Structural Exclusion in Park Slope

What is the actual envelope of a Park Slope rowhouse?

Not four walls. Two walls you own outright, two you own half of, a roof, a cellar in the ground, and an extension somebody added to the back of it.

Exclusion means making a physical boundary an animal cannot cross. On a detached house that is a legible exercise: walk the perimeter, list the openings, close them. On an attached nineteenth-century rowhouse that has been extended and subdivided, the boundary is not obvious and the perimeter is not a walk.

Here is what the building actually consists of.

A front elevation onto the street, above an areaway, with a stoop rising across it to the parlour floor. Whatever is on that elevation is the least important surface on the building for this purpose, and it is the only one anybody looks at.

A rear elevation onto the yard, which is where the extension is, where the rear yard drains, and where nearly all the alteration history is.

Two party walls, shared with the houses either side. The floor joists of both buildings bear into them, and the recesses that receive those joist ends were never sealed. Half of that wall is somebody else’s, and their contractor, their view about the cellar and their waste storage are outside your control.

A roof, generally flat or nearly so, with a parapet, a bulkhead, roof drainage and often a deck.

A cellar story partly below grade, with soil on two sides, the drainage connection, the service entries and the party-wall pockets all in one room.

A rear extension, which the majority of these houses have, added at some point after the original construction and joined to the back wall along a seam that runs the full height.

The list matters because scope is where this work succeeds or fails, and a scope written against a detached-house mental model will cover the front, the windows and the doors and miss every route that carries traffic. The pathways in this building are the seam, the cellar line, the chases inside, the pockets in the shared masonry and the roof.

Why is the rear extension seam the most productive single item?

Because it is a joint between two buildings of different ages, made from the outside, running the entire height of the house.

Almost every house in this neighborhood has been extended at the back — into the yard, at one or more stories, at some point in the last century and a quarter. Sometimes it is a Victorian rear ell that was there from the beginning. More often it is a later addition: a kitchen pushed out in the sixties, a full-width extension in the nineties, a glazed rear on a yard flat done last decade.

Whenever it happened, the same thing had to occur. New construction was built against an existing masonry rear wall that had already been standing, settling and weathering for decades. Whatever detail was used to make that joint, it is a junction between two structures that will move differently for the rest of their lives.

What we find at that seam, repeatedly:

A gap that has opened since. Differential settlement between an old wall on old footings and a new structure on newer ones is not a defect; it is what buildings do. Over thirty years it opens a crack, and on a rowhouse that crack runs from the cellar to the roof.

Flashing that was never right or has failed. Where the extension roof meets the original wall, and where the extension’s side walls meet the neighbors’, a flashing detail was required. On a great many of these it was done in a hurry, in the wrong material, or by turning a lead into a mortar joint that has since failed.

Voids inside. The extension’s floor structure had to be tied into or hung off the original wall. There is nearly always a void where new joists meet old masonry, and it is inside the building, invisible from either room.

Services routed through it. Waste from the new kitchen, a gas line, an electrical drop, a condensate pipe from a modern boiler or an air-conditioning line set. Each of those was cut through a wall to get from the old part to the new part, and none of them was sealed at the time.

A different roof at a different level, which means a parapet or an upstand and a change of plane at the neighbors’ party walls on both sides.

Working the seam is a proper job, not a caulking exercise. Open joints get raked out and repacked in an appropriate mortar. Flashing gets remade where it has gone. Service penetrations get closed at the wall face in something that cannot be chewed, with a sleeve where the pipe needs to move. Voids inside get closed wherever a ceiling or a wall is genuinely open, which usually means during other work rather than as a standalone visit.

What does a terraced garden do to a rear wall?

It puts soil against masonry that was never intended to be below grade, and it creates a row of burrow-ready structures along every property line.

The ground here falls continuously from Prospect Park West down toward Fourth Avenue and the Gowanus lowland. That gradient does not stop at the house line; it runs through the block interiors. So the rear yards step downhill across a block, and the steps are held by retaining walls — at the rear boundary, along the side property lines, or partway down a yard where somebody built a terrace.

Three consequences for exclusion work.

Buried wall. Where a yard has been built up against the back of a house — and a hundred and forty years of paving, planting, mulch and re-landscaping tends to raise ground rather than lower it — the rear wall may have soil against it well above the original grade. Masonry below grade behaves differently: it stays damp, its mortar joints deteriorate faster, and any opening in it is inaccessible for inspection and repair. This is also where cellar damp on the lower blocks usually originates.

Retaining walls as habitat. A wall holding back two or three feet of soil is, from below, a vertical face with drainage behind it, and from above, a raised bed with a soft edge. Dry-laid stone and old brick versions contain voids by construction. Poured versions have a drainage layer and weep holes. Either way it is sheltered, well-drained ground with cover over it, and there is one on nearly every boundary on the slope.

Water arriving where the gradient is interrupted. The slope moves surface water fast, which is generally good, but every terrace, wall, raised patio and paved area interrupts it and concentrates it somewhere. On a lot that has been re-landscaped repeatedly, that somewhere is usually a corner nobody chose. It is found by standing outside in rain rather than by looking on a dry afternoon.

Correcting the ground is part of an exclusion scope even though none of it looks like pest work. Lowering a bed that has been built up against the wall so there is clearance between soil and any timber. Clearing and testing the areaway drain and the rear yard drain. Carrying a downspout away from the wall rather than letting it discharge at the base of it. Rebuilding a failed section of retaining wall properly rather than filling the void behind it. On the slope, this is often the largest single line on the proposal, and it is the item most likely to be cut.

What is in the void under the stoop?

More than most owners expect, and it is one of the least inspected places on the building.

The stoop is a masonry structure sitting against the front of the house, carrying steps from the sidewalk to the parlour floor, with cheek walls either side. Underneath it, at cellar level, there is a space. What is in that space varies enormously and matters in every case.

On some houses it is a genuine room or store with a door from the areaway, used for bins, bicycles or nothing at all. On others it is a void enclosed by masonry with no access at all. On many it is filled with rubble, ash, builders’ debris or a century of accumulated material, sometimes deliberately, sometimes because it was easier than carting it away.

Whatever is in it, three things are true. It sits directly against the cellar wall of the house, so any opening in that wall inside the void is an opening into the building. It is enclosed, sheltered and dark, which makes it attractive harborage. And it is very rarely opened, so nobody has looked at the cellar wall behind it in living memory.

The associated features are the areaway itself — its floor, its drain, the cellar windows onto it, the door into the cellar or the lower flat, and the gap where the front steps meet the building. The areaway is also where bins usually live, which puts a food source against the most important surface on the house.

An honest exclusion survey opens what can be opened here, reports what cannot, and does not quietly leave it off the list. Where a stoop void is genuinely sealed masonry with no access, that is worth knowing too: it is a compartment, and provided the cellar wall inside it is sound, a compartment is not a problem. The problem is the unexamined one that turns out to have a hole in the back of it.

What does retrofitted insulation hide?

The surfaces the survey needs to read, without closing anything that an animal cannot get through.

A great many houses here have had insulation retrofitted in the last two decades — spray foam or batts into the joist bays at the cellar ceiling, into the rim and band joist, into a roof structure, or blown into wall cavities where there are any. There are good reasons to do that, and none of this is an argument against it.

But it interacts badly with this work in three ways.

It covers the diagnostic surfaces. The band joist, the top of the foundation wall, the sill line and the joist pockets are where the survey reads a rowhouse. Covered, they cannot be read at all, and a route that was obvious to anyone who looked in 2010 is invisible now.

Cured foam is not a barrier. Rodents chew straight through it. Insulating the rim joist does not exclude anything from the rim joist; it insulates it. The two are constantly confused, including by people selling the work.

It destroys the record. Rub marks, droppings and gnawing on old timber are the map of where animals actually travel. Covered or removed, that record is gone, and the building has to be diagnosed from monitoring rather than from evidence.

The practical answer is not to remove insulation. It is to do the exclusion work at the same time as any insulation work, in the same access window, in the correct order: read the structure, close the openings in material that cannot be bitten, then insulate over the top. Where insulation is already in, expect the survey to open selected areas rather than accept the covering at face value, and expect that to be part of the cost.

The same logic applies to boxed-in chases, to bath panels with no access hatch, to boarded ceilings in a cellar and to any finished basement. Anything that covers the structure covers the problem. On a house of this age, the willingness to open something is what separates a survey from a walk-round.

Which materials survive on 1880s masonry?

The ones an animal cannot bite, matched to a substrate that is soft, irregular and a hundred and forty years old.

The substrate is the constraint here. These cellars are brick or stone laid in lime mortar. A proportion of that mortar has crumbled at the face or fallen out of the joint entirely, in places that are not predictable and cannot be listed in advance. Brownstone on the front spalls in a characteristic way: the veneer was set with the grain running vertically, so water between the bedding layers freezes and forces them apart, layer by layer. The Landmarks rowhouse manual identifies water as the eventual cause of most masonry deterioration, and it is right.

What works:

Repointing, where mortar has failed. Raking out the failed material to a sound depth and repacking the joint in an appropriate mortar. This is a preservation repair and an exclusion measure at the same time, and the two purposes do not conflict. Running a bead of sealant across the face of an open joint is neither; it lasts one winter and comes off with the frost.

Hydraulic cement or mortar at penetrations in masonry, packed properly around a sleeve rather than smeared over the front of a hole.

Sheet metal at chewable edges. Timber edges, door bottoms, the underside of a cellar door, anywhere an animal can get a tooth onto a corner.

Copper or stainless mesh where ventilation has to continue. This is the distinction most often got wrong in this trade: an opening that exists to let air or water move is screened, not sealed. Closing a foundation vent, a weep, an areaway drain or a chimney flue that needs to breathe creates a moisture problem in place of a rodent problem.

Rigid closure at soffits, returns and cornice ends, cut to fit rather than stuffed.

Purpose-made firestopping where a service passes through a floor, which is the correct material for the chase penetrations and does the exclusion job as a side effect.

What fails:

Cured expanding foam in any position whatsoever, for the two reasons already given. We do not keep it, so it cannot quietly become the closure on the one opening nobody got a photograph of.

Plain steel wool, which corrodes and bleeds a rust stain down whatever is beneath it. On a brownstone front that stain is permanent and visible.

Silicone and acrylic caulk as the barrier. Fine as a finish over something structural, useless on its own.

Cementitious patching over a moving joint, which cracks in the first season and then holds water in the crack.

Does designation change what can be done?

It changes the timetable on a small part of the work and nothing about the rest — and the first question is whether your block is even in it.

The Park Slope Historic District was designated by the Landmarks Preservation Commission in 1973, listed on the National Register in 1980, extended to the south in 2012 and to the north in 2016. The combined district covers roughly 2,575 buildings across about forty blocks, which makes it New York’s largest landmarked neighborhood by building count. On the National Register side, 1,802 contributing buildings are counted, all of them raised between 1862 and roughly 1920.

That is a large area and it is not the whole neighborhood. A good deal of the South Slope, and the blocks toward Greenwood Heights and the Prospect Expressway, sit outside it, and those streets have a different building history anyway — generally smaller and slightly later houses, more frame and brick, less brownstone, and a different pattern of alteration. Establishing which side of the line a specific address falls on is a five-minute question and it is worth asking before anything is scoped.

Where a house is inside the district, three points hold.

The productive locations are not the protected streetscape. The cellar and its ceiling line, the areaway floor and drain, the rear elevation onto the yard, the party wall pockets, the service penetrations, the roof, the bulkhead and the inside of a cornice return. That is where the traffic is, and none of it is the front of the house as seen from the sidewalk.

Where the front is involved, material choice does the work. Repointing an open joint appropriately is a repair the district wants anyway. Mesh is set back inside an opening rather than fixed across the face of it. A weep or vent that must stay open gets screened.

Review changes the schedule, not the answer. If a scope touches the visible exterior, the timeline becomes part of the program, and a job that would have been done in October gets done in March. The response is to survey early and to do the concealed work, which needs no review at all, while any exterior element is in process.

What order does this run in, and who signs it off?

Reduce, correct, close, verify — and in a house with tenants, the sign-off question has to be settled before any of it.

Reduce first. Closing a building that still holds an active population removes the animals’ route out and concentrates them in wall voids and party-wall pockets, where anything that dies is behind plaster and brick. That is a materially worse problem than the one you started with, and in an attached house it is the neighbors’ problem too. The reduction side is rodent control in Park Slope.

Correct the conditions. Drainage, grading, refuse storage, interior water. This is the part that decides whether the closure work has to be repeated.

Close, comprehensively rather than conveniently. The openings that matter on a house of this age are behind fifty-year-old planting, under a stoop, at the back of an areaway, behind stored goods in a cellar corner, or on a roofline that needs a ladder set on an awkward surface. What can be reached gets closed. What cannot gets written up as inaccessible, with a reason, rather than omitted. Nine tenths of a perimeter closed does not buy nine tenths of a result; the remaining tenth is all an animal requires.

Verify after weather. A hundred-and-forty-year-old masonry building moves, and a winter of freeze-thaw opens joints that were sound in autumn.

The authorisation question is specific to this housing band. In a managed building a board or an agent instructs the work. Here, the owner instructs it, but the work happens in and around units other households live in, and access to a tenant’s flat for closing a chase penetration requires notice and cooperation. The party wall raises a further question, because closing joist pockets properly is a physical alteration to a wall the neighbor half owns, and the only realistic moment to do it is when one side or the other has a ceiling or a wall open during other work. Raising it with a neighbor who is renovating is worth more than any material, because that access will not exist again for years.

Where obligations between owner and resident are unclear, landlord and tenant pest responsibilities sets out how they usually divide. The sealing and water-source requirements that Local Law 55 places on multiple dwellings describe this work almost exactly, and the general duty sits at the NYC Housing Maintenance Code and pests. Anyone comparing proposals should read choosing a pest control contractor, because on this housing stock the difference between two quotes is almost always a difference in how much of the building is in scope.

What does the finished scope look like on paper?

A located list, not a description — and the difference is what makes verification possible.

An exclusion scope on a house like this should read as a set of individual items, each with a location precise enough to find again, a description of what was found, the material used, and a photograph. Openings that could not be reached appear on the same list with the reason. Conditions referred elsewhere — a suspected drainage defect for a plumber, a failed cornice for a roofer, a retaining wall for a mason — appear as referrals rather than being quietly dropped.

That document does four things. It lets somebody check the work without taking anybody’s word for it. It survives a change of ownership, which in this neighborhood happens more often than the work does. It gives the next survey a baseline, so a year later the question is what has changed rather than what is there. And it is the record that a multiple dwelling’s investigation and remediation obligations are actually being met, rather than asserted.

For the discipline in general, see structural exclusion. The animals this work is aimed at are covered under house mouse and Norway rat in the library, with roof-level entry handled under wildlife management in Park Slope. For the neighborhood picture across every service, see pest control in Park Slope and the New York City hub. Brooklyn Heights is the older rowhouse version of the same discipline, and Garden City on Long Island is the detached contrast where the perimeter really is a perimeter.

If you want the seam, the cellar line and the roof read properly rather than a price for sealing a house, tell us what the building is and what has been added to it.

Related

• Structural Exclusion — how we approach it
• All pest control in Park Slope
• Rodent Control in Park Slope
• Cockroach Control in Park Slope
• Bed Bug Treatment in Park Slope