Because landscape design and construction is often carried out adjacent to or as an extension to buildings, the interaction between the landscape and buildings must be considered carefully. If done incorrectly, landscaping work could have a detrimental effect on the performance or durability of the buildings themselves
Below are important elements to consider before designing and constructing the landscape surrounding a variety of buildings.
One of the performance criteria in Building Code clause E2 External moisture is the requirement to prevent surface moisture from outside entering a building.
To achieve this, the ground floor level inside a building must meet the specified minimum distance above finished ground or paving levels outside the building (Figure 1).
Soil or mulch material must not be built up against the exterior wall cladding of a building, as this can:
Lead to premature deterioration of most cladding materials.
Cause paint coatings to blister and peel.
Obstruct drainage from veneer or cavity-fixed claddings and low-level windows.
Wick water up into framing or floors.
Strip drains may be required immediately beside buildings, where there is ramped access at door thresholds for full mobility access.
Figure 2 outlines the support requirements when excavating the likes of trees, walls, pools, and poles adjacent to a typical timber-framed building. Separation distances between the excavation and a foundation wall will increase with foundation depth. The detail also applies to the positioning of a retaining wall or cut ground to avoid surcharge.
Plants and buildings
Trees planted too close to buildings can cause major problems, including cracks in foundation walls or slabs, and lifting or settlement of floors that could result in internal damage and jamming of doors. Mature trees, for example, are capable of removing large amounts of water from soil, causing shrinkage in expansive soils.
Thick vegetation that is too close to a building may intensify dampness problems, if it blocks air movement around the structure. It also makes maintenance work, such as the regular repainting of timber weatherboards, more difficult.
The danger of falling branches should also be considered when selecting trees to be planted in the vicinity of buildings, structures or pathways. Some species are more susceptible than others to breakage in high winds.
Plants that climb up building structures have a significant effect on the performance of the materials they attach themselves to. The root filaments or suckers can weaken the material they bond to by damaging the structure of the material, damaging protective coatings, or separating joints between materials or elements. Other effects are:
Trapping moisture against the building material.
The increasing weight of the plants on the building.
Filaments or roots migrating into building cavities.
Removal of essential moisture from older porous materials (lime mortars).
Filaments or roots blocking stormwater and foulwater drainage systems (pohutukawa trees are notable offenders in this regard).
Blocking of ventilation openings to subfloor spaces.
Added risk of fire spread.
Plants also require certain depths of topsoil to be bedded into.
The effect of plant growth and root systems on buildings, drains and other structures such as fences, driveways, paths, roads and retaining walls must also be considered. Damage from inappropriate plant choice or planting trees too close to buildings is common.
Building materials and finishes that remain continuously damp (except those specifically manufactured for ground contact or use in wet conditions) will deteriorate much faster than those that are allowed to dry out.
A common example of this in landscape design is mulch or soil built up higher than the bottom edge of the cladding material.
This article was first published in BRANZ’s Landscape Construction book, Second Edition and is reproduced with permission.
The book covers the design, building and planning rules as they apply to landscape construction and provides guidance on design and construction to ensure good practices are followed. It can be purchased at www.branz.co.nz.