Can You Fit a Bigger VELUX in the Same Roof?

In a pitched roof the rafters run from ridge to eaves, and a roof window is fixed between them. Making the window taller means lengthening the opening along the same pair of rafters. Making it wider means moving the rafters that bound it.

Height is the easier change. The rafters stay put, and what moves are the horizontal timbers across the opening (the head and sill trimmers) and the roof battens. The opening is cut up or down the slope and the new frame sits in the longer gap.

Width is the harder change. A rafter in the way has to be cut, and the load it carried has to be passed to the rafters either side. That is done with doubled timbers and trimmers fixed in a particular way, and it is the part that Building Control wants to inspect.

VELUX windows come in a ladder of widths and heights. Reading the code, the first letter gives the width (M is 78 cm and P is 94 cm, for example) and the digits give the height (04 is 98 cm and 06 is 118 cm).

Our answer on VELUX size codes goes through the full range. Which step-up is possible depends on the roof, and the survey gives the answer.

Three readings decide it. The first is the clear gap between the rafters either side of the old window. The second is the rafter centres on the rest of the slope, which show how far a wider window would run into the next timber. The third is the position of anything that limits the height: the ridge above, the eaves below, a chimney or a valley nearby.

As an example, take a roof with rafters at 600 mm centres. The clear gap between two rafters is smaller than 600 mm by the thickness of the timber. A window code whose width is greater than that gap needs a rafter cut. A narrower code may drop in with no cutting.

How those widths line up with rafter gaps is worked through in the rafter spacing answer.

Enlarging an opening is a staged job. It starts on the roof, because the covering around the old window must come off to the full extent of the new opening.

1. The old window and its flashing are removed.
2. The roof covering and battens are stripped back to the new outline.
3. From inside the roof void, the rafter is cut, and trimmers are fixed to carry its load.
4. The new frame is set in, then the underlay collar and flashing kit for the new size are fitted.
5. The covering is relaid and the sash is opened and closed through its range.
6. Inside, the plasterboard is cut back to the new size and the lining is reformed.

The old flashing cannot be reused, since the new window has different dimensions. A kit for the new size goes in, matched to your covering.

Cutting a rafter is a structural alteration. It needs Building Regulations approval before work starts, and the trimmers are normally inspected before they are covered. In Bournemouth, Christchurch and Poole the council's building control team is BCP's; outside that area it is the district's own, and a registered building control approver is the private alternative.

A new window must also meet the Part L figure for a roof window, which is a U-value of 1.4 W/m²K or an energy rating of Band B. For planning, rooflights in a roof slope can fall within permitted development, with conditions, and a listed building or an Article 4 direction changes that. Read our planning guide and check with the council.

We check the position and advise at the survey, and the council gives the answer on planning.

Sometimes the roof says no, or says not quite. A chimney stack beside the window, a valley gutter below it, a hip rafter or the ridge close above can all stop an opening growing in that direction. A purlin crossing the slope, a heavy timber that carries the rafters, sits in the way of a taller opening and is not moved lightly.

In these cases the survey shows the next size up that does fit, or a different position for the window in the slope. A window moved sideways to a clear bay can be larger than one forced into a crowded one.

A wider window can also be a poor fit with the room. The light well, the sloped lining between glass and ceiling, has to be reshaped, and a very large opening can sit awkwardly against a low eaves wall or a built-in cupboard. Standing in the room with the survey sheet shows whether the extra size helps.

A bigger window changes the light well, the sloped lining between the glass and the ceiling. It is widened or lengthened, re-boarded, plastered or taped and filled, and then decorated. Your quote says how much of that we carry out.

Blinds are made to the window size, so a blind from the old window will not fit the new one. If you want a new blind, order it at the same time.

For the full account of fitting, see VELUX window installation.