From the top down there are four layers to permeable paving.
Permeable seamless roof surface.
With the new seamless barrier.
Consider permeable paving in large walkways or driveways and make sure that a 6 inch gravel bed is immediately below the porous material.
Permeable surfaces also known as porous or pervious surfaces allow water to percolate into the soil to filter out pollutants and recharge the water table.
After all water can gather under an existing roof surface and then over time erode and ruin the roofing components underneath the surface layer.
In this particular job the roof is vented.
But the attic floor air seal is terrible there s a huge attic stair that leaks like a sieve.
Other types of roofing including cedar shingles and concrete tile roofing are fairly permeable.
Impermeable impervious surfaces are solid surfaces that don t allow water to penetrate forcing it to run off.
Each of these different layers acts as a filtration system to clean storm water.
Seamless roof coating solutions take care of this immediately though in many cases.
Many types of roofing including epdm and standing seam steel roofing are virtually impermeable to water vapor.
Even smaller roof leaks can lead to bigger problems if they are left untended.
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The top layer is the actual paving material.
These steel purlins are also used in steel sided buildings that have osb wall sheathing.
Regardless of your roofing underlayment choice your insulated roof assembly will not dry to the exterior unless the roofing is vapor permeable.
A breathable 34 0 perms but waterproof membrane like siga majcoat membrane and elevating the standing seam metal roof off of the decking to allow any water than may be in the osb sheathing to dry.