Section B Substructure
Requirements for Foundations and Rising Walls
Foundations to be provided for all brick and block walls, chimneys and load bearing partitions including load bearing stud partitions. All internal foundations are to be the same thickness and depth as the external wall foundations.
Diagram B8 - Typical strip foundations
Standard Dimensions Required for Foundations
Diagram B9 - Standard dimensions required for foundations
Depth: Depth of foundation excavation to be at least 600mm below ground level at completion.
Width: Foundation must be at least 3 times the width of the wall it supports.
Thickness: Concrete to be at least 300mm thick.
The above are min. figures; special foundation design may be required and greater dimensions may be required. Larger foundations are required for larger elements such as chimneys.
As raft foundations are a specialist form of foundation design it is important that a qualified engineer designs them. The engineer must be qualified by examination, be in private practice and possess professional indemnity insurance.
Before raft design commences a site investigation must first be carried out, the results of which are to be taken into account in the design. Once designed the construction should be supervised and completed to the engineer’s satisfaction.
Excavation and Filling
Excavate area to a suitable level while ensuring all soft layers are removed.
Diagram B10 - Excavate to suitable bearing
Fill area using appropriate granular fill. Fill in layers not exceeding 225mm in depth. The filling and compaction process must be overseen by a qualified engineer. The engineer must be qualified by examination, be in private practice and possess professional indemnity insurance.
Diagram B11 - Fill using an appropriate granular fill material and compact
Once filled and compacted, first place reinforcement, then shutter raft, pour concrete, vibrate and cure. Steel and concrete to engineer’s specification.
Diagram B12 - Provide reinforcement, shutter, pour, vibrate and cure
DPMs and Screed
DPM must be provided along with insulation and a minimum of 65mm thick screed. DPM must be 1200 gauge; never use recycled material.
Diagram B13 - DPM to be provided, insulation and screed
When placed on top of insulation, screed should be at least 65mm thick with light mesh reinforcement incorporated. Insulation should never be placed under the raft. Water and heating pipes must be above the level of the structural slab.
When drains run near Foundations
When drains run close to foundations they can be subject to loads from the foundation. In order to prevent settlement of the foundations and/or fracturing of the drains it is necessary to take precautions.
There are 2 general precautions to be taken depending on the distance of the drain from the foundation when the drain is at a lower level:
If trench is within 1m of the foundations, trench is to be filled with concrete to the level of the bottom of the foundation.
If trench is greater than 1m from the foundations, the trench is to be filled to the level of the bottom of the foundation less the distance from the foundation less 150mm.
Diagram B14 - Drain less than 1m from foundation
Diagram B15 - Drain more than 1m from foundation
Services through rising walls
Clearance for service pipes and ducts is important as inadequate clearance or excessive rigidity can cause settlement or fracture. The opening should provide at least 50mm clearance all around the pipe. This opening should be masked with rigid sheet material to reduce the risk of vermin or fill entering. The void should also be filled with a sealant, the purpose of which is to prevent gas seeping in. For further guidelines see TGD H of the building regulations.
For clarity, the ope masking is not shown in the sketches.
Diagram B16 - Services passing through rising walls
50mm clearance should be provided for all services passing through the rising wall. Ducts to be provided where required.
Openings and Lintels
In the case of a large opening, a lintel should be provided. Use of metal lintels is not permitted in rising walls.
Diagram B17 - Lintels to be provided for large openings
Protecting Water Service Pipes from Frost
Guidance on adequately protecting service pipe and fitting and all cold water pipes against damage from frost is outlined in Clause 1.9 of Technical Guidance Document G – Hygiene, Building Regulations 2008 as amended July 2011:
Underground service pipe from external meter/stopcock must have a cover of at least 600mm. This cover is to be maintained along the entire length of the pipe. When near an external wall the pipe must be insulated with insulation which is impervious to water vapour.
Insulation for a cold water supply through a floor which is in contact with the ground.
Diagram B18 - Protecting water service pipes from frost - floors in contact with the ground
Insulation for a cold water supply through a suspended ventilated floor.
Diagram B19 - Protecting water service pipes from frost - suspended ventilated floors
Rising Walls Changes in Level
When rising wall is 215mm wide, change in level should be between 150mm and 860mm.
Diagram B20 - Rising walls - changes in level 150-800mm
For changes in level between 800mm and 1260mm and when the rise exceeds 4 times the wall’s thickness, the wall thickness should be specified by the engineer. The engineer should be qualified by examination, in private practice and be in possession of professional indemnity insurance.
Diagram B21 - Rising walls - changes in level 800-1260mm
In cases where the change in level is large between both sides of a rising wall, the wall, particularly in the partially built state, will act as a retaining wall. Consequently, wall thickness needs to be increased and great care needs to be taken also when compacting the fill material.
The height of a rising wall should never exceed 4 times the wall’s width. Where the cavity is not filled the thickness of the wall is taken as the sum of both wall leaves as illustrated.
Diagram B22 - Maximum height of rising wall with respect to its width
Diagram B23 - Maximum height of rising wall with respect to its width
DPC must be stepped with changes in site level or slopes as shown below.
Diagram B24 - Damp proof membrane installation on site with level changes or slope
Diagram B25 - Typical stepped DPC detail on sloping site
Diagram B26 - Level changes within a house
Use 1200 gauge polythene laid with joints sealed on binding material which will not damage the membrane in all cases where the DPM is laid under the concrete.
Diagram B27 - Detail A - Typical vertical joints in the DPM or DPC
Minimum lapping of vertical joints for dpm or dpc should be 150mm along the length of the wall. Joints should be taped using double-sided proprietary sealant tape.
In order to prevent dampness travelling beneath the floor of a higher house penetrating through a party wall above the floor level in the next house it is advisable to make these party walls cavity walls i.e. wall between House A and House B below. This wall must be drained to the outside face of the building.
Diagram B28 - Path of flanking dampness
Diagram B29 - Level changes in adjoining houses
Diagram B30 - Detail X - Typical party wall detail with a change in level
It is important in a situation where the ground level is greater than the floor level that measures are taken to eliminate the risk of moisture penetration. When designing the house the engineer or architect should provide barriers sufficient to prevent this.
If a case occurs where part of the house is below ground level, it is recommended that at least the extent of the wall which is underground be built as detailed below or alternatively the entire wall is to be fully tanked where there is a possibility of water pressure building up.
Diagram B31 - Internal floor level partially below ground level
When an entire floor is below ground level, as in a basement, tanking should be provided. In this situation, there should be no build-up of water pressure and the depth of cavity below ground level should be drained.
Diagram B32 - Internal floor level partially below ground level
Diagram B33 - Detail A - DPM installation where internal floor level is below ground floor level
In order to reduce the risk of clogging by migration of fines it is recommended to wrap the perforated drain in a geotextile filter.