saving is usually a plus point with clients.

Views from the house can be seriously affected by the deck and balustrades; a deep deck always impedes the view. Glass infill panels can help to minimise obstruction, but rarely match the landscape environment. Aspect is also important try to site the deck in a sunny location.

If this cannot be achieved adjacent to the house, consider locating the deck away from the building. Shade encourages algal growth on the deck.

Consider prevailing winds and if necessary construct wind breaks, but remember wind breaks create turbulence on the lee side and thus a low windbreak can be counter-productive. Overall, aim to achieve a warm and sheltered microclimate.

CONSTRUCTION MATERIALS
A wide variety of materials are available to the designer—hardwood, treated softwood, cedar and even plastic. Hardwoods are expensive and relatively difficult to work with compared to softwood and cedar. The high strength of hardwood allows thin sections but this in turn can have an adverse affect on section stability, leading to cupping and twisting. Hardwoods can become polished in use and slippery, particularly when wet.

Treated softwoods are widely available but the treatment fluids are toxic and care needs to be taken during construction to safeguard the health of workers and the householder. Cutting or drilling into treated softwood can destroy the treatment locally so that timber needs to be retreated on site.

Treatment fluids retained within the timber can corrode metal fixings. Specialist advice should be sought before specifying treated softwoods.

Western red cedar is considered by many to be the best timber for deck construction. It is naturally resistant to decay, needs no re-treatment, and weathers to a pleasant silver grey. It is a very stable timber and easy to work.

Small timber 'tiles' are becoming widely available for low-cost deck construction, but they cannot match the appearance of a classic timber deck.

A recent TV programme advocated using old railway sleepers to construct a deck. No mention was made of tar leakage onto clothing, or infection caused by creosote-saturated splinters. If you want to use sleepers, first check your professional indemnity insurance policy!

All timbers mentioned are resistant to decay but not wasp attack! The only effective method to eradicate wasps is to destroy the nest.

All timber deck boards should be specified with 'pencil' radii to all corners (i.e Canadian Lumber Standard). Not only does CLS timber look better than square-edged board, it also masks bruising of edges and helps prevent the corner from shedding splinters.

Some deck boards have grooves indented parallel to the length of the timber to provide extra grip, but these tend to prevent water draining off the surface.

Boards remain wet and collect organic material, leading to algae growth. Surface grooves also inhibit deck cleaning, which is essential to prevent the build-up of algae. Chicken wire netting nailed on to deck surface improves grip, but in most situations it is visually unacceptable.

The top surface of decking is subject to sunshine, drying winds and rain; whereas the underside remains shaded and relatively dry. To prevent cupping of deck boards, it is prudent to use a robust section with a width to depth ratio of 2 1 and for the boards to be securely fixed at the edges to underlying joists.

In a mistaken quest for economy, some clients request timber is purchased loose, for design and construction to evolve on site. There is no substitute for working out all details at the design stage.

STRUCTURAL CONSIDERATIONS
Structures have to be able to transfer load without the structure being over-stressed or excessively deforming in any part. There are many recorded cases of deck collapses in the USA. For most decks it is prudent to consider using the services of a structural engineer. You should aim to achieve a robust construction, well braced, with all elements properly sized and connected.

If the deck surface is 600mm or more above ground level, the National Building Regulations require a balustrade of 1100mm—all decks tend to look better with an edge feature of some sort.

The deck construction should always achieve the same load-carrying capacity as a timber floor in a house. If settlement occurs, it is not too difficult to jack-up and repack the posts. Bracing should be provided under the deck parallel and perpendicular to deck joists. Once completed, decks do not receive much maintenance and a flimsy construction will deteriorate rapidly. Aim to create a robust construction without stifling creative design.

Deck fixings have visual and practical impact on the deck. Many hardwoods and cedar are very susceptible to black staining when in contact with iron, and it is essential to use hot dipped galvanised fixings as a minimum (not plated fixings which quickly corrode and generate staining—a situation to be avoided at all costs).

Never rub down a hardwood or cedar deck with steel wire wool the next day you will have a black deck! Ideally stainless steel fixings should be used throughout. The initial expense will be worthwhile in the long term. There are various options for fixing deck boards nailing looks crude and can create bruised areas.

Smooth nails have limited grip and tend to split the boards particularly at ends near joints. Ring-shank nails with ridges along the length are virtually impossible to remove once driven.

It can be necessary to remove a deck board and thus nailing is the least satisfactory option. Properly sized screws hold the deck well, but the screw heads, like nailing, deface the surface. This can be overcome by countersinking and pelleting all screws, a labour intensive and expensive operation. Screwing on back-fixed battens produces sturdy deck panels with unblemished top surfaces.

This is the basis of the Leisuredeck method of construction. Leisuredeck has also developed a unique stainless steel clip, providing a blemish-free deck.
Stairs are expensive elements and always require careful design. Consider installing stepped ramps which are economical and visually attractive. Stairs and ramps should meet National Building Regulation standards.

DECK BOARD LAYOUT
Deck boards have to be laid with a 3-5mm gap between boards to allow for lateral movement (three per cent movement across the board width throughout the season is not unusual). This gap generates very strong linear patterns, which have a major impact on the finished deck appearance.

Boards laid parallel to the line of view appear to lengthen the deck, whereas laid perpendicular to view they widen it. Long parallel boards very clearly show slight deviations in joint width crossboards at intervals help to break up this effect.

Take care when using diagonal boards—the pattern can quickly become confusing and creates havoc with the layout of the substructure. If you joint boards on the diagonal, maintain 45 or 90 degree cuts any other angle will result in asymmetrical joints, with board edges mismatching.

Curves look good on paper but rarely work in practice and are wasteful of materials. If you want curves then consider cutting into the deck after construction using a router. Cut to just a few millimetres to avoid weakening the board. Consider creating curved areas set at different levels, filled with smooth cobbles to contrast with the deck. The opportunities for unique design input are infinite.

FINISHES
Some clients request that the deck should be stained or painted try to discourage this. Modern stains are translucent and when partly worn and recoated appear very patchy unless completely stripped to bare wood before re-coating—something that has to be done every few years. The silver-grey colour of untreated timber always harmonises with the landscape and obviously is the easiest and most economical finish.

Oiling is costly, needing several coats applied 'wet on wet'. If the first coat is allowed to dry, subsequent coats cannot penetrate into the wood. Painting hardwoods and cedar can present problems if tannin leaches through to stain the top surface. When considering furnishing the deck, the silver-grey colour of weathered timber contrasts well with smooth pale coloured furniture.

The prime aim of maintenance is to keep the deck clean and free of algae growth. Every six months a wash with Armillatox or similar is a simple and effective method of algae control. More frequent washing will be required for shaded decks.

Ensure the surface under the deck is covered with a weed inhibiting fabric topped with a layer of shingle to prevent weed growth. When metal furniture is used, ensure leg ends are covered with rubber ferrules to prevent indentation or iron staining. If the deck is dented make a small dam around the dent with Blu-Tack and saturate with water.

The dent will be reduced, and may even disappear. Make certain your client is aware that timber will contain splits, cracks and knots and will move with changing weather conditions. You are creating an element of landscape, not a piece of fine furniture.

STATUTORY APPROVALS
A deck would not normally require planning permission or building regulations approval. However if the deck overlooks neighbours, or if the main building is listed or in a conservation area, then planning permission may be required.

If the deck is set at a first floor level, planning and building regulations approvals should be sought. To avoid problems before proceeding too far with the design it is prudent for all deck proposals to approach the local authority for an informal comment on regulation requirements. Advise your client accordingly if permissions are required.

To customise your design, look out for unusual items or create them yourself—a focal point firepit, a spa pool and automatically watered flower boxes topping balustrades. Decks are here to stay!

PETER LARDI IS A CHARTERED CIVIL AND STRUCTURAL ENGINEER
AND A CONSULTANT FOR LEISUREDECK

THIS ARTICLE IS OFFERED IN GOOD FAITH BUT THE GARDEN DESIGN JOURNAL AND THE AUTHOR CANNOT BE HELD RESPONSIBLE FOR THE ACCURACY OF THE INFORMATION CONTAINED IN IT