Window Install and Drainage

Gibson St shed · two west-wall salvage sash · consolidated decisions and research through August 27, 2026

Current snapshot

What JCS actually provides—and omits

The purchased Jamaica Cottage Shop plans use vertical board-and-batten siding and provide a very simple barn-sash installation. They specify a rough opening ½ inch larger than their nominal sash, 1×3 stops flush with the interior face of the framing, exterior 1×4 casing, top hinges, and an outward/upward-opening sash.

They do not detail exterior plywood, an interior liner, WRB, sill-pan flashing, jamb flashing, a head-flashing sequence, a sloped sill, a drainage cavity, or concealed-water inspection. Those omissions are understandable in the original board-wall system: leakage is visible and the wall can dry readily.

How the window fits

The sash fits inside the rough opening and closes against stops near the interior side. It does not overlap a smaller hole like a surface-mounted flap. For a fixed installation, interior and exterior retaining stops can hold it permanently.

A fixed sash simplifies the assembly: no hinges, hooks or operating sweep; no need to leave the window safely cracked; and the top and two sides can be bedded and sealed permanently. The bottom remains a drainage route into the pan. Opening the doors supplies occupied ventilation; matching north/south screened gable vents supply continuous passive ventilation.

The sill pan: what it is and is not

The pan is a waterproof tray beneath the sash. It spans the rough-opening sill, catches water arriving from the sash or jambs, and empties onto the exterior drainage plane.

• Back dam: an upturn at the interior edge that prevents water entering the room/wall.
• End dams: upturns at both jambs that prevent water escaping sideways into the two vulnerable bottom corners.
• Positive slope: approximately ¼ inch across the sill depth is a practical target.
• Exterior apron/drip: extends over the WRB/felt below and remains open at its lower edge.

DIY aluminum direction

A pan can be made from factory-painted aluminum trim coil or sheet. Use approximately 0.019-inch minimum thickness; 0.024 inch is preferable if available. Very thin disposable flashing roll is easier to dent and crease.

• Allow roughly 7–8 inches of blank width: back dam + 3½-inch sill + exterior apron + hem.
• Leave extra length to form end dams. Fold the back-dam/end-dam corners where possible; keep relief cuts off the horizontal pan floor.
• Support the pan continuously on a tapered solid substrate; aluminum alone does not create or hold the slope.
• Isolate aluminum from PT lumber with compatible membrane.
• Prefer fasteners through vertical portions, not the horizontal pan floor.
• Water-test each completed pan and both rear corners before installation.

A local gutter/siding shop can brake-form the long bends cheaply if the three-dimensional end folds prove difficult. A shop bend does not eliminate the need to inspect and test the corners.

The water-management hierarchy

Water behind cedar shingles is expected occasionally. The underlayment is supposed to stop it from reaching plywood and direct it down its exterior face. Both felt and housewrap can serve as that drainage plane. A physical gap or textured wrap improves drainage/drying; it is not what first introduces water behind the shingles.

Cedar tannins and wood sugars can act as surfactants and reduce the water repellency of some sheet WRBs, particularly when wet cedar is held tightly against them. That is the durability argument for standardized felt, a named surfactant-resistant/drainable WRB, or a physical rainscreen gap—not a claim that ordinary housewrap is categorically forbidden.

Underlayment options and full project costs

CSSB does not require plastic rain mesh. Its current sidewall guidance recommends No. 30 felt conforming to ASTM D226 Type II or ASTM D4869 Type IV. USDA/FPL guidance also recognizes locally approved housewrap. Drainable WRBs and mesh/furring are enhanced options.

Pricing basis at research date: APOC felt ~$36.24/200 sf; generic wrap ~$118; HydroGap 5×100 ~$185–217; Slicker Classic ~$176–197/150 sf, three rolls; packers based on two ¼-inch exterior-glue plywood sheets. Tax, shipping and flashing common to all options excluded.

Flashing sequence: the non-negotiable physics

Whatever underlayment is selected, upper layers must overlap lower layers. Water cannot be asked to travel uphill, and the pan outlet cannot be sealed shut.

1. Install exterior plywood.
2. Create the sloped, waterproof sill-pan assembly with back and end dams.
3. Integrate both pan corners into the jamb-flashing plane.
4. Set and retain the fixed sash; seal the exterior top and sides while preserving bottom drainage.
5. Install jamb flashing so it overlaps the pan corners.
6. Install head flashing so it overlaps both jamb flashings and projects beyond them.
7. Lap the field underlayment above over the head flashing and the pan apron over the underlayment below.
8. Install casing and shingles without sealing the bottom drainage route.

Cutting the plywood window openings after dry-in

This is a detail for the modified plywood wall, not the original JCS board-siding system. The west-wall W2 and W4 panels may be installed as solid rectangles over the window locations and opened after the roof is on. This avoids lifting a full-height panel with only narrow plywood webs remaining beside a precut 34-inch opening.

1. Mark an access opening at least 1–2 inches inside the anticipated finished opening. Verify that no fasteners lie in the cut path.
2. Set the circular-saw depth only slightly beyond the 0.451-inch plywood. Cut the straight portions and stop short of the corners.
3. Finish the corners with a jigsaw or oscillating tool and remove the center waste.
4. If the sill was intentionally deferred, install and fasten it now at the final elevation.
5. Working from the exterior, place the trim router in the access opening. Use a bottom-bearing flush-trim bit whose bearing rides on the full-depth studs, header and sill behind the plywood.
6. Rout to the actual framing. Finish the small corner radii with the oscillating tool.
7. Complete the plywood perimeter fastening into both studs, the header and the sill before installing the pan and flashing.

If #30 felt is selected

Do not simply cover the wall with one sheet and leave raw cut edges around the window. The published building-paper method uses a loose sill apron, separate jamb paper pieces, corner patches, jamb/head flashing, and later field courses tucked or lapped shingle-fashion. Ordinary horizontal felt laps are not seam-taped; localized flashing still is.

GCP Vycor Plus 6×75 (~$38) was the best-supported readily available flashing found: GCP lists PT wood, plywood and metal as suitable substrates and asphalt felt as chemically compatible. However, chemical compatibility is not a guarantee of strong adhesion to every rough felt surface. Critical bonds should be to clean structural substrates/metal, with felt integrated by laps rather than relying exclusively on tape-to-felt adhesion.

If a drainable WRB is selected

Use one named manufacturer family for WRB, sill/corner membrane, jamb/head tape and compatible sealant. Follow that system's illustrated sequence. This avoids inventing a hybrid detail between asphalt felt, unrelated tape and a custom pan. Confirm in writing that the selected WRB is approved in direct contact with the exact cedar shingle finish—especially if the shingles remain unfinished.

Sealant strategy for fixed sash

Sealant is a first defense, not the drainage system. Do not run a continuous exterior bead across the bottom in a way that turns the pan into a bathtub. Any sealant touching rubberized-asphalt flashing must be confirmed chemically compatible.

Decisions rejected or superseded

What must be decided before cuts and purchases

1. Confirm fixed versus operable. Current recommendation: both fixed, plus a matching south gable vent.
2. Re-measure both sash. Record maximum width/height and squareness; retain the current ½-inch-total clearance rule.
3. Choose underlayment: compliant #30 felt or one named drainable-WRB system.
4. Draw and mock up the window section using real 2×4, sash, 1×3 stop, 0.451-inch sheathing, proposed pan metal and exterior casing.
5. Resolve the visible back dam and retaining-stop attachment without puncturing the pan floor.
6. Set usable opening height. Preserve ¼-inch nominal sash clearance above the finished pan—not merely above the wood sill. Only then fix header elevation.
7. Select compatible flashing and sealant from the chosen WRB system or document the hybrid compatibility.
8. Build and water-test one pan before producing the second.

Source trail

• Cedar Shake & Shingle Bureau Sidewall Manual — compliant felt, sidewall spacing and fasteners.
• CSSB Installation FAQ — felt preference and cedar application guidance.
• Building Science: Pan Flashing — back dams, end dams, positive drainage and open sill outlet.
• Building Science: Window with Building Paper — apron/jamb/head sequence.
• Building Science: Residential Water Management — cedar surfactants and drainage gaps.
• GCP Vycor Compatibility — PT wood, structural substrates and asphalt-felt compatibility.