How do I fit or replace a garage side door?

A garage side door is an external door in a masonry opening, usually with no porch, no overhang and often facing the weather on the least-maintained elevation of the property. That combination is why they rot at the bottom, stick in winter and end up with a lock that no longer lines up. Replacing one properly is ordinary work, and the parts that make the difference are the threshold detail, the frame fixings and getting the frame genuinely square.

Safety

What you'll need

• Doorset — door and frame together — sized to the opening
• Frame fixings rated for masonry
• Packers — plastic or hardwood, not softwood offcuts
• Long spirit level, at least 1.8m
• Low-modulus exterior sealant and expanding foam
• SDS drill, chisels and a reciprocating saw
• Cable and pipe detector
• DPC strip and a weatherbar or drip
• Threshold or sill section (optional)

Step by step

1. Establish what is above the opening.There must be a lintel over the door and it must be sound. Look for a visible lintel, a soldier course or an arch. If you are changing the opening size in any way, stop — that is structural work needing design and building control, not a door replacement.
2. Check whether the door has a fire rating requirement.A door between an integral garage and the living accommodation has specific fire resistance and self-closing requirements, and a garage side door onto a passage close to a boundary may have its own considerations. A door in an external wall of a detached garage generally does not. Establish which situation you are in before choosing a door.
3. Measure the structural opening properly.Width at top, middle and bottom; height at both sides and the centre; and both diagonals. Take the smallest width and height as your working dimensions. Openings in masonry are never square and the smallest figure is the one the frame has to fit through.
4. Order the doorset undersize.10 to 15mm smaller than the smallest opening dimension in each direction. That gap is for packing, plumbing and foam. A doorset ordered to the opening size cannot be adjusted, and adjustment is the whole job.
5. Take the old frame out without wrecking the reveal.Cut through the old frame fixings with a reciprocating saw, cut the frame in two places on each jamb, and collapse it inwards. Levering a whole frame out pulls the reveal, the render and sometimes the plaster inside with it.
6. Prepare and check the opening.Clean the reveals back to sound masonry, check for a damp proof course at the jambs and at the threshold, and scan for cables and pipes before drilling anywhere. Make good any broken brick before the frame goes in, not after.
7. Sit the frame on a proper threshold detail.The frame's cill or bottom rail must not sit directly on wet concrete. A strip of DPC under the frame, the cill bedded and sealed, and where possible a small step up so water is not standing at the joint. This detail is the reason most garage side doors rot at the bottom.
8. Fix the frame plumb, square and straight — in that order.Wedge it in with packers, plumb both jambs in both directions with a long level, check the head is level, and measure both diagonals equal. Only then start fixing. Frame fixings through the jambs at around 450mm centres, always packed at the fixing point so the fixing cannot pull the frame out of line.
9. Check the door before you foam anything.Hang the door if it is not pre-hung, close it, and look at the margin all round. An even gap top, bottom and both sides means the frame is right. Correct it now — after the foam has gone in, adjustment means starting again.
10. Weather the head and the outside joint.A drip or a weatherbar over the head if there is no overhang above, and a low-modulus sealant joint between frame and masonry all round the outside, on a properly cleaned and dry surface. Foam is an insulator and a gap filler, not a weather seal.
11. Fit the ironmongery and finish the timber.Lock, handles, hinges and a threshold seal as required. Then finish every face of a timber door, including the top and bottom edges, before it takes any weather. Unfinished end grain at the bottom of an external door is the single commonest cause of early failure.

Tips

• Buy a complete doorset rather than a door and a frame separately. A factory-hung set arrives square, with the hinges and lock already positioned correctly, and it removes most of the ways this job goes wrong.
• A composite or uPVC doorset is a reasonable choice for a garage side door precisely because the exposure is high and the maintenance usually is not.
• If the door is on a passage between houses, think about security before choosing. A garage side door is often the least-watched entrance on a property.
• Photograph the opening with a tape in it before ordering. Suppliers ask questions you will not have thought to measure.
• Do the outside sealant joint on a dry mild day. Sealant applied to a damp or cold surface does not bond and the joint fails in its first winter.

Common mistakes

• Changing the size of the opening — The lintel is sized for the span it was built for. Widening an opening without a new correctly sized lintel is a structural alteration and it is dangerous as well as unlawful without approval.
• Fixing the frame without packers at every fixing — Tightening a frame fixing against a void pulls the jamb towards the masonry and bows it. The door then binds, and nothing you do to the door will fix it.
• Relying on expanding foam to hold or seal the frame — Foam fills and insulates. It is not a structural fixing and it is not weathertight — it degrades in sunlight and absorbs water at an exposed joint.
• Leaving the bottom edge of a timber door unfinished — It is the edge that gets wettest and the one nobody paints because the door is hung. It wicks water straight up into the door and the bottom rail rots first.
• Ignoring the fire requirement on an integral garage door — The door between an attached garage and a house is a fire separation element with specific requirements including self-closing. Fitting a standard external door there removes protection people depend on.

If it doesn't work

Questions people ask