Notes Extensions, chapter 2 of 4
The opening into the house
Most of an extension is new. The part that decides the structure is the gap you make in the house to reach it.
The photograph is an ordinary two-storey house cut through in section. Everything drawn over it is the engineering.
Step by step, on the model
How the opening is made, and where the load goes
Illustrative image
The outer wall, today
The extension will join the house on the right, so start with the outer wall there. The floor joists run into the page, past it, so it holds up no floor. It carries the roof where the rafters land, and the first-floor wall standing on it, down to its own foundation and into the ground.
Illustrative image
The wall above is held up first
First the wall above is held up on temporary supports, so nothing drops while the slot for the beam is cut. We design them and the order they go in and come out, and the builder works to that design. The wall article draws it step by step.
Illustrative image
The beam goes in, above the old wall
A pair of steel beams goes in at the head of the opening, one under each leaf, packed tight to the wall above. They run into the page, so you see their ends, each a solid I. They bear on the house out of this view. Once the packing has had the time our drawing sets, the props come out.
Illustrative image
The extension is built round it
The model changes. The extension is built with its own slab, walls and flat roof, while the old outer wall still stands between it and the kitchen.
Illustrative image
The wall below goes
Only now, with the beam working and the extension closed in, does the ground-floor wall come out. The dashed outline is where it stood.
Illustrative image
Two separate routes
The roof and the wall above now come down onto the beam, which carries them sideways to its two ends. Each end needs something solid under it, all the way down to a foundation. The extension roof takes a separate route: down its own end wall to its own foundation, then the ground. And which way does the extension roof span? If its joists run from the house to the end wall, the house end of every joist lands on or beside this beam, and the beam carries that too.
Illustrative image
What decides the beam
Three things: how far it spans, how much it carries, and how far it may sag. The span lesson in the five pounds a kilo note shows why span matters so much.
Illustrative image
When to call us
While the opening is still a line on a sketch. Its width, where it sits and which way the roof spans all decide the beam, and each is cheap to change before the drawings are fixed.
Illustrative image
Which way does the extension roof span?
It is the question an architect’s drawing can leave open, and it changes the beam. put one end of every joist on or beside the beam. land on the side walls, and the beam carries only the wall above.
Illustrative. Not a calculation and not a design.
Key to the drawings
- Existing masonry A flat tone: everything already there, cut or seen.
- Brickwork, new Single diagonal hatch. The outer leaf, and new brickwork in elevation.
- Blockwork, new Cross-hatch, so it can never be read as brick beside it.
- Concrete Aggregate triangles and fine dots, scattered at random: foundations, slabs, padstones, lean-mix cavity fill, encasement.
- Undisturbed ground Basket weave, as a band that follows the ground line and fades out.
- Hardcore Large open zigzag, the depth of its own layer.
- Insulation board The looped line, touching both faces, inside the board outline.
- Timber, cut A box with a cross. Seen along its length, an outline only.
- Steel, cut Solid, at the section's true proportion.
- Pressed steel Lintels, hangers, straps: a line at the wide weight.
- DPC Heavy dashed line, as Approved Document C draws it.
- Membrane, tray Heavy solid line: DPM, cavity tray, roof membrane.
- Load path Where the load goes, from the roof to the ground; dashed where it runs beyond the cut.
- Load marks Arrows at the symbol weight: loads down onto a member, bearing up under a foundation.
- Hidden, beyond or removed Out of the cut, beyond it, or taken out.
- Cloud Confirm on site. Never a revision mark.
- Break line The drawing stops; the building does not.
What does each end of the beam stand on?
On the model the beam is seen end on, so its ends are out of view. Turn it through a right angle and they are the whole story.
Diagram drawn for this site. Not to scale, and not a design.
In an older house the masonry each side of the opening is often narrower than you would like and its foundation shallower than you would guess, so we ask what is under the floor at each end before we size anything above it.