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Constructing your Subtrate or Under Building
The substructure or underbuilding is that largely
hidden section of the building which is built off
the foundations to the ground floor level. |
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Under Building Construction
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Key Points |
1 |
Always cover the sand, bricks or blocks with a tarpaulin
to keep them dry. |
2 |
Always use a proprietary brand, never washing up liquid
as it has a detrimental affect on the compressive strength
of the mortar. |
3 |
Brick or block work must
be positioned centrally on the foundations. If the positions
are close to the inner or outer edge of the concrete foundation
when setting out the positions, consult your architect
or engineer. |
4 |
Always ensure that at least 150mm from the finished
ground level to the damp course is maintained. |
5 |
If you are having a timber frame erected the under building
size is critical as the tolerances must not exceed 12.5mm
on any length,
width or diagonal and 20mm on level. |
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The standard form of underbuilding construction uses two
skins of dense concrete blockwork for all external walls.
This is built centrally off the foundation concrete to the
ground floor or damp proof course level otherwise know as
DPC. If a facing brick is to be used for the external leaf
it is usual for the outer leaf of the blockwork to stop
just below ground level with the inner leaf taken up to
DPC level to support your chosen or specified flooring system.
Special foundation blocks are available which negate the
need for a cavity to go up to ground level but in most cases
the two skins of blockwork are built with a cavity that
reflects the width of the intended superstructure walling.
The two leaves are tied together with metal wall ties to
provide structural integrity and this is further increased
by filling the cavity with lean mix concrete to ground level
or at least 225mm below DPC. Care must be taken with backfilling
trenches before this fill has been installed and allowed
to go off.
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Internal Walls
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Internal loadbearing partition walls are
built as single skin walls, which are founded in just the
same way as for the external walls, although the width of
the concrete can sometimes be lessened. Where a suspended
timber ground floor is to be employed, sleeper walls are
built to provide interim support. These can be founded on
normal strip foundations but it is more usual for them to
be built off a concrete slab known as oversite or solum.
The oversite is built on the levelled
out and compacted ground within the building and it is thickened
out beneath the dwarf or sleeper walls. These are built
in ‘honeycomb' fashion with air holes to allow a free
flow of air within the void. Drains or services that pass
through walls must be sleeved or allowed to pass freely
through openings created by the use of concrete exit lintels.
You will need to determine precise positions and levels
of all of these.
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Basements
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Basements need to be carefully thought
out and designed by qualified engineers. There are various
means of construction including pre-formed concrete, poured
concrete, blockwork and hollow Styrofoam blocks filled with
concrete. These need to be reinforced and integrated with
any flooring system. All basements must be tanked or waterproofed
and once again there are various methods. Most involve the
application or building in of a waterproof membrane but
others employ a sump and pump system, which channels moisture
harmlessly away.
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Ventilation
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All suspended ground flooring systems
must be ventilated. With timber flooring systems employing
a concrete solum this is provided by means of air bricks
built at intervals and sleeved
through the external cavity wall. Beam and block or suspended
concrete systems, allow the oversite to remain as compacted
subsoil, and cranked ventilators are used. This allows the
free passage of air without letting light in, thus prohibiting
any vegetable growth.
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