Guide · Roof Window Installation
Underlay and sarking around a pitched rooflight
The underlay is the layer nobody sees and the layer that decides whether the room below stays dry. Tiles and slates shed the bulk of the water, but they are a loose covering with hundreds of open joints, and in driven rain a proportion of what lands on the roof gets past them. The underlay catches that and carries it down to the gutter. Cut a hole in it for a roof window and you have interrupted the drainage path of the whole slope above.
So the question is never whether the membrane gets cut. It always does. The question is what happens to the water that was travelling down it. This page covers how the membrane is opened, where it is dressed, what the collar in the box is actually for, and how the answer changes between a 1930s bitumen felt roof off Springfield Road and a breathable membrane on a house at Beaulieu Park. It sits under roof window installation, which covers the wider job.
What the underlay is actually doing up there
It is a secondary weathering layer, and the British Standards treat it as one. Its job is to deal with what the primary covering lets through: wind driven rain forced up under a lap, fine snow blown through a joint at the ridge, and any moisture that condenses on the underside of the tiles on a clear night.
It has a second job that gets forgotten. During construction it is the only thing keeping the building dry between the roof going up and the tiles going on. On an installation that means the membrane is restored and lapped correctly before the covering comes back, not afterwards, because the covering is not what is keeping the water out at that moment.
Sarking, and what the word means in an English roof
The word is used two ways and the difference matters on site. In Scotland, and on some older houses in the south, sarking means timber boarding laid across the rafters under the covering. Elsewhere in England the word gets used loosely for the underlay itself, as in “sarking felt”.
They produce completely different jobs at an opening. A membrane is cut with a knife and folded. A boarded roof has 19mm or 25mm of softwood to saw through, and that board sets the depth the window frame has to be packed out to. Establishing which one you have is one of the first things a survey settles.
The layers in order, from plaster to tile
A pitched roof is a stack, and the rooflight has to be threaded through every layer of it in the right order. Getting the order wrong is how water ends up inside a construction it was supposed to run over.
| Layer | Typical build-up | What it does at an opening |
|---|---|---|
| Internal lining | Plasterboard and skim | Forms the reveal, finished last |
| Vapour control | Foil-backed board or separate membrane | Returned and sealed to the frame |
| Insulation | Between and under rafters | Stops at the frame, collar makes up the gap |
| Structure | Rafters, trimmers | Carries the frame fixings |
| Underlay | Membrane or felt | Cut, dressed and drained around the unit |
| Battens | 25 x 50 or 25 x 38 | Cut back, supported off the frame brackets |
| Covering | Tile or slate | Cut and re-laid to the flashing kit |
Where the membrane has to be cut
The membrane is not cut flush to the opening. It is cut back in a cross or an H, so that generous flaps of material remain on all four sides, and those flaps are what get dressed onto the frame. Trimming tight to the trimmer leaves nothing to work with and no way of making the drainage continuous.
The head is the critical corner of this. Water arriving from the slope above has to be taken sideways around the opening and returned onto the membrane below the window. If the top flap is cut short, that water has nowhere to go but into the rafter void, and the wet mark appears on the ceiling a metre below the window rather than at it, which is why the diagnosis is so often wrong.
Dressing the underlay into the frame
Each side of the opening is treated differently. At the head, the membrane from above laps over the top of the window’s drainage gutter, so gravity carries water onto the gutter and not behind it. At the sides, the flaps are dressed down onto the frame and held, so any water running down the side of the unit lands on top of the membrane. At the sill, the underlay below the window has to be continuous and clear, because everything the window sheds is discharged onto it.
Every one of those laps runs high over low. It is a simple rule and it is broken constantly, usually because a flap was fixed before the sequence was thought through.
Water does not care how good the window is; it only cares which piece of material is on top.
The underlay collar supplied with a roof window
Most manufacturers supply an underlay collar as a separate item, and it is not an optional extra. It is a pre-formed apron with a moulded drainage gutter that sits at the head of the frame, takes water arriving from above, and channels it around both sides of the unit and back onto the underlay below.
Ordering the window without it and improvising with offcuts is a false economy of a few pounds against a detail that has to work for twenty five winters. The collar also gives the insulation collar something to close against, which is where the thermal side of the detail begins.
Bituminous felt in older Essex roofs
Reinforced bitumen felt was the standard underlay from the 1930s until breathable membranes took over, and it is still what is up there on the interwar semis across Chelmsford, Brentwood and Billericay. After sixty or seventy years it is dark, stiff and often torn where it sagged between the rafters and finally split.
Cutting an opening in that material takes care, because it tears further than you intended if you are rough with it. The working method is to cut back to material that is still sound, then lap new membrane generously under and over it, so the old felt drains onto the new rather than the other way round. Where the felt around the opening has gone entirely, the sensible move is to take the new material wider and pick up the next rafter bay either side.
Breathable membranes, and the ventilation they assume
Modern breathable membranes are air and vapour permeable, which lets moisture from the house pass out through the roof rather than collecting on the underside of the covering. Some are rated for use without a ventilated void at all, and that rating is the whole basis of the roof’s moisture strategy.
That matters at an opening because the rating usually depends on the laps being correct and on the membrane being supported so it drapes properly between rafters. A membrane that has been cut, pulled tight and stapled flat around a window is no longer performing the way its specification assumed. Restoring the drape either side of the opening is part of the work.
Non-breathable membranes and the ventilated void
Where the underlay is not vapour permeable, the roof depends on moving air through the space above the insulation instead. The conventional figures are a continuous 25mm free path at the eaves for a pitched roof, with a 5mm continuous equivalent at the ridge on the steeper pitches.
A roof window occupies most of a rafter bay, and it interrupts the airflow up that bay. On a small unit in a well ventilated roof that is rarely significant. On a wide unit, or two units in adjacent bays, the ventilation path deserves a moment’s thought rather than an assumption, particularly in a bathroom or a kitchen where the moisture load is higher.
Counter battens, drainage channels and the gap behind the tile
A counter batten is a strip run up the line of each rafter, on top of the underlay, with the tiling battens laid across it. It lifts the tiling battens clear of the membrane and creates a drainage and ventilation channel behind the covering.
Where a roof has counter battens, the window sits higher relative to the membrane and the flashing has more depth to accommodate. It is a build-up worth identifying before the kit is ordered, because a standard flashing set assumes the tile sits directly on battens laid on the underlay. Counter battens are common on re-roofed slate work and on roofs where the covering is fixed in an exposed position.
Battens around the opening, and how they are set out
Tiling battens are cut back to the sides of the opening and lose their bearing, so the loose ends are carried by the brackets and the frame itself. The manufacturer’s instructions give the fixing positions for this and they are worth following exactly, because a batten end that is only nailed to the membrane is holding tiles up with nothing underneath it.
The batten gauge across the opening also has to line through with the gauge on the rest of the slope. If it drifts by even 10mm, the courses either side of the window will not meet, and the correction has to be made over several courses rather than at the window itself.
Sarking board roofs, and the extra thickness
A boarded roof, common under old slate in the Victorian terraces of Chelmsford and Colchester, changes two things. The opening has to be cut through the board as well as the covering, and the additional 19mm to 25mm of thickness raises the plane the flashing works from.
Frames are packed and set to suit, and the flashing has to be dressed onto the covering rather than sitting proud of it. Where the boarding is original and has taken a century of nail holes, the cut edges want supporting off the trimmers rather than left cantilevered. It is slower work than a membrane roof and it is worth knowing about before a date is agreed.
Where condensation forms in the void
In a cold roof, where the insulation sits at ceiling level, the void above is close to outside temperature and any warm moist air that gets into it condenses on the coldest surface it finds. A roof window changes that geometry, because the frame and the glass are now part of the envelope.
The insulation collar that goes round the frame is the item that stops a cold ring forming at the perimeter, and the vapour control layer returned to the internal lining is what stops room air reaching the void in the first place. Get both and the unit performs the way its U-value suggests. Miss the vapour control and you will see moisture at the top of the reveal in January, and the glass will get blamed for it.
Wind uplift on a loose membrane
An underlay that is not restrained balloons under wind, and a ballooning membrane lifts battens and works nail fixings loose over time. Around a fresh opening the effect is amplified, because the cut edges give the wind something to get under.
The countermeasures are ordinary and effective: laps of the recommended width, fixings at the correct centres, and taped or sealed laps where the roof is exposed. Exposure is a real variable in this county. A sheltered mid-terrace in central Chelmsford and a detached house on open ground near Burnham or Bradwell are not the same roof, even with identical coverings.
Handling a membrane that has gone brittle
Old felt does not forgive being walked on. The working approach is to keep loads off it, work from the outside where possible, and take the covering off in a wider band than the opening so that the membrane can be cut and lapped in still air rather than wrestled through a small hole.
Where the felt has degraded across the whole slope, the honest conversation is that the underlay is at the end of its working life and the roof will want re-covering at some point regardless of the window. That is a statement about the building, and it is better said at survey than discovered halfway through a day’s work.
Sequencing on the day, and keeping the roof closed
The order on site is fixed: strip the covering back only as far as the detail requires, cut and fold the membrane, set the frame, fit the underlay collar and dress the flaps, replace the battens, then re-lay the covering to the flashing kit. Insulation collar and internal lining follow.
Nothing in that list is left half done overnight. The roof is closed and weathertight at the end of every working day, whatever the forecast says, because an open membrane and an Essex night are a poor combination. On a straightforward window in a tiled slope the whole sequence is a single day.
What to ask about the underlay detail
Three questions separate a quote that has thought about this from one that has not. What underlay is on the roof now, and what condition is it in. Is the manufacturer’s underlay collar included as a named item. And how is water arriving from above being returned onto the membrane below the unit.
Any installer who has done the work will answer those in about thirty seconds. The same detail applies whether the unit is a standard VELUX roof window, a slimmer conservation rooflight, or a like for like replacement into an existing opening, because the membrane behaves the same way regardless of what is set into it.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Soakers, aprons and head flashings explained
Read the guide →Rooflight reveals and why splaying them matters
Read the guide →Two rooflights side by side on one slope
Read the guide →Rooflights on a shallow pitch below 15 degrees
Read the guide →
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