Fig. 38: Cold deck pitched root cold root space (screed)
Fig. 39: Cold deck pitched root two ventilation cavities
Fig. 40: Cold deck flat root accessible root space
Fig. 41: Cold deck tlat root air cavity in root construction
Fig. 42: Warm deck pitched root one ventilation cavity
Fig. 43: Warm deck flat root no air cavity
Apart from the fundamental protective function ot the root, i.e., providing shelter tor human beings, keeping the water out is the main task ot the root. External influences (sunshine, rain, wind) but also those from inside (water vapour pressure) and the resulting problem ot water vapour diffusion give rise to further strains in the root construction. In order to do justice to these diverse demands, a multi-layer structure is necessary, which has led to two layering principles. One ot these systems is chosen depending on the given overriding conditions, the root form, the loadbearing structure, the conditions at junctions with other parts ot the structure and at the edges ot the root.
In the cold deck the waterproofing layer is so tar removed from the layer ot thermal insulation that a dry air cavity is formed between the two. This captures the water vapour dittusing out ot the insulation and carries it away.
A pitched cold deck has two air cavities: one between the root covering and the secondary waterprooting/cov-ering layer, and one between this latter layer and the i nsulation, although it is this second cavity that actually qualities the root to be called a "cold deck" (see "Pitched root" on p. 218).
In the warm deck the waterprooting layer or a dittusion-retardant layer, e.g. in a pitched root a secondary wa-terprooting/covering layer, is laid immediately above the thermal i nsulation. The water vapour diffusing out ot the insulation could theretore condense on the non-ventilated cold side ot the insulation and saturate this. A vapour barrier installed on the inside prevents the warm, vapour-saturated air entering the insulation and thus prevents any damaging condensation.
Flat overlapping elements
Profiled overlapping elements Flat sheets Profiled sheets
Sheet metal Flexible sheeting
Straw, reec Loose stone slabs Wooden shakes/shingles Fibre-cement slates Clay/concrete tiles Clay/concrete tiles Glass
Fibre- cement Metal
Metal (standing seam) Bitumen
Fig. 44: Relationships
Additional fixing (wind loads)
Seamless secondary waterproofing/covering layer (risk ot ponding) Root covering material
Couple root Purlin root
In situ reinforced concrete root with queen struts with queen posts trussed
Fig. 44: Relationships with queen struts with queen posts trussed
Relationships between roof pitch and roof covering material
The pitch ot the root depends on the root covering material, the root form, the fixings and the type ot jointing. A flat root must exhibit a seamless, waterproof root covering.
On the other hand, a root covering ot overlapping elements with its high proportion ot joints is better suited to a pitched root. The more watertight the root covering element and its joints with neighbouring elements, the shallower is the allowable pitch.
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