Searching for a beam and block floor detail usually means one of three things: you are drawing up plans, your Building Control officer has asked how the floor works, or your groundworker wants to know how it all fits together. This page walks through the standard details. Your own floor’s layout drawing will confirm the exact sizes for your job.
1. Suspended ground floor at an external wall
This is the key detail. The beams bear on the inner leaf of the cavity wall, with a ventilated void below and the insulation and screed on top.
What the detail shows
- Beam bearing: each beam sits on the inner leaf with the bearing shown on the layout drawing. Beams are never notched or cut to get a bearing.
- DPC under the bearing: a damp-proof course in the inner leaf, under the beam ends.
- DPC in the outer leaf: at least 150 mm above the outside ground level, so splashback and rising damp stay below it.
- Ventilated void: at least 150 mm clear below the beams, ventilated by air bricks with sleeves through the cavity.
- Insulation and finish: rigid insulation on the deck, with a perimeter strip at the walls to cut cold bridging, then a screed or floating board finish.
- Cavity: insulated above the DPC, and filled with lean-mix concrete below ground where the design calls for it.
2. Beams bearing on an internal wall
Where the floor spans onto an internal load-bearing wall, beams from each side bear on the wall head, again on a DPC. Insulation and screed run continuously over the top, so there is no cold spot over the wall.
Internal walls that carry beams need their own foundations, and the layout drawing shows which walls are load-bearing. Lightweight partitions that sit on top of the finished floor are a different case: they are carried by doubled beams under them.
3. Plan of a typical beam layout
On plan, beams run across the shorter span at the spacing set by the design, with infill blocks between. Doubled (or trebled) beams go under partitions, baths and other heavy loads, and slip or cut blocks close up the gaps at the walls.
Insulation options
| System | How it works | Good for |
|---|---|---|
| Rigid board + screed | PIR or EPS boards on the deck, then a sand and cement or liquid screed | Most houses; works well with underfloor heating |
| Rigid board + floating floor | Insulation boards with tongue-and-groove chipboard on top | A dry finish with no screed drying time |
| Insulated infill system | Expanded polystyrene panels in place of blocks, with a structural concrete topping | Very low U-values in a shallow build-up |
Which one you choose affects the overall floor depth, so it needs deciding before the walls are built to level. See suspended ground floors for more on the choices.
Common detailing mistakes
- Blocking air bricks with paving, soil or a patio laid too high
- Missing the DPC under the beam bearing
- Not enough void depth on clay near trees
- Moving a partition without checking the beam layout
- Leaving out perimeter insulation, creating a cold bridge at the wall
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Frequently asked questions
How much bearing does a beam need on the wall?
It depends on the beam and the wall, and the required bearing is shown on your layout drawing. Around 100 mm on masonry is common, but always follow the figure on the drawing rather than a rule of thumb.
How deep should the void under a beam and block floor be?
Building Regulations guidance generally asks for a clear ventilated void of at least 150 mm below the underside of the floor. On shrinkable clay, especially near trees, a deeper void is often needed to allow for heave.
Where does the DPC go in a beam and block floor?
There is a DPC in the outer leaf at least 150 mm above outside ground level, and a DPC in the inner leaf under the beam bearings. They are linked across the cavity where the detail needs it, for example with a cavity tray.
Do I need a DPM under a beam and block floor?
Not usually for damp, because the floor is suspended over a ventilated void. A membrane may still be needed for radon or other ground gases, or a vapour control layer under a screed, depending on your site and insulation system.
How is the void ventilated?
Through air bricks in the outer leaf with sleeves or periscope vents through the cavity into the void, spaced along opposite walls so air can cross the void. Ventilation needs to stay clear of soil, paving and the cavity.