Bitumen's slight flexibility enables roads to handle vibration from traffic, and enables roofing felts to bend, as long as they're not too cold.
When heated with a torch it melts, and molten bitumen is normally used at around 200C, making it a definite risk to handle. It can soften to a degree in full sun on the hottest summer days. In midwinter it becomes stiffer still, making unmodified bitumen based roofing felt a problem to work with. This behaviour change over a wide temperature range occurs because bitumen is a mixture of over 200 different chemicals.
Bitumen sticks to almost anything, even polythene, but few things stick to it once its dry.
Emulsified bitumen in water is usually acidic, sometimes strongly so. This adds handling issues and limits its uses. This doesn't apply to other bitumen products.
Bitumen is an effective glue, and unlike most glues its 100% waterproof. But its not one of the stronger glues. Its liquified with heat or solvent.
The slow drying time of solvent bitumen is an issue, and it lacks grab.
Hot bitumen was widely used for parquet floors, and is still good for the job.
When regluing to existing bitumen is wanted, most glues won't stick to it, but more bitumen will. Specialist bitumen compatible glue is also available, though more bitumen is often just as good.
Bitumen in solvent has very little grab, and is slow drying, making the vinyl prone to creep or curl up. Weights on the vinyl prevent curl, but should be left on for days. I leave them in place for a week to avoid any risk of curl.
Bitumen used as paint isn't prone to peeling off, and it sticks the surface of the substrate together. It lasts extremely well out of the sun, and well in the sun when on a firm porous surface such as masonry. When it has no hard immovable substrate under it it tends to crack & split over time.
The black colour isn't the most popular. Adding red pigment (iron oxide) can make bitumen brown or a very dull red. Iron oxide is sold as red cement colouring powder.
Bitumen emulsion is sometimes used to renovate worn tarmac drives. It wears off after a bit.
Roofing felt, with fibres
Many roofing felts are simply bitumen and fibres. When a small patch is wanted its also possible to use bitumen in solvent & synthetic fibre cloth. Natural fibres don't last well.
There are 2 types of tarmac, hot lay and cold lay. Cold lay has added paraffin, diesel or white spirit to soften it, so it doesn't need heating to apply it. Its slower setting than hot tarmac of course.
Tarmac must be rolled to compact it, and needs to be laid on a well compacted surface for it to last, eg hardcore and gravel rolled firm. A couple of inches of tarmac laid on soil by a dodgy cowboy can look great at first, but weeds grow through and it soon breaks up.
Pay good attention to level and slope, any water puddling and freezing causes tarmac to fail early.
Tarmac is much cheaper bought as tarmac than mixing on site. Scalpings (used road surface) can be had for around £200 per 20 ton truck load in 2010. Diesel is used to stop it sticking to things.
Rigid roofing sheets such as corruline and onduline are fibres stuck together with bitumen.
Bitumen is sometimes used as mortar between paving setts or slabs. Its a very long lasting option for paving, but not for cars travelling at speed. It resists weed growth & cracking, and looks good with most paving. The higher cost than sand limits its popularity.
Bitumen paint is sometimes used as a remedy for damp. It is waterproof, but painting the outside of historic houses with it can make damp worse by preventing evaporation. Painting the interior of basements and retaining walls produces a dry interior surface, but increases the water content within the wall, which can occasionally lead to joist rot.
Asphalt is a mix of bitumen and clay, sometimes with added colourant, usually iron oxide. Its laid and polished. The result is a very tough hardwearing long lasting floor with some antibacterial properties.
Bitumen is sometimes used to delay rotting of timber by waterproofing it. A classic example is bitumen coating the bottom section of fenceposts to keep them from touching the wet soil. Coating the foot or so from ground level down delays rot, further down the wood is less rot prone.
Coal tar has long been used to treat psoriasis, and is very effective. Concerns about its safety have led to bitumen being used in its place. The resulting appearance discourages most people from using it this way, despite its effectiveness.
The paper roof is a historic curiosity that was once a standard roof type in some parts of the country. It was made of layers of bitumen coated paper, and damage was often repaired with a household iron. Its a predecessor of modern roofing felt.
Bitumen comes in various forms
- Just bitumen. Heat to melt and apply.
- Bitumen in solvent. Pour/brush on and let dry
- Bitumen emulsion. Emulsified in water
- Roof repair gloop. Bitumen with fibres & solvent
- Tarmac. Bitumen with stone & chalk/clay. Cold lay also contains a little solvent.
- Roofing felt. Bitumen and fibres, capped with sand or stone waste
- Roofing sheet. Bitumen and fibres
Roof repair gloop
Bitumen is used to repair cracks and splits on roofs, both felt roofs and permanent roofs. The result doesn't last long term, but it has its uses.
Mixing chopped fibres into bitumen enables it to last better on a roof. The fibres reduce its tendency to crack. Synthetic fibres last much better than rottable natural fibres, and glass fibre lasts even better.
Petrochemical solvents are used with bitumen. Choice of solvent affects drying times. From slowest to fastest:
- Diesel - very slow
- Paraffin - paint dries out in a couple of days in summer
- heating oil & lamp oil - as paraffin
- White spirit - relatively quick
- Petrol - not safe to use
- Lighter gas - evaporates in seconds, usable for removing small spots of bitumen, but other solvents are preferable.
Applied with any of the following:
- Blow torch to melt the underside, then apply it, pressing it down
- Stick it down with bitumen in solvent
- Clout nails
Most better quality roofing felts use modified bitumen, which stays flexible in cold weather. The longer life of good and mid-quality felts comes primarily from the synthetic fibre content, whereas cheap felt uses mixed rag, which contains a high percentage of rottable natural fibres. The best felt uses glass fibre.
The tarred roof is the ancestor of roofing felt.
When hot bitumen's wanted, its usually heated to at least 200C in a bucket on a gas ring. The high temperature plus its sticky nature makes it a serious burn risk, requiring protective clothing.
Playing a torch flame on the surface of bitumen to melt it makes it stick. This is a common technique used with torch-on roofing felt.
Fibres are often added to make the bitumen tougher. Roofing felt and rigid sheets are the prime use for this approach, but fibre can also be added to painted bitumen to reduce its tendency to crack. Fibres can be mixed into the paint can, or sprinkled on the painted surface then over painted.
Bitumen can be filled with low cost powders such as chalk & clay to reduce cost. Many bitumen products already contain fillers.
- the black solid fraction of crude oil that fails to boil at 400C
- similar to bitumen, but made from coal. Not generally used now
- nowadays this is stone, bitumen and clay or chalk. Tarmac was once made with tar rather than bitumen, they look the same.
- Pronounced ass fault or ash felt, this is a mixture of bitumen and clay often used for flooring in large buildings
The differences in US terminology can cause confusion on the internet
- What we call bitumen, the US calls asphalt.
- What we call tarmac, the US calls pavement or asphaltic concrete, so road surface is sometimes called pavement.