Fixing a Sagging Floor Before Installing a New Kitchen
You are renovating a 1970s house and about to install a new kitchen. The existing timber floor in the kitchen area sags about 25 mm over a 3.6 m span, and the floor is noticeably springy near the middle. The bearers are exposed in the subfloor and appear sound, but one bearer has a notch cut into its top edge about 40 mm deep where a previous plumber ran a pipe. The new kitchen includes a 900 mm stone-topped island in the middle of the room. The client does not want the floor level raised at the doorway to the hallway.
1. What is your diagnosis of the sag and springiness, and what would you check before deciding on a repair?
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A 25 mm sag over 3.6 m plus springiness points to a structural cause, not just wear. The likely candidates are undersized or over-spanned joists, a bearer that has settled or been weakened, or a notch that has reduced a member's capacity.
The notch is the first thing to assess. A 40 mm deep notch in the top edge of a bearer is in the tension or compression zone depending on the member and support, and it concentrates stress at the corner. A notch that deep in a bearer is a serious defect and may explain local deflection. Check the notch against the member depth: a 40 mm notch in a 200 mm bearer is 20 percent of the depth, which is well beyond what is normally allowed without engineering.
Before deciding, check: joist size, span and spacing against a span table; bearer size and support spacing; whether the bearers are founded on adequate piers or have settled; whether the subfloor is dry and free of rot; and whether the springiness is uniform or local to the notch. Also check the stone island load: a 900 mm stone-topped island is heavy and point-loads the floor, so the floor must be assessed for the new load, not just brought back to level. Get an engineer involved if the notch or the spans are outside the tables.
2. The client will not accept a raised doorway. What repair options remain, and how would you choose between them?
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The constraint is that the finished floor level at the hallway door cannot rise, so you cannot simply pack the floor up 25 mm across the room. Options:
- Jack and re-level the existing structure: if the sag is settlement of a pier or a bearer, jacking the bearer back to level and re-packing the pier may recover most of the 25 mm without adding height. This is the least invasive if the cause is settlement.
- Sister or replace the weakened bearer and any over-spanned joists: this restores stiffness without changing the floor level. It is the right answer if the notch or undersizing is the cause.
- Add a mid-span support (a new bearer and piers) under the island zone: this shortens the span and stiffens the floor where the heavy island sits, again without raising the floor.
- If the joists are simply too small, replace or deepen them from below, or add a steel beam, rather than build up from above.
Choose by cause: settlement is fixed by jacking and re-founding; weakness is fixed by strengthening or adding support. Do not use self-levelling compound to hide a 25 mm structural sag, because it adds weight and does not fix the springiness. The stone island is the deciding factor: it needs a stiff floor, so the repair must address deflection under the island, and the engineer should confirm the chosen option.