Roofs structures (vapour barrier, ventilation, sarking etc.)
In a pitched roof, do the gaps at eaves below the sarking boards provide sufficient ventilation of the roof?

Usually they are not sufficient alone. The gaps under the eaves provides replacement air for ventilation. The exhaust vents should be located as high as possible, either on the gable end below the ridge, or alternately suction vents may be installed on the ridge. If there are no exhaust vents, warm and humid air will collect at the top of the roof space, thus leading to condensation and absorption of moisture in the timber structure as the weather gets colder.

Why is the role of the vapour barrier emphasised in all instructions?

Problems other than a leaking roofing, account for about a half of roof problems. Condensation problems are very common. When a sealed vapour barrier is installed as part of the roof structure (also lead through sleeves must be vapour-tight) these problems are almost totally eliminated. The narrower the ventilated space in the roof structure, the more important it is that the vapour barrier provides a tight seal.

How is rough-sawn T&G boarding fixed to the roof rafters?

Every rough-sawn T&G sarking board should be nailed to every rafter with two sufficiently long hot-dip galvanised nails (usually 75 mm). When using dry timber, small gaps (1-2 mm) should be left between the boards to account for the expansion of the wood. Wood expands and contracts (width and thickness) by as much as 5-10% depending on weather conditions. Board end joints should always coincide with a roof rafter. If using rough-sawn T&G boarding with end T&G joints, the end joints in two consecutive board courses should not be in the same gap between two consecutive rafters; two-to-three solid board lengths should be left in between.

Can rough-sawn T&G boarding be substituted by some other material?

The rough-sawn T&G board sarking under bitumen felt roofing may be substituted by moisture resistant plywood sarking and even rough sawn boarding, provided that certain conditions are met. In the use of square, rough-sawn boarding, an excess amount of knots or oblique grain are potential risk factors. Manufacturer’s instructions should be followed when installing plywood. Rough-sawn square sarking boards should be thicker than T&G boards as they do not support one another. A decking of rough-sawn square boarding also develops proud edges as boards deform when they dry or load is directed only on a singular board, e.g. when walking on the roof.

Is it necessary to remove snow from the roof?

Under normal circumstances there is no need to remove snow from the roofs of residential buildings (detached or terraced houses and apartment buildings) if your roofing material is bitumen felt with a granular coating. The structures in these types of houses can withstand snow loads without difficulty. There is also no need to remove snow from the roof because of the risk of snow falling off the roof, as snow does not slide off bitumen felt roofing with a granular surface coating. With tiled and sheetmetal roofs there are situations when snow and ice must be removed from the roof in a controlled manner to ensure the safety of people in the street or courtyard areas below.  Otherwise these represent a potentially life-threatening risk for those moving around the building.

When the square, rough-sawn sarking boarding has indentations and buckling boards, does the decking have to be renewed if the reroofing material is bitumen felt or bitumen roofing shingles?

If it is possible to install a suitable size timber (board, 50×50 batten or similar) along the centre of each space between the roof rafters and every sarking board is screwed down to these, the indentations can be removed, the decking acts as a single structural element and does not need to be replaced. If there is any rot damage in the boards they obviously have to be renewed.

Why is the detailing at the foot of the chimney in a log building different from other buildings?

The frame of a new log building will settle by as much as several centimetres over the course of the years and therefore the waterproofing cannot be attached to both the chimney and the sarking. A standard brick or blockwork chimney does not settle with the log frame. Therefore a collar fixed to the roof must be built around it and the roofing upturns are attached to that. The upper edge of the sleeve and the upturn are protected by apron flashing fixed to the chimney and this may not be attached mechanically to the upturn or the roof surface. The lower edge of the apron flashing should be approx. 200 mm below the upper edge of the upturn (allowance for structural settlement).

Bitumen roofing shingles
Why do the nails have to penetrate through the sarking?

If the tip of a roofing nail remains inside the board, the expansion and contraction of wood caused by moisture content changes gradually starts to dislodge the nail from the wood. In a bitumen shingle roof the symptoms (leaks) caused by this usually start after 10-15 years. If the present roofing shingle is of elastomer bitumen, with a life span of several decades, the significance of the wrong nail length is emphasised, as at worst it shortens the life span of the roof remarkable.

Why should nails not be driven in along the very top edge of the shingles?

The shingles are nailed above the notches so that they penetrate through both overlapping shingles and press the adhesive surface of the shingle tightly against surface of the previous shingle. This secures the bonding of the joint; each shingle is fixed with eight nails and there is no risk of shingles becoming torn at the places where the nails are driven in. If nails are along the very top edge there is a danger that the thin top edge (without adhesive) gets torn away in a storm, possibly resulting in the roofing falling off the entire roof slope.

Why should you not install roofing shingles on roofs with a pitch less than 1:5?

The situation where water pressure causes problems can occur on roof with a pitch lower than 1:5 and the weathertightness of the roofing cannot be guaranteed. Roofing applications with weathertight joints should be used on roofs with a lower pitch.

Why is an underlay needed even on steep roofs?

Even on steep roofs situations may arise where water gets locked in a certain area and cannot run down the roof in a normal manner. Most typically such situations occur after a winter of heavy snowfall when the snow melts in the spring. In such a situation snow and ice may prevent the water from flowing off the roof and the seams of the roofing shingles are subjected to water pressure. The water that may then pass through the joint would be absorbed into the sarking in the absence of an underlay. A similar phenomenon may occur during heavy rainfall and strong wind. Pushed by the wind, water can even run up a steep roof slope and cause pressure at the joints. As the present style of building favours roofs with a complex geometry, the risks involved in the execution of connecting details have increased. A correctly installed underlay ensures the weathertightness of a roof in these situations, too.

How to remove moss from roofing shingles?

In some conditions moss may grow over bitumen roofing shingles. The likelihood of moss growth is increased by the presence of trees near the roof. Also the pH of the surrounding soil and other vegetation play a part. The surface of the roofing shingle is of granular stone material and moss grows naturally on stones and rocks. It is advisable though to remove moss from roofs regularly. The first measure is a brush in the spring or autumn (not in the hot summer). If this is not enough or cannot be done, moss can also be removed with chemicals. As there are many species of moss, there is no one product that can be applied to remove all of them (at least there are differing experiences). Moss removal products may be obtained from hardware stores. Please contact your local dealer.

My bitumen shingle roof has been fine for the past twelve years. Why are there now all of a sudden leaks here and there?

The most probable reason is the use of roofing nails or staples that were too short. If the tips of the nails (or staples) do not penetrate through the sarking, they start to get dislodged due to the moisture movement (expansion and contraction) of the wood and they start to push up the tabs of the shingles. i.e. the bond of the joints starts to disintegrate. Water finds its way under the shingles and along the nail end into the sarking, thus making it swell, which of course worsens the problem.

Can valleys and chimney roots be waterproofed with eaves starter sheets?

Weatherproofing of valleys and chimney roots should be done with Super-Pintari (70 cm wide elastomer bitumen sheet supplied in ten-metre rolls). The eaves/ridge shingles may not be used for the weatherproofing of valleys or chimney roots. More detailed installation instructions can be found on the packaging and on our website.

Why is sheetmetal flashing not installed along the bottom of valleys?

Valleys should be weatherproofed with an underlay felt and bitumen top sheet according to instructions. Sheetmetal flashing should not be installed in the valley. The reason is the high thermal expansion of metal. The surface temperatures of roofs may vary by as much as 130 degrees centigrade between scorching summer days and the coldest winter days. The significant thermal movement of sheetmetal places great strain on the fixings of the metal and the bond between felt and sheetmetal. As valleys are the places where most snow and ice collects, there are too many strain factors involved at the point where there will be most water.

Why at the verges is the felt not simply turned over the edge and nailed down?

The reason is long term experience of poor performance and the knowledge that a gable detailed in the above way will not endure the life span of modern elastomer bitumen products. At the eaves the nails driven into the side of the sarking will gradually become dislodged and the membrane turned over the edge can then tear more easily. From the root of a loosened nail, water running over the eave can become absorbed into the edge board, gradually rotting it. We absolutely recommend the use of a sheetmetal drip along the eaves and verges.

Can roofing shingles be installed in the winter or otherwise in cold weather?

In principle roofing shingles should be installed in warm conditions, which guarantees the best results. However, sometimes it is necessary to carry out roofing work even in cold weather. In such cases the following matters should be observed: 1) The roofing shingles will not bond until the following summer without additional heating. 2) Due to the stiffness of the shingles caused by the cold temperature, they may remain partially detached from the shingle below, and consequently the adhesive surface may dry and oxidise, which prevents proper adhesion without heating the shingles. 3) Usually south and west facing slopes bond gain properly without any problems in the following summer but especially in the bond of the north slope there may be problems. 4) The sealing of penetrations, upturns, valleys and other places where Sealing Compound K-36 is needed will be difficult to perform. If roofing work is scheduled for autumn or winter, we recommend the use of Super Underlay Sheet as underlay and leaving the installation of roofing shingles until the next spring or summer.

Will underlay on its own protect my roof over the winter?

If you use either Super Underlay Sheet or K-MS 170/3000 underlay membrane as underlay it can be left without any additional roofing over the winter. If K-EL 60/2200 is used as underlay, special measures are required to ensure the integrity of the membrane.

Can old bitumen felt be used as underlay for bitumen roofing shingles?

If the pitch of the roof is less than 1:3, old bitumen felt roofing is not a sufficient underlay under any circumstances. On steeper roofs, the issue may be considered, depending on the roof pitch and the quality of the old felt.

How can roofing shingles be installed more easily on a steep roof?

Moving on a steep roof is difficult and when installing roofing shingles, as ladders and scaffoldings tend to get in the way. A simple way to provide an anti-slip surface under your feet is the following: 1) There should be scaffoldings to the level of the eaves and these should be used as far as possible to carry out installation work. 2) A straight plank (50×100) is fixed over roofing shingles that have already been installed and 10 mm holes are drilled in the centre at 1000 mm intervals. These holes should be aligned with the tabs of one of the shingle courses. The said tabs are then folded up and the batten is screwed with 8×80 mm deck screws along the bent tabs to the sarking through the lower course shingles. The screws should be tightened so that the batten is stable. When stepping on the batten, take care not to trample and damage the upturned shingle tabs. Similar planks can be fixed to the roof slope at suitable intervals as work progresses. When the first slope has been roofed up to the ridge, work your way downwards and dismantle the planks carefully and seal the holes left under the tabs with Sealing Compound K-36. Ensure that the shingles bond  especially if debris has stuck to the adhesive surface or the shingle ends have had to be upturned for a long time. The roofing of the second slope is installed in the same way and the planks should be removed only after the ridge sheets are in place.

Are snow barriers needed with bitumen felt roofing?

Usually no snow barriers are needed even on steep bitumen roofs. However, if they have been specified in the master plans, the planning authority may justifiably demand them as with other items specified there. If the roofing material changes from the original plan, the effects of the change have to be taken into account. In other words, if sheetmetal roofing is changed for bitumen felt roofing, it is worth removing the snow barriers from the drawings as well. With bitumen membrane and shingle roofing, the fall of snow is prevented by the granular surface. The surfacing is so efficient that even if someone wants to get the snow down off the roof it will not fall unless it is shovelled or melts.

Self-adhesive bitumen membranes
Is an underlay felt needed with TopTite 6° or TopTite 3° roofing?

There is no need to install an underlay felt below roofings with sealed joints (TopTite 6° and TopTite 3°) with the exception of valleys, where one roll width is needed to ensure the durability of the roof’s most critical point. A prerequirement is of course that these membranes are used on roofs with permitted slope. The minimum pitch for TopTite 6° is 1:10 and for TopTite 3° correspondingly 1:20.

Why do the nails have to penetrate through the sarking?

If the tip of a roofing nail remains inside the board the expansion and contraction of the wood caused by moisture content changes gradually starts to dislodge the nail from the wood. In a bitumen shingle roof the symptoms (leaks) caused by this usually start after 10-15 years. If the present roofing shingle is of elastomer bitumen, with a life span of several decades, the significance of the wrong nail length is emphasised as at worst it shortens the life span of the roof remarkable.

What are the risks involved in installing TopTite 6° in cold weather?

There are two risk factors involved: 1) the joints will not bond and 2) the membranes get creased easily after the weather warms up. The bonding can be helped by warming the adhesive surfaces at the joint with a hot air gun and similarly the creases may be prevented by installing the membranes tight enough (a cold membrane should be tightened far more than is normally the case).

What is the correct direction for installing the membranes?

The membranes may be installed both vertically and horizontally. The most important thing is that the joints are properly sealed. In order to make a weathertight seal, any consecutive runs of the bitumen membrane should be parallel and equally tightly installed. On steep roofs we recommend vertical installation to prevent creases forming. If the membranes are installed over old felt roofing we recommend they be installed parallel to the old ones.

What causes the small creases in a horizontally installed membrane?

In horizontal assembly, it is difficult to fix the membrane in an absolutely straight line – the steeper the roof, the more difficult it is. Also on steep roofs tightening the membrane in the horizontal direction is  difficult. Consequently, if the membrane is even slightly loose or not absolutely flat, small creases will appear.

There are small creases in the membrane joints or they are not bonded. How can this be remedied?

If the roofing is installed in place and small creases appear (they sometimes appear after a few days or after a spell of cold weather when the weather warms up again) and may lead to small open gaps in the seam, they can be resealed easily with Sealing Compound K-36. Press Sealing Compound out of the cartridge into the joint and press the membranes tightly together.

Are snow barriers needed with bitumen felt roofing?

Usually no snow barriers are needed even on steep bitumen roofs. However, if they have been specified in the master plans the planning authority may justifiably demand them, as with other items specified there. If the roofing material changes from the original plan, the effects of the change have to be taken into account. In other words, if sheetmetal roofing is changed for bitumen felt roofing it is worth removing the snow barriers from the drawings as well.With bitumen membrane and shingle roofing, the falling of snow is prevented by the granular surface. The surfacing is so efficient that even if someone wants to get the snow off the roof, it will not fall unless it is shovelled or melts.

Bitumen felt roofing with triangular battens
Is an underlay felt used with bitumen roofing assembled over triangular battens?

In a traditional bitumen roofing assembled with triangular battens to create upturn, an underlay felt is not needed except over valleys. Today roofs with triangular battens are also installed over an underlay, but in this case the battens are made of plastic and the membranes are of a weldable type (products for professional use requiring hot work installation). A wooden triangular batten will rot between the membranes before the end of the membrane life span.

Can roofing shingles be installed on old bitumen roofing with triangular battens?

It is not possible to install bitumen shingle roofing on old felt roofing with triangular battens. The triangular batten roofing should first be removed and the shingles installed after that. At the same time it is advisable to inspect the condition of the sarking boards and the rest of the structure. If necessary the rotted boards should be renewed. The old sheets of the old triangular batten bitumen roofing are under no circumstances acceptable as underlay, even if the battens are removed.

Professional products
Why are professional products not recommended to DIY-builders?

The installation of professional products usually involves hot bitumen bonding where the roofers are required to have a valid hot work licence for roofing and waterproofing applications. If hot work is performed on any site against regulations and this results in a fire (or other comparable damage) insurance companies are not likely to pay compensation for the damage.

What do the letter and numerical codes in conjunction with the bitumen product names signify?

The letters and numbers in the membrane names are based on the old Finnish Standards SFS 5010 and 5011. The letters and numbers are still applied as they do still describe almost all the essential data about the product. The letters stand for the bitumen quality, membrane type and the material of the reinforcement layer. The numbers stand for the reinforcement and total weight in grams per square metre.

Why is elastomer bitumen better than blown oxidized bitumen?

Elastomer bitumen is modified bitumen. SBS elastomer is used for the modification. Elastomer bitumen is elastic even in cold conditions (with most products the elongation after fracture at a temperature of -30º centigrade is over 30 per cent), it can withstand UV radiation significantly better than blown bitumen and its softening point is significantly higher than that of blown bitumen. The life span of elastomer bitumen products is often as much as double that of blown bitumen products. Katepal today manufactures all its bitumen membranes and shingles of elastomer bitumen.

Is it true that elastomer bitumen roofing can be applied over a low pitch roof with sheetmetal roofing that has welted seams?

This claim is true, and in fact it a very sensible way of renewing sheetmetal roofing. If provides many technical advantages at lesser cost. On an old sheetmetal roof hard insulation is installed between the raised welts. At the edges high-pressure preservative impregnated battens of equal (or a few mm smaller) thickness are installed, to which the membranes and sheetmetal flashings are anchored. Over the insulation felt roofing (usually K-MS + K-PS) is installed, and it is mechanically fixed to the roof beams along the seams of the underlay. This makes the roof more silent, fully waterproof and even condensation problems usually disappear. There is no need for snow barriers on the roof, as snow and ice will not slide down the roof endangering people below.

Most common mistakes
The product is installed on a surface that is not steep enough

Leaks because the water pressures on low pitch roofs are higher than on steep roofs. The minimum roof pitch of any roofing material should always be checked when choosing it.

Too short nail length

Nails that are too short will gradually become dislodged from the sarking due to back and forth movement and at the same time push up the membrane or roofing shingle above and force the seams open.

Fixing with staples

Staple guns do not have a regulator for the depth of penetration; the depth of the staples is controlled by the compressor pressure regulator. In soft wood the staple will be pushed too deep into the wood material (cuts through the membrane or shingle) and in case of harder wood or knots, remains proud of the surface and pushes against the membrane or shingle above.

Insufficient vapour barrier

Moisture gets into the building fabric (especially in winter), which may lead to problematic condensation and/or rot damage.

Insufficient or poorly functioning roof space ventilation

Parts of the roof structure absorb moisture and swelling of the sarking is increased. In the worst case rot damage can occur in a short period of time. There may also be condensation leaks.

Shingles are not bonded to the valley sheet along the valleys

Leaks often occur in the valley area, at first primarily in the spring when the snow melts and later also during heavy rainfall throughout the year.

Boarding has been nailed too tight

In the worst case sarking boards start to buckle up, at first during a rainy autumn, and gradually this leads to tears in the waterproofing that cause leaking.

Sarking boards not thick enough or extension joints off the roof rafters

The roof cannot withstand normal walking on the roof and the waterproofing ruptures because of people moving on the roof or due to snow and ice.

Chimney abutment not sealed according to guidelines

Chimney abutment leaks.

Bitumen roofing shingle nailed at the top edge

The nails only penetrate one roofing shingle and do not press the adhesive surface of the shingle against the previous shingle. In the worst case the roofing over an entire slope may get detached under stormy conditions.

Difficult-to-shake false claim


Roofing nails should not penetrate through the sarking.

FALSE! The nail must penetrate through the sarking!

T&G boarding should be assembled tight when building the sarking.

FALSE! You should leave gap (1-2 mm) for moisture expansion between boards when using dry timber.

In pitched and hipped roofs, air circulates through the gaps in the soffit boards, hence the ventilation works

If there are no ventilation exhaust pipes at the ridge, moisture accumulates in the non-ventilated top part of the air space.

With a steep roof, where the ceiling follows the profile of the roof, gaps in the soffit provide sufficient ventilation.

FALSE! There should be exhaust vents on the ridge and air must be able to escape from all compartments between the roof rafters (if necessary a ventilation channel should be made along the ridge).