Good craftsmanship Guide Foundations

1 good craftsmanship GuideFoundations, Substructures and Ground FloorsThis good craftsmanship Guidehighlights key problems withfoundations, substructures and groundfloors and gives guidance on how toavoid them. The problems are thosemost commonly identified during NHBC sinspection of homes under photographs are of defects and weretaken on real building Guide , based on the NHBCS tandards, is intended for use by NHBC registered builders andinspection staff. NHBC 2004No reproduction without NHBC prior permission in (strip and trench fill)Foundation depth2 Building near trees4 Setting out (foundation)6 Excavation8 Heave precautions11 Concreting13 SubstructureSetting out (substructure)16 Position of DPC17 Access and thresholds18 Drains/service entries20 Ground bearing floorsGround below fill22 Fill supporting ground bearing slabs24 Damp-proofing25 Insulation27 Suspended ground floors - blockand beam and precast planksDPC position28 Heave precautions29 Ventilation of underfloor voids30 Bearings31 Radon and other ground gases32 Contents1 Foundation depth2 What to do: excavate foundation to design depth (refer toAppendix of NHBC Standards when building onclay soil near trees)Problem to avoid.

2 Foundation movement3 Volume change potentialModified Plasticity Index Minimum depth (m)High 40% and 20% to less than 40% 10% to less than 20% Engineer s advice if foundation is more than deep in frost susceptible soils ensure minimum foundationdepth is 450mm to avoid frost action in soils that are subject to volume change on clay the following table should be observed4 Building near treesProblem to avoid: Foundation movement due to shrinkage or heaveof clay soilWhat to do: identify species of existing, removed and proposedtrees and their water demand measure height of trees and distance from Foundations (including trees outside the plot boundary) determine volume change potential of soil (if notknown, assume high ) determine the minimum acceptable foundation depthin accordance with NHBC Standards Chapter Building near trees 5 incorporate heave precautions see page 11 NHBC FOUNDATIONDEPTH CALCULATOR6 Setting out (foundation)Problem to avoid: Foundation in wrong place Foundation wrong size Foundation not square Foundation out of levelWhat to do: check all measurements carefully on sloping sites check measurements horizontally check diagonals are equal check levels carefully7boundarytrenchwidthdiagonalsdi stance fromboundarytrench lengthtrenchlengthExcavation8 Problem to avoid: Foundation movementWhat to do.

3 Foundations should bear onto consistent andcompetent ground investigation should prove that the ground iscompetent below the foundation for a distance of at least times the width of a strip or trench fillfoundation ensure trenches are in the right place and straight excavate trench to correct depth and width ensure trench sides are vertical and bottomshorizontal make sure any steps in trench are in the right place make sure any steps are of the right height andprovide for the correct overlap9 ToverlapSSToverlap the overlap should be not lessthan: 2 x S, or T (maximum 500mm), or 300mm, whichever is the largest the overlap should be not lessthan: 2 x S, or one metre,whichever is the largercontinued over obtain Engineer s advice if unexpected groundconditions or significant quantities of roots areencountered re-bottom trenches if affected by ground water or drying line of trench bottomstep not greaterthan depthground level in clay soils near trees steps should not exceed to avoid: Foundation movement due to heave of clay soilsWhat to do: provide heave precautions to the inside faces ofexternal wall Foundations that are more than deep11continued overHeave precautionsHeave precautions12 Note: 1 For compressible material the void dimension is the amount thematerial should be able to compress to accommodate heave.

4 The actualthickness of compressible material required should be established from themanufacturer s recommendations and is generally in the order of twice thevoid dimension shown. ensure correct thickness and type of compressiblematerial is used check manufacturer srecommendations and volume change potential of soil ensure compressible material is held in place firmlybackfill500mm450mmmaxverticalface tofoundationcompressible material(see table below)Void(see table below) Note: It is essential that:- compressible material isprovided to the entirearea shown, and- the foundation excavationhas a vertical face. Wherethe excavation is batteredor if there is overbreak orconcrete overspill it maybe necessary to consultan side of foundation and ground beamVolume change Void dimensionpotential[mm]1 High35 Medium25 Low0 Concreting13 Problems to avoid: Contamination of concrete Reduced strengthWhat to do: remove all water and loose material from base of trench re-bottom trenches if affected by ground water or drying make sure any reinforcement is correctly placed inaccordance with the design with at least 75mm cover(or as specified in the design)continued overConcretingWhat to do.

5 Ensure reinforcement is adequately lapped and tied make sure the correct mix of concrete is used inaccordance with the design check delivery notes forready-mixed concrete ensure the depth of concrete is in accordance withthe design and not less than 150mm if construction joints are unavoidable they should beformed away from returns in the foundation by oneof the methods opposite75mmsidecoverat least75mm cover14 JOINT USINGCORRUGATEDMETAL FORMERJOINT WITHEXPANDEDMETAL LATHJOINT WITHEXPANDEDMETAL LATH15 JOINT USINGREINFORCINGBARSS etting out (substructure)16 Problems to avoid: Inadequate support Substructure in wrong place Substructure wrong size Substructure not square Substructure out of levelWhat to do: make sure substructure wall is located centrally onfoundation to avoid eccentric loading check all measurements carefully check diagonals are equal check levels carefullyPosition of DPC17 Problem to avoid: Rising dampWhat to do: locate dpc at least 150mm above external ground orpaving levelat least150mm18 Access and thresholdsProblem to avoid: Unacceptable rampsRAMPED APPPROACH900mmmin1:121200mm max1200mm max1200mm clearLevel landingsWhat to do: ensure ramps to principal entrance do not exceed 1:12and ideally 1:201:1519 Note: The design of the threshold is critical in terms of accessibility andbeing THRESHOLD5mmmax15mm maxCill 15 max slope10-18mm50mm slab125mmmin75mmmax15mm maxDrains/service entries20 Problem to avoid: Entry of vermin and gasesWhat to do: mask service entries to prevent vermin entry.

6 Rigidplastics board is a suitable material50mm spacearound pipeopeningmaskedboth sidesPIPES PASSING THROUGH LINTELLED OPENING21lintel over multiple servicesflexiblejointflexiblejoint600mmm ax600mmmax150mmmax150mmmaxPIPES BEDDED IN WALLS where required, seal around drains and serviceentries to prevent entry of gases alternatively services can be built in provided thatmovement is allowed forGround below fill22 Problem to avoid: Movement of floor due to inappropriate fillWhat to do: strip all vegetable soil and organic matter ensure depth of fill will not exceed 600mm,otherwise use a suspended floor compact fill in layersground bearingfloor whereinfill is lessthan 600mmproperlycompactedfill and backfill23fill compactedin equal layerson both sidessuspended floorwhere infill ismore than 600mmWhat to do: ensure ground is suitable to support floor loads andany other loads compact trench backfill carefully and equally on bothsides of substructure avoid constructing ground bearing slabs on clay insummer and autumn unless NHBC is satisfied the soilis not desiccated24 Fill supporting ground bearing slabsProblem to avoid: Movement of floorWhat to do: ensure depth of fill will not exceed 600mm,otherwise use a suspended floor use well graded inert fill without hazardous materialswhich passes a 150mm x 150mm screen mechanically compact fill in layers not exceeding225mm25 Problem to avoid: Rising dampWhat to do: provide smooth blinding, sand use suitable dpm material 1200 gauge ( )polythene sheet ensure dpms are lapped at least 300mm and sealedwhere necessary link floor dpm with wall dpcsDamp-proofingcontinued over26at least225mmconcretecavity filldpcDamp-proofingInsulation27 Problem to avoid: Cold bridging and condensationWhat to do.

7 Provide insulation to floors in accordance with design protect insulation during construction to avoiddamageinsulationprotected byboarddpm trimmedto avoidbridging cavity28 What to do: provide dpc below end bearings of floor beams orplanks. Alternatively position dpc on top and providea dpm to top surface of floorDPC PositionProblem to avoid: Rising damp through floorsHeave precautions29 Problem to avoid: Movement of floor due to heave of clay soilWhat to do: provide void under floors to accommodate heavewhere required, to comply with NHBC StandardsChapter : 1 Measurement from underside of floor/beam to ground level (includes75mm ventilation allowance). Precast concreteSoil heave potentialVoid dimension[mm]1 High225 Medium175 Low12530 Problems to avoid: Condensation and dampness Build-up of gasesWhat to do: provide a void of at least 75mm (see page 27 for minimum void where heave precautions are necessary) ventilate underfloor void: provide clay air bricks (225mm x 75mm) at 2m centres provide clay air bricks (225mm x 150mm) at in Scotland Ensure air bricks / ventilators are unobstructedVentilation of underfloor voids31 Problems to avoid: Inadequate bearing Bridging the cavityBearingsWhat to do: ensure minimum bearing of 90mm at each end ofbeam/plank ensure beams do not obstruct the cavity32 Radon and other ground gasesProblems to avoid: Radon/Gas leaks Damp penetrationWhat to do.

8 Use correct materials carefully lap and seal membrane carefully seal around service entries (preformedcomponents are available) ensure membrane is not damaged position correctly across the cavity - stepped notdiagonal ensure good workmanship particularly at corners anddoor openings (preformed components are available) provide cavity trays and weepholes33 CORRECT INSTALLATIONINCORRECT INSTALLATION ground levelfloor trappingradon barrier above(or below) slabconcretefloordpccavity trayweepholesground levelfloor trappingradon barrier above(or below) slabconcretefloordpccavity trayweepholes34_____Notes35_____Notes36_ ____Notes37_____NotesNHBC, Buildmark House, Chiltern Avenue,Amersham, Bucks HP6 5 APTel:0870 241 4302 Fax:01494 735201 03/04