Who this guide is for
- Self-builders who will be on site while the substructure is formed
- Owners employing a contractor and wondering what to ask to see
- Owners assembling a handover pack at the end of a build
- Buyers asking what documentation exists for a recently built house
- Anyone planning an alteration to a building whose substructure is undocumented
The window is short and it does not reopen
A formation is at its best at the moment it is first exposed. It is a made surface rather than a natural one, and how long it stays open and what happens to it while it does, exposure, rainfall and site traffic, all change it after it has been formed.
So the sequence below is not a set of milestones that can be caught up on later. Each one is a moment when something is visible that will not be visible again, and the record made at that moment is the only account of it that will exist.
At the open excavation
This is the point at which ground assumptions are either confirmed or not. What was found at formation, and when it was covered, is worth having recorded, and the question of who inspected it before covering is one the reference entries put directly to professionals.
It is also where a decision may be taken to go further. What would trigger excavating deeper, and who makes that call, is worth agreeing before anyone is standing in a trench in the rain.
- What the formation is, and whether it matches what was expected
- Whether groundwater was encountered at or near the level
- How the formation is protected between exposure and the next operation
- Who inspected it, when, and what was recorded
Where the foundation is not a simple trench
Different foundations have different things to verify, and each of them happens once. Installed piles are verified rather than assumed, because the component sharing the work is the one nobody can see. Improved ground is verified once the treatment is complete. Pile positions are recorded because the cap design is resolved against where they actually finished.
For a raft, the whole of the make-up beneath the plate, barrier, insulation, service positions and joints, has to be right before the concrete arrives, because almost none of it can be revisited afterwards without breaking the structure.
Before the barriers disappear
The damp and gas barriers are laid early and then walked over, loaded, drilled and covered by other trades. Damage happens before completion and is concealed by the very construction that follows it, which is why verification happens while the plane is still visible.
The lap between the floor barrier and the wall damp-proof course is formed at the moment when neither trade regards the work as theirs. It is the least visible part of the arrangement and the one most often left out of both packages, and it cannot be checked afterwards.
Before backfilling and after it
A drainage network is conventionally checked after backfilling rather than before, because the fall achieved at laying is not necessarily the fall present at handover. The runs are set before most of the site exists and are then buried, driven over and built around by later trades.
That makes the drainage record a slightly different animal from the foundation record: one captures what was found, the other captures what survived.
What the record is worth later
Every use of a substructure record is in the future. A later alteration has to be assessed against something; cutting into a raft without as-built information on services, joints and barrier arrangement risks damaging reinforcement and the continuity beneath in the same operation.
For piled work, installation records, as-installed positions and test results are what a future extension or a boundary discussion will be judged against. For an underpinned building, what was found at formation and how the joint was made are recoverable only from records.
Who holds it
A record that exists in a contractor's files and not in yours is a record you do not have. The handover conversation is worth having before the final invoice rather than after it, and it is worth being specific about what is wanted rather than asking for the paperwork.
- The ground investigation report, factual and interpretative
- The foundation drawings actually built from, including revisions
- Records of what was found at formation and who inspected it
- Pile positions, test results or treatment verification where they apply
- Service and drainage positions, joint locations and barrier arrangement
- Which external openings belong to any ventilation or gas strategy
Substructure inspection and record checklist
- 1Agree at the outset which stages will be inspected and by whom.
- 2Agree how the formation will be protected between exposure and covering.
- 3Agree what would trigger excavating further, and who decides.
- 4Have the open excavation looked at and recorded before concrete is placed.
- 5Where piles are used, obtain as-installed positions and test results.
- 6Where ground is treated, obtain the verification testing results.
- 7Check the lap between the floor barrier and the wall damp-proof course while it is visible.
- 8Photograph the barrier arrangement and every penetration before it is covered.
- 9Record service and drainage positions before the slab or plate is cast.
- 10Have the drainage checked after backfilling rather than only at laying.
- 11Ask which openings belong to a ventilation or gas strategy, and record them.
- 12Collect everything into one pack before the final account is settled.
- 13Store it where a future owner would look for it.
Common mistakes to avoid
- Treating inspection as a formality, when each stage is the only chance to see something that will be buried.
- Leaving a formation open through weather and traffic and assuming it is the surface the design was based on.
- Relying on the contractor to hold the records, so the owner ends up with none.
- Asking for paperwork in general rather than for named documents.
- Photographing the finished slab instead of the barrier and penetrations beneath it.
- Assuming a drainage test at laying represents the network at handover.
- Discovering at the point of an extension that nobody recorded where the pile heads are.
- Filing the substructure record separately from the deeds and losing it at the next sale.
When to involve a professional
- What inspection is required on a project, at which stages and by whom, is set by the design, the contract and the relevant authority.
- Whether a formation is suitable is a ground-engineering judgement made on site with investigation data, not something a reference can answer.
- Verification of piles, treated ground and gas protection is specialist work carried out by qualified people at defined points in the programme.
- Cutting or coring completed substructure affects the structure and the continuity of anything buried within it, and is not done without design advice.
Frequently asked questions
Questions readers ask about this topic
Why does the open excavation need looking at?
Because it is the point at which ground assumptions are either confirmed or not. The formation is a surface in a condition rather than a depth, and two excavations to the same depth can arrive at very different formations, so it is judged when it is exposed.
What if the formation sits open for a week in the rain?
An exposed formation can soften, and a softened formation is not the surface the design assumed. Whether that has happened is a matter for site inspection, and an unprotected formation can deteriorate to the point where the level has to be taken lower.
Why are drains tested after backfilling?
Because the fall achieved when a run is laid is not necessarily the fall present at handover. Runs are buried, driven over and built around by later trades, so the network is conventionally checked once that has happened rather than only at laying.
What should be in a substructure handover pack?
The ground investigation, the drawings actually built from, what was found at formation and who inspected it, any pile positions or verification results, service and joint positions, and which external openings belong to a ventilation or gas strategy.
Does any of this matter if I am not selling?
It matters at the first alteration. Cutting into a raft or a ground beam without as-built information risks a structural element and a buried barrier in the same operation, and a later extension is assessed against what was recorded rather than what is remembered.
Keep reading