Underpressure vents

If the roof's ventilation space does not allow for ventilation holes to be set close enough to the roof ridge to end triangles or the ridge's length is more than 15 m, the ridge needs underpressure vents. This situation is almost always present in row houses, where fire interruptions also prevent ventilation parallel to the ridge. Often also in condominiums (always in one with sealed roofs), the ridge needs removal ventilation with underpressure vents or similar structures. 

The roof's boarding

  • The platform of the bitumen roof shingles is usually close boarding or alternatively made from roof board, which needs to be sufficiently sturdy, non-bending, smooth and dry (table 1).

  • We recommend building the shingles from T&G board (width approx. 95 mm). When using rough-sawn boards (width approx. 100 mm), the boards need to be stronger, full edged, non-knotty and equal in quality to ensure that the adjacent boards do not bend differently.

  • The lengthening pieces of the boards must be located at the roof trusses or be supported separately. When using boards with T&G ends, with an equivalent roof truss gap, the lengthening pieces must have at least 3 unbroken boards.

  • The expansion caused by the alteration of the moistures and temperatures of the boards is noted by leaving a sufficient gap between them. When using a T&G roof board, the shingle manufacturers’ instructions regarding the thickness and the attachment of the board need to be taken into account.


Ventilation of a timber roof plays an extremely important role for its functioning and durability. There must be a ventilation gap of a minimum of 100 mm beneath the roof decking. The exhaust air vents must be as high as possible (at ridge or in gable end), and if necessary, ridge ventilators should be used. There must be sufficient replacement air vents at the eaves. All ventilation openings, gaps, grilles, and vents should be equipped with netting where needed to stop birds, squirrels, etc. getting into the ventilated space. The need for ventilation is further highlighted if there are gaps or holes in the vapour barrier.

Improving roof ventilation

If the ventilated spaces are formed as “channels” between the roof rafters (roof joists) leading from the eave to the ridge, there must be a continuous open longitudinal channel under the ridge so that air from ventilated spaces between all the roof rafters can escape through it. If ventilation in the direction of the ridge cannot be made to function otherwise, the roof structure must be altered so as to bring the ridge level higher

Underlay membranes and their installation

  • Katepal underlay membrane must always be installed under bitumen roofing shingles.
  • If roofing shingles are not installed immediately after the application of the underlay membrane (e.g. delays caused by work on chimneys, flues or other penetrations), we absolutely recommend the use of Super Underlay Sheet. This is also recommended if roofing work is postponed over the winter and the waterproofing consists of the underlay, or the roof geometry is complex or otherwise challenging (See table "Choosing the underlay membrane for bitumen roofing shingles" in separate pdf file).
  • The underlay membrane can be installed parallel or perpendicular to the roof deck boarding (Fig. 1 a and Fig. 1 b).The sheet is unrolled, positioned correctly in a straight line, any slack is removed, and the sheet is nailed to the deck with roofing nails along the edge that will be overlapped by the following sheet (also hidden ends are to be nailed).
  • On steep roofs, vertical installation is easier as there is less risk of creasing. To avoid creasing, the membrane must be absolutely straight and sufficiently tight. If installation work is carried out in cold weather, particular attention must be paid to tightening the sheets.

Metal drip trims

  • A metal drip trim is installed over the underlay membrane, at eaves without exception and usually at the verges (if a blocking piece is not incorporated into the verge detail). Metal drip trims are fixed to the decking through the underlayment with roofing nails or alternatively with flat or half-round headed screws at 10 cm spacings in an alternating zigzag pattern.
  • If it is unacceptable having the nail tips show through the eave soffit, sufficiently short screws with wide and flat heads (KFR screw) are to be used.
  • It is recommended that at the verges the drip edge be folded to an angle of 90° so that the fascia edge points straight down.
  • Drip edge extension joints are overlapped by 50 mm and sheetmetal shears are used to cut away the folded edge that forms the drip and a bevelled piece from the bottom edge of the section that is pushed under the previous section. Two nails (or two screws) are driven through both metal sections at the overlap

Special roof products

Pass-through seals
Manufactured EPDM-rubber seals are installed over the underlay, and the shingles are carefully bonded over the flange. The upper edge of the seal is tightened over the penetrating pipe with a metal clamp at such a height that the seal does not pull the shingles up if the pipe moves. Rectangular pass-through seals are also available, but we recommend the use of circular pass-through fitments

Ridge ventilators
Plastic ridge ventilators are the most common type. The ventilator is usually installed on the ridge or right next to it. The ridge ventilator types generally supplied include ridge types as well as slope types for various pitches, with straight or angled flanges.

A ridge ventilator should be installed over the underlay and fixed with adhesive and nails. A piece the size of the ventilation pipe is cut out of the shingles and they are carefully bonded over the flange. The size of the ridge ventilator (the diameter of the pipe) is determined by the necessary ventilation rate.

Ridge vent
If the roof ridge's vents cannot be implemented with the details on page 8, factory-made roof vents can be installed, generally for the entire length of the roof ridge. Typical targets of use include the repaired wooden structures, where the airing under the roof has not been taken into account during the building phase. It is essential to ensure the removal of air for each roof truss gap for the entire height of the roof. The old bitumen roof and the underlay boards are removed from the ridge from the width of 100 mm and the ridge vents installed according to the installation guide. The gap between the old roof and the vents is sealed with Sealing Compound K-36. Finally, the upper surface of the fans has roof shingles or a self-adhesive bitumen strip installed

Walkways should always be installed as high up on the slope as possible. We recommend the installation of pressure impregnated wood attachments under the metal supports. They should be installed upright on the roof so that water does not accumulate behind them. The joint between the wood attachments and the roofing should be sealed with Sealing Compound K-36Snow barriersOn elastomer bitumen roofing, with granular mineral surfacing, no snow barriers are usually needed. Snow does not slide down even from steep roof slopes. If snow barriers are installed, particular care must be taken to seal the fixing points securely

Chimney abutment of a log building

In a building with a log frame it must be noted that, as the frame settles, the roof descends in relation to a masonry chimney. The membranes may not be fixed to the chimney, instead a frame is made, against which the membranes are turned up. The flashing of the chimney is fixed to the chimney, overlapping the upturn by a minimum of 200 mm. The flashing may not be fixed to the roof surface or the membranes. In this way the frame and the upturn can settle with the roof without damage. The chimney flashing covers the gap between the upper edge of the upturn and the chimney, despite settling of the building.

Installing shingles on existing bitumen shingle roofing

A roof with old bitumen shingle roofing can be reroofed in three different ways

1. New shingles directly over the old ones - a quick and cheap alternative.
2. The old roofing shingles are dismantled, and a new underlay and new shingles are applied. A thorough alternative in connection with which it is easy to renew the whole decking or repair parts of it.
3. A new underlay and new shingles are applied over the old shingles.

Installing new shingles directly over the old ones
New roofing shingles of a matching model can be installed directly over old KL, Jazzy and Katrilli bitumen shingles under the following conditions:

  • The decking under the old roofing is in good condition and the roof space is sufficiently
  • The going/shingle row of the new and old shingles is almost the same (max. deviation 10 mm).
  • An underlay has been installed under the old shingles (min. pitch 1:5), or without the underlay, the pitch is no less than 1:3.
  • New shingles are fixed with roofing nails that penetrate through the decking. Minimum nail length is usually 35 mm.

*) gap of 5–10 mm should be left between the tip of the old shingle and the back edge of the new shingle. The vertical joint of the new shingles should be aligned with the centre of the tip of the old shingle.
**) Nail length should be specified so that the nail penetrates the roof deck (usually 35 mm roofing nails)