The basic construction of a flat roof starts with roofing timber that is laid across two upright items usually two walls.
Flat lead roof build up.
Others would treat even steeper pitches as flat roofs.
However there is a lot more to them than a normal flat roof.
In a cold roof build up the vcl normally ends up on the underside of the joists or rafters and is punctured many times when covered over by the internal finishes.
When building a flat roof it is vital that a degree of falls is built into the construction to ensure rainwater run off.
The vcl in a warm roof.
Where insulation above the roof void allows the air underneath to stay warm avoiding condensation but also allows for an air flow should one be deemed to be required through the joists.
They require a root barrier to stop the plants interacting with the waterproofing beneath and a flat roof in particular requires a drainage layer to retain water.
If you build and flash one right and keep it clean a good flat roof won t leak says toh general contractor tom silva who had seven different flat roofs.
Having a manual for warm flat roof construction with details and a summary of rules to follow helps.
Should the ceiling below also have downlighters the vcl could be seriously damaged and therefore virtually useless.
Warm flat roof fully packed with insulation to prevent any condensation build up.
The definition of a flat roof according to bs 6229 is having a pitch less than 10 o to the horizontal however some in the industry would consider up to 15 o as their definition of a flat roof.
Investigative work carried out on warm roofs covered in lead has shown that in rare instances moisture can also be introduced by the development of sub atmospheric pressure in the roof space.
Coupled with some edging features to stop the vegetation leaking away this makes a green roof build up quite.
Lead is commonly used for flat roofs because of it s long life and ability to be formed to fit many positions lead must be laid and formed correctly to achieve this long life however.
More about lead as a building material.
When a hot roof surface is suddenly cooled by rain the pressure in the area between the vapour barrier and roof covering can fall below atmospheric.
Correctly laid lead roof greyfriars roofing note the height of steps between bays the rolled joints and adequate overlaps problems.
The fixing of lead sheet is carried out in many different ways all of which are dependent on the particular application.
Fortunately modern materials for covering flat roofs have improved considerably over the past two decades.
For instance a lead bay on a roof with a pitch of up to 3 degrees differs in the arrangement and position of its fixings when compared to the same bay on a higher pitch.
A further benefit of this type of flat roof build up is that the waterproofing layer is hidden and thereby protected from the effects of solar gain.
Given that the specially selected insulation layer is loose laid it will require ballasting to guard against wind uplift or floating in wet conditions.
