How to Install a Door Casing: A Step-by-Step Guide

A door casing is the timber frame that lines a wall opening and carries the door, with a rebate that forms a built-in stop. Get it plumb, square and flat, and the door closes evenly into that rebate. Rush it and you cannot hide the mistake by moving a planted stop later, because the stop is already part of the frame. This guide covers fitting a solid wood casing set, such as our Solid European Oak Door Casing Set, into a masonry or stud wall. If you would rather fit a thinner lining and add the stops afterwards, see our guide on how to install a door lining.

What's in a casing set

Our oak casing sets come as two legs (jambs) and a head, with a rebate that forms the door stop. The liners are 32 mm thick. On the standard sets the rebate is 35 mm wide and 12 mm deep and will take doors up to 35 mm thick. Prestige sets have a 44 mm by 12 mm rebate: the European Oak Prestige Door Casing Set (42mm doors) is made for 42 mm doors, and the American oak Prestige sets are made for 40 mm doors.

The single door set has two 2100 mm legs and a 1000 mm head and suits openings up to 2100 × 1000 mm; the double set has three 2100 mm lengths for openings up to 2100 × 2100 mm. Widths are 95, 108, 125, 138, 145, 170, 195 and 220 mm, to suit walls of the same depth. Widths over 138 mm are laminated from two or three pieces of oak for stability. Fixings are not included.

Tools and materials

• Tape measure, pencil, combination square and a long spirit level
• Fine-toothed saw or mitre saw, and a straight edge
• Drill/driver, masonry bits for block or brick, and screws with plugs to suit the wall
• Packers or folding wedges
• Treatex Hardwax Oil or your chosen finish

Step-by-step: how to install a door casing

1. Measure the opening and the wall

Measure the width of the opening at the top, middle and bottom, and the height on both sides. Then measure the wall thickness, including plaster, in several places and note the largest figure. Our guide on how to measure for a bespoke oak door explains how the door, frame and opening relate.

2. Choose the liner width

Pick the liner width that matches your wall thickness so the casing finishes flush with the plaster on both faces. That gives the architrave a flat surface to sit on. We don't offer cut-to-width liners, so if your wall falls between two stock widths, take the next width up and plane or pack it so it finishes flush with the plaster.

3. Work out the internal size

The inside of the casing, at the rebate where the door will sit, needs to be the door size plus a small clearance gap at each side and at the head. Check the door maker's recommended gaps, and ask your fitter what gaps they work to. As a general trade practice, allow about 2–3 mm at the sides and head, plus an allowance under the door for the floor finish. Because the stop is built in, this size is fixed once the frame is cut and fitted, so measure twice.

4. Cut the legs and head

Cut both legs so the head sits at the correct height above the finished floor level, not the bare screed or subfloor. Cut the head to the internal width from step 3. The legs and head are square-cut, and are cut and joined on site by the fitter.

5. Assemble the frame

Join the head to the legs and check the frame is square by measuring the diagonals; they should match. Keep the rebate on the same face all the way round, on the side the door will close against. A temporary brace across the bottom of the legs helps keep the width true while you lift it into place.

6. Set it in the opening

Stand the frame in the opening, centred so both edges line up with the plaster, with the rebate facing the side the door will close against. Pack the head level first, then work down each leg.

7. Pack and fix the hinge side

Fix the hinge-side leg first. Put packers behind the leg at each hinge position and at top, middle and bottom, and check it is plumb on both faces. Screw through the casing at the packers into the wall, so it does not bow when you tighten them.

8. Fix the other leg and the head

Pack and fix the latch-side leg so the width is the same at top, middle and bottom. Use a straight edge across the opening to check the casing isn't twisted — a twist will show when the door meets the built-in rebate. Fix the head, packing any gaps so it stays straight.

9. Hang the door in the rebate

Hang the door into the rebate and check it closes evenly along its whole length. Adjust the hinges and, if needed, plane the meeting edge so the door sits flat against the stop without binding. Unlike a lining set, you do not fit the stops after hanging the door; they are already there.

10. Fill, finish and fit the architrave

Fill screw holes, sand lightly and apply your finish. Our sets are supplied untreated; for a natural, clear look we recommend Treatex Hardwax Oil, which also protects oak against moisture, spills and scratches. Then fit the architrave to cover the joint between the casing and the wall; see our hardwood architraves, including Solid European Oak Architrave.

Tips for a better finish

• Always set heights from the finished floor level.
• Check plumb on both faces of each leg, and put a packer behind every screw.
• Take extra care that the two legs are coplanar, because you cannot move the stop later.
• Finish the casing, architrave and skirting with the same product so the timber tones match.
• Bring the set into the room under normal living conditions, once the plaster is dry, before fitting.

Frequently asked questions

What width of door casing do I need?

One that matches the full thickness of your wall, including plaster. Our oak sets come in widths from 95 to 220 mm.

What's the difference between a lining set and a casing set?

A lining set is 20 mm thick with separate 44 mm by 12 mm stops, which you usually fit after hanging the door. A casing set is 32 mm thick with a rebate that forms a built-in stop (standard 35 mm by 12 mm, Prestige 44 mm by 12 mm). See our guide on how to install a door lining.

Which casing do I need for a thicker door?

The standard oak casing sets suit doors up to 35 mm thick. For thicker doors, use a Prestige casing set: the European oak Prestige set is made for 42 mm doors, and the American oak Prestige sets are made for 40 mm doors.

Are fixings included?

No. Choose screws and plugs to suit your wall type.

Can I use the casing with a double door?

Yes. The double door set suits openings up to 2100 mm high and 2100 mm wide.

Do you make casing sets in other timbers?

Yes. Besides European oak, we offer Solid American White Oak Door Casing Set and Solid American Red Oak Door Casing Set, including Prestige versions. Ash sets are in Ash Door Casing and Lining Sets and Douglas fir sets in Douglas Fir Casing and Lining Sets. The standard ash and Douglas fir casing sets use the same 32 mm thickness, 35 × 12 mm rebate and 95 to 220 mm widths as the standard oak sets. Their Prestige versions are made for 42 mm doors, like the European oak Prestige set.

Ready to start?

Browse our oak door casing and lining sets, or design your new door and see the price as you go in our timber door configurator.