Windows, Explained Simply

Every term on this page, in plain language · Gibson St shed · west wall, two salvage barn sash

The one idea

You build a hole. Then you hang a window in the hole. Those are two separate jobs, months apart. Everything else is detail.

Rough opening (people say “RO”) is just the carpenter's name for the hole. It is rough because it does not have to be pretty — trim covers it later. It does have to be the right size and square.

What makes the hole — four pieces

On most houses the studs are short stubs between the sill and header. Yours are the other way round — the uprights run full height and the sill and header fill in between them. That gives the liner one continuous edge to land on instead of three short ones.

Why these pieces stick out

Your walls have two kinds of wood in them:

The window framing has to be proud, and here is why. That wood is the jamb — the surface the window closes against and the little stop strips nail to. If it were ripped and buried, the edge of your window opening would be the sawn edge of a sheet of plywood. Nothing solid to nail to, and a raw ply edge showing all the way round the glass.

The window itself

How the glass stays in

Prime the bare rabbet before you putty. Dry wood sucks the oil straight out of fresh putty and it cracks and falls off in a couple of years. One coat of oil primer first, let it dry, then glaze.

How the sash stays in the hole

1. Hinges at the top — it swings out like an awning
2. Stops — thin strips of wood nailed inside the opening, flush with the inside face of the wall. The sash closes against them, the way a door closes against a door stop. They are what makes it stop in the right place
3. Hook and eye — holds it open, and holds it closed
4. 1x4 trim outside, covering the joint between the sheathing and the framing

Jamb vs. stop vs. casing, since these get muddled. The jamb is the wood lining the hole — on your build that is the framing itself. The stop is the small strip the sash rests against. The casing (or trim) is the decorative board covering the joint on the outside. Three different pieces, three different jobs.

Keeping water out

Water will get past an old sash — wind-driven rain, cracked putty, condensation running down the glass in winter. The job is not to stop it. The job is to give it somewhere to go.

A sill pan is a shallow tray under the window. Water lands in it and runs back outside instead of soaking into the framing. It has a lip at the back so water cannot run indoors, walls at the ends so it cannot run sideways into the corners, and an open front so it drains.

The shingle rule: every layer laps over the one below, like roof shingles, so water can only ever run down and out.

1. Sill pan onto bare framing — first, before the window
2. Window in, sitting on the pan
3. Jamb flashing over the pan's ends
4. Head flashing over the jamb flashing
5. Housewrap over the head flashing

Do it in the wrong order and you have built a funnel.

The whole glossary, one table

Your numbers

Measure your actual sash before cutting anything. Salvage sash vary. The rule is RO = sash + ½″, measured off the two you are keeping — not off this page.

More detail: Window Install and Drainage for the sill pan, flashing, underlayment choices and current fixed-sash direction · Wall Framing for the framing itself · Barn Sash Search for the sash condition and restoration · Build Guide Phase 4–5 for sequence.