Many hut designs will incorporate decks and stairs externally and some may have internal galleries and stairs. The variety of possible designs for these structures is almost infinite but it is important to consider the structural integrity and safety aspects of each. High wind speeds and the wet climate in many parts of Scotland pose particular challenges in the construction of these useful appendages which, if not carefully designed, can become a danger to users and a maintenance nightmare.
The principles of accessibility are laid out in Access and accessibility of this Guide and should be applied on a site specific basis.
In the most general terms, access routes to a hut, which include stairs, ramps or walkways, should never be less than 900 mm in width and will ideally be wider. Providing robust structures, safe surfaces and good handrails will be the most important considerations and guidance to achieve these is given below.
Whilst not a blueprint, the principles, deck and balustrades illustrated in Appendix 3 can be followed but you should seek professional advice if you have any doubts about the safety of your design.
A building warrant is required for any work associated with open raised external decking at a height of more than 1.2 metres
When building a deck, two aspects of structure need to be considered: (1) raised substructure (2) balustrade structure.
The structural stability of a deck is always important and you should follow the principles in figure Appendix 3. The deck illustrated has been engineered and you should follow the design and fixing details shown.
Where deck structures are built on sloping ground, as they often are, supporting structure can get unexpectedly tall and structural stability will become an issue.
Many of the same principles as apply to post and beam framing of a building apply equally to a deck structure. It is easy to under-estimate the loads that decks and especially their balustrades are subject to so always consult an engineer if in any doubt. As noted above, if the deck surface is more than 1.2 metres above the surrounding ground at any point you will need an engineer’s certificate to support a mandatory Building Warrant application.
Decking boards should be a minimum of 22 mm thick (preferably 28-30mm) to span joists on 400-600 mm centres. They should be fixed to every joist with at least two proprietary decking screws or hot dipped galvanised nails.
PROTECTIVE BARRIERS TO DECKS, WALKWAYS, GALLERIES, RAMPS AND STAIRS
The section on Pedestrian Protective Barriers details the way in which compliance with regulation 4.4 can be achieved. This is mandatory to qualify for exemption from building warrant for your hut design. This Section repeats much of what is stated there but in the context of external access structures and arrangements.
Almost by definition, decks will be raised off the ground and the design of protective barriers at deck perimeters should follow the following specification and the balustrade design in Appendix 3
The most critical design requirements are:
Protective barriers are required wherever the difference in height between the deck surface and the adjacent ground is more than 600 mm
- Protective barriers must be 1100 mm minimum in height for decks and level walkways
- Balustrades providing edge protection for stairs and ramps must be 840 to 1000 mm in height
Openings in a protective barrier should prevent the passage of a 100 mm diameter sphere. The only exception to this is at the base of a stair balustrade, which must be no more than 50 mm above and parallel to the pitch line.
In addition, measures should be taken to discourage climbing of the barrier by children. Solid horizontal rails are, for instance, relatively easy to climb whereas balusters (verticals) are not. Meshes (e.g of wire netting) are acceptable where the 100 mm rule is met. Horizontal wires are less safe than vertical but may be acceptable where falls are not excessive.
External timber decks and stairs can become very slippery over time and the wetter the local climate, the worse the situation. A variety of deck finishes and treatments are available to reduce the risk of slips and falls. These range from profiled/grooved surfaces to inserted grips and complete gritty coatings.
The choice of what measures to take to improve the grip of feet on decks will depend to some extent on the local climate, the degree of protection and the amount of maintenance and day to day care that can be given to the condition of the surface. The lowest maintenance and probably highest capital cost option is inserted non-slip grips. Decking boards can be purchased with these already inserted or they can be inserted by a joiner.
DURABILITY AND MAINTENANCE
Many of the same issues around the durability of timber decks also apply to timber cladding except that horizontal surfaces are subject to more severe u/v degradation, more wear and longer periods of wetting. Decking should therefore always be made of naturally durable timber such as larch or oak, preservative impregnated timber, acetylated or heat treated timber products. This goes for both the decking boards and the substructure. Surface coatings with high quality paints and stains incorporating preservatives will further help. Tropical hardwoods are often very durable but should be sustainably sourced (Durability of Timber). The high embodied energy of timber imported from the tropics does not make it a good choice for huts.
Ideally, fix decking boards with proprietary decking screws so that individual boards can be easily replaced if they show signs of failure. Use only galvanised bolts and exterior grade fixings for the substructure.
At the least, decks should be scrubbed or pressure washed from time to time to remove slippery algae. There is an increasing range of brush or spray applied coatings to reduce and remove algal and fungal growth and there are also some gritty coatings available.