External cladding and rainwater goods are exposed to weather every day the house stands, which means shortcuts in the specification show up as real problems years later rather than immediately. A ventilated cavity behind cladding and correctly sized guttering are the two details that decide whether the system performs, and both are easy to omit from a quote without the homeowner noticing until there's a problem.
Ventilated cavity behind cladding
Timber and composite cladding systems need a ventilated air gap between the cladding and the breather membrane over the wall structure, to let moisture that gets past the boards evaporate rather than sit against the wall build-up. The spec should state the cavity depth (typically at least 25mm), the batten specification creating it, and the vermin mesh or insect screening at top and bottom of the cavity.
Cladding fixed directly to a wall with no cavity is a common shortcut that traps moisture behind the boards and accelerates rot, particularly on timber cladding, and it should be treated as a defect in the specification if found.
Cladding material and fixing
The spec should name the cladding species or product (treated softwood, cedar, composite), the profile (shiplap, board-on-board, feather edge), and the fixing method — stainless steel fixings are preferable to prevent staining, particularly on lighter timbers. Where render is being applied instead, the render system and base coat/mesh specification should be named rather than just 'render finish'.
Rainwater goods sizing
Gutter and downpipe sizing should be calculated from the actual roof plan area being drained, not selected from a standard size regardless of roof size. An undersized gutter on a large roof overflows in heavy rain even when installed perfectly, and this is a genuine design calculation the spec should reflect, particularly on larger extensions or new roof areas being added to the drainage system.
The material — aluminium, cast iron effect, or UPVC — and the fall (gradient) to outlets should also be stated, since an inadequate fall causes standing water and premature failure at joints.
Fascias, soffits and interfaces with the roof
Fascia and soffit material (timber, UPVC, aluminium composite) and their fixing into the rafter feet should be specified, along with how they interface with the roof underlay at the eaves to maintain the ventilation path into the roof void — this connects directly to the roof's own ventilation strategy and shouldn't be specified in isolation from it.
What cheap cladding and guttering quotes tend to skip
Ventilated cavity detailing, correctly calculated gutter sizing, and disposal of old fascias and guttering are common gaps. Access equipment (tower scaffold or scaffold hire) for full-height cladding work is another area sometimes quietly excluded from a cheaper price.
- Ventilated cavity with batten spec and vermin mesh
- Gutter and downpipe sizing calculated to roof area
- Stainless steel fixings for timber cladding
- Disposal of removed fascias, soffits and guttering
- Scaffold or tower access for full-height elevations
Common questions
Why does timber cladding need a ventilated cavity behind it?+
Without a ventilated gap, moisture that gets behind the boards from driving rain or condensation has nowhere to escape, which accelerates rot in the cladding and can affect the wall structure behind it over time.
How is gutter size actually calculated?+
It's based on the plan area of roof draining into that length of gutter, using published rainfall intensity figures for the region — not simply matched to whatever size gutter was there before.
Can render be used instead of timber cladding on an extension?+
Yes, and the choice affects the wall build-up and specification significantly — render needs its own base coat, mesh and finish system named rather than treated as a simple like-for-like swap.
Get a cladding and guttering specification with sizing calculated
We calculate cavity, fixing and rainwater sizing to the actual building before quoting cladding or guttering work.