Constrution - siding, roofing and weather barriers

Keeping Water Out

Liquid water (rain, groundwater) is the number one enemy of buildings (vapor being a distant second), because water inevitably leads to mold.  While builders have always used siding, roofing and flashing to keep water out, sometimes, small amounts often got in anyhow.  buildings historically leaked a lot of air, so they also dried out, but now that they are less leaking, we pay much more attention to keeping water out.  Its useful to remember that anywhere water can go, it will go--and depending on winds, even places that seem impossible.

Water can enter a building in any one of the following ways:

  • Exterior leaks (flashing, caulking, ice dams, gutter leaks, etc)
  • Wind driven rain (thru joints in roofing or siding)
  • Capillary action (wicking up thru concrete)
  • Plumbing leaks

Because the components of a building are significantly different, its easiest to think about water problems on that basis.


foundation drainage

Masonry foundations, although largely resistant to the flow of water (except via cracks), are however absorbent, and so not only will they soak up water, but it will move upward in them via capillary action. In a dry climate this might be of no concern, but in a wetter one, at the minimum you want to use a sill gasket to keep the moisture out of the framing.

Foundations have dual exposures--the above ground portion is exposed to the rain, although it also can dry to the air.  The part below ground is exposed only to the wetness of the soil, which is hopefully more like a damp sponge than a wet or soaked one.  In order to try to keep the soil from getting soaked, first try to avoid building in wetlands, low areas where water pools, or in the path of a subsurface stream (building in those areas is beyond the scope of this site).  Next, the land should be sloped away from the building, about 6" drop in 10', or a 5% grade.  Admittedly this is often more than you'll find on existing buildings, so clearly you can get away with less, but that also means that surface water won't run off as well.  Large roof overhangs (2' or so) help keep rain away from the foundation.  Downspouts should have extensions or be put onto splash blocks so that water is expelled at least two feet from the house. Ideally the soil itself should be sandy loam or any soil with a low clay content so that it doesn't hold water.  To encourage the soil to drain, coarse gravel is placed all around the footing, and a drain (typically 4" perforated pipe) is placed in the gravel (where the drain either goes into the sewer or is daylighted somewhere far away).

 Since the soil will still be wet sometimes, a water resistant coating is applied to the exterior--typically an asphalt coating, or a cementious coating or some kind of elastomeric coating.  If there is a chance the soil will be waterlogged, a drain fabric is then placed top of this.  The drain fabric is a dimpled plastic sheet (for example Delta Drain), attached and sealed at the top right at ground level. The dimples create an air gap against the foundation that drains rapidly into the foundation drain.

Slabs, whether basement slabs or slabs on grade, will almost always have a vapor barrier under them, and that will sit on coarse gravel--potentially with rigid board insulation between the two.  Unlike vapor barriers used elsewhere, the common material for under a slab is 6-10mil thick polyethylene sheets (often called Visqueen, which is a brand name), which is almost completely impermeable to water movement.

In the past downspouts were sometimes connected directly to the cities sewer system, but many cities no longer allow this as heavy rains cause the sewer system capacity to be exceeded, and result in raw sewage overflowing somewhere.  Alternative solutions including Green Roofs & Cisterns (see the site page). 


Siding is only a primary rain barrier--its not going to keep 100% of the rain out, although depending on the siding type, it often will come quite close.  Virtually every siding type is installed over a secondary barrier--historically it was tar paper, then housewrap and now it might be a larger number of materials.  This secondary layer is called the "Weather resisitant barrier" or often just WRB (see below for more on WRBs).

Bevel siding (wood, fiber cement, aluminum or vinyl) is installed from the bottom up, just like roofing, with the upper course overlapping the lower one so that water needs to defy gravity to get behind it.  Still there is still vulnerability where it joins windows, door or other openings, and when there are butt joints in the siding.  The WRB is there because the caulk in those joints will eventually fail.  Because wood and fiber cement will absorb water, its best to prime the back of them so they are less absorbent.  It will make the paint on the front last longer also.  While vinyl siding is not an environmentally preferred product, it does have the advantage of providing a minimal rain screen, since it is hollow. This would be the case for aluminum siding as well.

Masonry siding (Brick, stucco, faux stone etc) is continuous in that the peices are glued together, and unless painted, is porous and so can absorb significant amounts of water and hold it for a relatively long period of time, so although it stops water, it leaves walls exposed to water vapor for a prolonged period of time.  In damp climates, it is imperative that these materials be installed on a rain screen or not used at all.  In fact, brick is almost always installed with a 1" air gap behind it.  If the air gap is not used (as it typical with stucco), its important to use a very good vapor barrier, because the sun will drive any moisture held in the stucco into the building (although note that if the vapor barrier lets moisture out, it will also let it in, so if you have frequent sun on wet stucco, you need a rainscreen).

Sheet goods (cement board, EIFS (stucco coated rigid insulation), MDO or other wood paneling, resin composite) are generally installed without overlap, which pretty much insures they will leak.  Often sheet goods are being installed with small air gaps around the sheet and on a rainscreen, essentially acknowleging they will leak, so better allowing them to dry.  Expecting caulked joints not to leak is clearly futile, as evidenced by the many failures of EIFS systems and resulting lawsuits. Sheet goods need a quality WRB behind them.

Board and batten - although there is overlap in this system, the overlap doesn't take advantage of gravity since its vertical, not horizontal, although, unlike sheet goods, there is at least some overlap.  With enough overlap, and regular maintenance of the joint (caulk, paint), or just the lack of wind blown rain, this system won't leak.  Still you want a WRB, because gravity is not on your side in this case.  If you put it on a rain screen and back prime the wood, the siding might even last a very long time.

Solar Driven Moisture

When the sun hits wet siding and warms it well beyond the inside temperature, it will drive moisture to the interior.  For claddings that hold very little water, most of the moisture is driven off into the exterior air fairly rapidly and hence not much will be driven inward. For cladding that do hold significant moisture (primarily masonry materials--brick, stucco), enough moisture can be driven inward to cause mold problem. the WRB does little to stop it because the moisture is moving as vapor, not liquid water.  A rain screen will allow at least some of the moisture to vent out, but may not fix the problem--the safest thing is to avoid those types of cladding in hot humid climates. 

Rain Screens

A rain screen is an air gap behind the siding and on top of the weather barrier.  There are three basic benefit to doing this: (1) the gap will reduce the wind driven air pressure on the WRB dramatically (2) even a slight flow of air across the WRB will dry it out faster (3) the back of the siding dries faster.

Rain screens are typically made with vertical strips of wood, which in theory don't need to be treated wood, but may need to be if the wood strips stay continuously wet (which alas probably means you'll stainless steel fasteners).  There are also sheet goods (mostly plastic) made either out of plastic mesh or heavy dimpled plastic sheets (for example Delta Dry, which is similar to their below grade Delta Drain fabric).  There is some question as to whether a gap smaller than 1/4" will allow for enough air flow--its safer to use a product that provides at least a 3/8" air gap.  Both the top and bottom of the siding need to have openings so that air can circulate thru the rain screen--and this means that you will need to install bug barriers at both--the typical product is a roll of plastic mesh fabric that looks a lot like the green scratchy kitchen pads for cleaning pots.


Roofing, like siding is made of pieces that either overlap each other or are glued together to form a single membrane.  While siding is virtually always installed on a vertical surface, the surface roofing installed on varies in pitch from nearly flat to very steep--and the steeper the surface the more gravity helps the roof shed water.  Roofing materials will generally reject nearly 100% of the rain, but are often not perfect, and so are typically installed on a secondary barrier, usually tar paper. The dramatic effect of roof pitch on how easily the roof sheds water can be seen in thatched roofs.  Thatch isn't even vagely water resisitant--at a low slope it would leak like a sieve, but at a steeper slope, a thick layer of thatch means that as the water drips from one layer to the next, it also runs downhill rapidly--enough so that it runs out the end before it has a chance to get outside.

Because the roof isn't vertical, it has two unique vulnerabilities that siding doesn't have: ice dams, and flooded gutters.  Ice dams are caused by snow melting on a roof (often, but not always due to low insulation levels), and then refreezing over the colder eaves forming a dam that holds water.  Besides adding enough insulation so that heat leaking from the house doesn't melt the snow on the roof, the common solution is a strip of some kind of peel-n-stick membrane along the gutter edge.  This would work for clogged gutters as well, although there is usually easier to just keep the clean.

Shingles (shakes, asphalt composition, metal, slate) - these rely on overlap and gravity to shed water, so there is usually a minimum pitch roof they can be installed on (manufacturer will specify), often somewhere around 3:12.  Because the pieces are relatively small, there will be many sideways joints, and hence shingles are installed in what effectively is a double layer so that the sideways joint sits on top of a whole single, or a whole shingle sits on top of the joint--that is the joints in each row are staggered.  Needless to say, the secondary membrane is itself made of pieces, so those peices also use overlap--at both horizontal and vertical joints.

Tile (concrete, clay, or metal) - are barrel shaped and hence have a lot of air under them.  Original tiles were handmade, shaped on a person's thigh, and a half circle, but most modern tiles are refered to as "S" tile.  In either case, they are installed with overalp in all directions, and function similar to singles, but due to all the air space, and the fact they tend not to fit very tight, makes them more vulnerable to wind blown rain.  While tile is often installed on fairly low pitch roofs, this is probably not a great idea in wetter climates.  As usual, follow manufacturers specifications.

Metal panels - these are long strips of metal, with ridges along the edges that allow the panels to interlock with each other.  The ridges create a hill that water has to go over in order to escape the panel.  Often the panels are pre-cut and shipped to be the height of the roof, ie each strip runs from the ridge down to the the eaves, and it it doesn't the panels are installed like shingles--from bottom up, so the upper panel overlaps the lower.

Metal panels come in two basic varieties - either exposed screws or hidden screws.  The exposed screws have big rubber washers on them to keep them from leaking, which means that eventually they will leak as the rubber degrades and fails.

As with other roofing, virtually every manufacturer will require that you install a secondary membrane under the roofing.

Membranes (peel and stick, torch down, heat welded, glued) - these are used on very low slope roofs where overlap and gravity won't work.  These rely on being joined together into a single piece that covers the entire roof. Previously common only on commercial buildings or in fairly dry climates, they are appearing more frequently in wetter climates.  When this is the case, its important that the roof membrane be kept in very good shape, since leaks are likely to be somewhat catastrophic.

Vented vs Unvented Roofs

Roof venting is done one of two ways depending on where the insulation goes.  If the attic is unconditioned, the entire area is vented, and if its conditioned, the insulation under the roof is installed with an air gap (typically around 1"), and that air gap is ventilated, for example with soffit vents and a continuous ridge vent.  In either case, this venting serves essentially the same purpose as a rain screen--if water moved thru the insulation, rather than condense on potentially cold roof sheathing, it is vented away. If you live in an area prone to fires, you will need to use special fire-safe soffit vents that screen out embers.

Unvented roofs are like siding without a rain screen, and hence run the risk  of condensation on the sheathing (see condensing potential), as with siding, the primary technique is to install enough insulation on the exterior of the sheathing that it is almost always above its dew point.


Everyone is aware of the function of roofing and siding, but flashing is equally important--in fact the most common way for rain to enter a building is due to failed or improperly installed flashing.

Flashing must be installed everywhere roofing touches any other surface, and in siding around windows and doors.

Small penetrations like vent pipes in the roof are often done with a tight fitting boot, while in siding are often just caulked.  Skylights and sun tubes usually come with their own metal flashing kit, which may require the use of plastic roof cement.

Chimneys are usually flashed with metal--step flashing along the slope and a straight strip where the roof runs flat.  This flashing is then attached via some kind of goop.  Sometimes this flashing has counter-flashing--this is flashing that laps over the roof flashing.  If so, the counter flashing is attached in the mortar when the chimney is constructed.

Roof-to-wall intersections are just larger versions of chimney flashing--ie step flashing is used along the slope, and a continuous metal strip is used where the roof intersection if parallel to the ground.

Windows and doors are flashed with strips of peel-n-stick membrane (or some equivalent).  The still flashing laps over the WRB, usually the jamb sides flashing also laps over the WRB, while the WRB laps over the head flashing.  As with everything else dealing with water, overlap is used to get gravity to drain the water away.  The flashing at sills should be "pan" flashing, which is often accomplished by attaching a slightly beveled piece of wood

Aside: If you live in a rural area with mice, you want to make sure there is virtually no gap in the sheathing around any penetration--if there is fill the gap with steel wool (mice hate it).  This is mostly a problem with tile roofs because the mice can travel under the tiles.

Weather Resistive Barriers

Weather resistive barriers (WRBs) are the materials that form the layer under the siding (and to some degree the roofing) that keep the wind driven rain out while also acting as a kind of back-up protection in case the outer layer fails.  Typically these layers are water resistant, but vapor permeable, meaning that won't let liquid water pass, but they will let vapor pass.  Since the goal is to keep the rain out, there is no requirement that it be vapor permeable, but given that the assumption is that its impossible to keep walls perfectly dry, you generally want them to be vapor permeable.  In the old days, there were no WRBs, but then there was little to no insulation or air sealing either, so huge amounts of warm, fairly dry air was constantly washing thru the walls and drying everything out.  Now we try not to let any moisture in at all, but when it does, we'd sure like it to be able to get back out eventually.  Given that plywood and OSB sheathing isn't especially permeable, and that drywall with paint on it isn't either, WRBs that are very permeable (which is many of them) aren't really buying you anything in terms of drying out.

The issue of vapor permeability is where walls and roofs often differ, since typically there is no insulation under the roof and the space is unheated, hence no vapor drive.  Vaulted ceilings on the other hand are just like walls, so all the concerns of WRBs apply in that case.

WRBs are installed in generally one of three ways: overlapped pieces using mechanical fasteners, non overlapped pieces attached to each other with tape, or liquid applied, where each component sticks to the other one. Overlapped pieces (tar paper, house wrap) rely on gravity to do their work, while the other versions rely on some kind of sticky stuff.

Building paper (tar paper, asphalt felt,15# felt, 30# felt etc), was the first WRB.  Its permeable enough and water resistant enough.  It comes in 3 basic grades with tar paper being the lightest and #30 felt being the heaviest.  While originally 15# felt and 30# felt were made with cotton fiber and a square (100sf) weighed 15 and 30 lbs, now the weight is less than that--sometimes much less.  Some builders double up the building paper to make a more durable WRB. When its doubled, usually the second layer is offset from the one below so the joints are overlapped.  While building paper is quite durable, it will eventually degrade if it stays wet and will degrade in the sun as well.

Building paper is installed with either staples, roofing nails or plastic cap nails, and of course each fastener results in a hole in the WRB, and sometimes also a tear (usually the result of a crinkle or some other lateral pressure in the paper).  Due to the paper being a little bit gooey it does seem to seal around these holes to some degree.

No building paper is waterproof for ever: they soak up water and become very permeable, although some take much longer than others.  The fact that wet building paper will transport far more moisture than dry is generally considered a benefit, but given that the underlying sheathing has a much lower perm rating, the benefit is limited to drying the sheathing itself.

Housewrap (Tyvek, Typar etc) - comes in two basic varieties: one breathes only between threads, and the other has tiny pin holes punched in it.  Housewrap comes in much wider rolls than building paper, and hence could go on faster, but makes it hard for one person to install.  Housewrap was orginally sold as an air barrier, but how attitudes have changed, and we no longer think of it as being one (see barriers for the background).  The two issues with housewrap are (1) whether it degrades over time, particularly when in contact with cedar or redwood, but possibly other materials and (2) that is very permeable even when dry, so solar driven moisture will go right thru it.

Housewrap is usually installed with staples, and hence like building paper means there are holes in the WRB, and likewise, sometimes staples will result in tears.

There are now some housewraps imported from Europe (examples; Proclima and Siga) that claim to have better water resistance, are more durable (they're thicker) while still being vapor permeable. These can be combined with tape to make the WRB into an air barrier also, although intuitively this configuration seems less durable than taping something more solid like sheathing (ie any motion at all in the WRB will put additional pressure on the tape).  With these more advanced housewraps, often the staples are taped.

Liquid applied membranes - these are field applied and are essentially very sticky, thick flexible paints that combine with other related products to seal not only the sheathing, but around penetrations and windows openings. The thinner ones are applied with a roller or sprayer, while the thicker ones like the window pan flashing are often put on with a plastic putty knife.  Two leading brands are Prosoco and Tremoco.  Because they are applied as a single membrane (or at least each part sticks to the other), the net result is also an air barrier.  Some products are also vapor barriers while others are vapor permeable.

Because these products are applied like paint, there are no nail or staple holes, and no overlap joints, although they do have to be applied at a consistent thickness.  You will generally have to use another related thicker more viscous product to seal around penetration and around window opening.

Sheathing+tape -  there are some type of sheathing, (OSB in zip wall, fiberglass backed gypsum in others) , coated with some kind of membrane that is similar to (or in some cases is) a a factory applied liquid membrane.   This category also includes rigid insulation board that can be taped (XPS,  polyiso or anything water resistant that tape will stick to.).  In all cases, the joints are then taped to make a continuous WRB which also serves as an air barrier.  Unlike building paper and housewrap, this technique relies on tape to function rather than physical overlap and gravity.

Final Thoughts

Although we worry most about how water can get in a properly functioning house, its important to remember the building components all eventually fail, and when they do you can get leaks.  Its not just roofing failures, but plumbing leaks also, which is why many builders put drain pans under hot water heaters, and sometimes under washing machines--because they are the most likely fixtures to leak.  The building is more resilient is components are backed up with others, if the design uses gravity to drain away water etc.   Its also always useful to think about how water will get out, given that inevitably gets in almost everywhere.


A more detailed explanation of WRBs is here:

Houe Wrap, Felt paper, and Weather Penetration Barriers, Paul Fisette, U Mass, 2001

Builders Guide Series, Joe Lstiburek, EEBA, 2000.



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