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Chimney fire burns through roof (C) Daniel Friedman Concealed Spaces in Roof Construction
Fire hazards, codes, solutions

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Concealed spaces in building structures:

Definition: a concealed space is broadly defined as a non-occupied space that is created by building construction.

It might be helpful to emphasize that by concealed space you can figure that you cannot see into the space or cavity. A concealed space might be accessible by a removable hatch or cover, or it may be completely inaccessible.

A concealed space in a building structure can permit rapid spread of a building fire and it can also make fire fighting more-difficult and more-dangerous. For that reason concealed spaces are addressed by building codes and by fire safety standards.

Our page top photo shows a detail from a house fire that began when a woodstove overheated.



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Concealed Space Fire Hazards, Codes, Solutions

Collapsing building © Daniel FriedmanIn building renovation or repair work some builders add a whole new roof structure over an existing roof, such as building a gabled roof over an existing flat roof, or building a steep-pitched roof over a lower-sloped roof.

Those multiple layers of framed and sheathed roof construction create a "concealed space" thus building a fire hazard into the modified structure unless specific steps are taken to address the fire risks involved. The fire hazards are a worry regardless of whether the concealed space is accessible or inaccessible.

The two photographs shown here and their companions below illustrate the conversion of a flat roofed single family home into a gable-roofed structure.

The building owners and their contractor agreed that the existing tar and gravel roof was at the end of its life, was ponding badly, and was going to continue to be a chronic leak source.

For their solution to this perpetually troublesome roof, the original flat roof and its covering materials were left in place while a new gabled roof structure was constructed above it (shown below).

Some building codes require the installation of sprinklers in concealed spaces but those same codes will typically permit the omission of sprinklers from small spaces where there are no ignition sources, no combustible materials, adequate enclosure, and fire stopping.

Below is the same one family home after gable roofs were installed.

Gable roof constructed atop original flat roofed building (C) InspectApedia

As we illustrate below, this modification created a concealed space - there is no access into the space from within the home, though it can be accessed by removing an external gable-end vent.

You also will note that in addition to the retention of the original tar and gravel roof, additional combustible materials are present: shingle scraps from the new roof.

Gable roof constructed atop original flat roofed building (C) InspectApedia

More about this roof structure is at LOW SLOPE ROOF CONVERSION.

Examples of Concealed Spaces in Buildings

Below: our photo of the remains of a chimney chase inside of a home is from a home at which a fire starting at the heating system flue or chimney caused extensive damage to the building.

Chimney chase indoor passage through living area (C) Daniel FriedmanExamples of concealed spaces in buildings are found at more than roof structure add-ons. Concealed spaces in old original construction, in remodeled or renovated buildings, and in new construction show up as

Below, using InspectApedia.com reader Bob's question and discussion below, we explain that concealed roof spaces can be a fire hazard and might pose structural loading questions too.

Reader Question: do little air gaps in my DIY foamed roof deck built atop an existing roof lead to any issues?

2017/10/26 Bob said:

I have 1 room. Between garage and house. The roof is connected but independent. It has cathedral ceiling with fiberglass between the tounge and groove (interior) and roof sheathing. I want to make it a hot roof to prevent condensation.

I will tear off shingles at foamboard. Frame with 2×4s and spray foam cavitys and cover with plywood and re shingle.

I will be able to airtight the interior suppifficently. My concern is after spray foam and applying new plywood.

If there is small air gaps between foam and roof deck will that lead to any issues? Help is appreciated thanks.

This question and our reply were posted originally at ROOF VENT if NO SOFFIT

[Click to enlarge any image]

Watch out: the roof design shown above is a sketch provided by the reader, is incomplete in that it does not show the details of the cathedral ceiling, and may require additional measures for fire safety and to meet building codes.

Reply: Possibly yes, both fire safety & design: a vented hot roof design may contain contradictions

I don't think foam gaps that do not communicate over any distance more than a few inches would be an issue as long as you are taking steps to be sure no indoor air/moisture leak into the enclosed roof cavity. You do that with a vapor barrier on the warm interior side and/or by using closed cell foam and taking care to seal around ceiling penetrations.

But I'm a bit worried about your design combining venting and multiple layers of roof structure that may be constructing a concealed space that is a fire concern.

To be accurate, I don't have a clear understanding of your structure. Be sure you're not building multiple layers of roof framing & sheathing in one roof - concealed spaces can be fire hazard and probably a code violation unless special fireproof measures are taken.

Reader follow-up: this is a self-created SIP roof, why's there a fire hazard?

Bob's DIY vented SIP roof design - Note: this is not a CODE-Approved design - at InspectApedia.comBob said:
Its pretty much a self created SIP roof. Why a fire hazzard?

This is close to what im doing. Just wondering if i need the furring strips if second layer of sheathing isn't 100 % in contact with spray foam

All in contact with each other: tounge and groove. Fiberglass batts.. roof sheathing.

Adding on top of that foam board frame with 2x4s closed cell spray foam new plywood then New shingles.
If there is space between the last two layers.

Spray foam and new ply wood then do i need to add furrimg strips between framed in spray foam and new roof. Then vented via ridge rent and vented flashing.

[Click to enlarge any image]

Reply: concealed roof spaces: fire hazards & structural concerns of a second roof structure atop the existing roof system

Thanks for the illustration, Bob. I'll add it here to permit other readers to comment.

To clarify our terminology, a "hot roof design" means that the roof is insulated and is NOT vented. See HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS

Watch out: The fire hazard against which I meant to warn is the construction of multiple layers of roof framing and sheathing, thus forming a concealed space that can interfere with the ability of fire fighters to cut a vent hole in a roof to release smoke when fighting a building fire.

This comes up on older buildings when someone leaves an existing roof in place then adds another set of framing and sheathing and then finish roof covering over or above the original structure. Fire spread through such spaces can be rapid and destructive and difficult for fire-fighters to control.

The usual method to address the concern is to use fireproof materials for sheathing, covering, and/or to completely fill the concealed space with non-combustible material such as fireproof insulation, such as rigid stone wool (PAROC WAS 25t is an example).

Construction of a new roof structure over an existing roof system, in addition to creating a concealed space that may require special methods and approvals to prevent spread of fire, there may be weight and thus structural issues for the building, depending on the weight of the new roof structure and its covering.

Your design is more like a structural insulated panel roof, but it looks as if it's site built.

If you strip the existing shingles and underlayment so that furring and insulation and then top sheathing and shingles are built atop that might be acceptable to your local building code compliance inspector.

However you are building this atop an existing 2x framed wood roof insulated with fiberglass. Your sketch doesn't make that clear.

In my OPINION that is really two complete roof structures in layers - and might be a fire concern.

I hope you'll check with both your building department and your fire department before proceeding, and do let me know what you're told.

Also, I'm not sure that this hybrid roof gains the advantages of a hot roof. You're putting venting above the existing insulated roof, separated from it by roof sheathing - that venting may not to do anything to avoid condensation - which is where we started, more so as you're spraying what, 1 1/2" of foam if your 2x4s are on flat, or 3 1/2" of foam if your 2xs are on edge -

Your drawing (that I will post along with this discussion at the bottom of the article above) doesn't show me how you are both adding foam insulation and adding an air vent path.

Vented structrual insulated panel for roofing - (C) InspectApedia.com

In contrast, an SIP or Structural-Insulated-Panel roof places a factory-built (or maybe site-built) structural panel - a plywood sandwich with foam core - on top of the rafters.

The ceiling and roof might achieve required rating by a layer of drywall on the inside surface of the SIP. Or that might be achieved by adding drywall - labor intensive- between rafters - more labor intensive if the owner wants to finish with a wood ceiling atop the drywall.

Concealed Space Articles & Codes: Reducing the Fire Hazards

Hidden problems above suspended ceilings (C) D FriedmanConcealed spaces are also created by the installation of a drop ceiling or suspended ceiling such as that shown in our photograph.

A suspended ceiling concealed space is also usually considered an accessible concealed space (different fire codes and rules may apply) and will often contain mechanical components.

The concealed space fire and weight hazards that may occur in adding a second roof structure over an existing one are discussed in these references.

It may be possible to proceed to build a second roof structure over an existing one if special measures are taken to prevent fire spread through that cavity, to avoid a fire-fighting issue, to avoid a structural and weight issue, and to obtain approval from local building officials.

There are, of course, ventilated SIPs, as we illustrate just above and as we describe at CATHEDRAL CEILING INSULATION.

See CATHEDRAL CEILING VENTILATION as well.

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Continue reading at LOW SLOPE ROOF CONVERSION - low slope roof re-cover procedure, converting a low slope roof to a steeper pitched slope: building a new roof over a low-slope or flat roofed building or mobile home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see CHIMNEY CHASE CONSTRUCTION

Or see FIRE RATINGS for ROOF SURFACES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see FIRE STOPPING at CHIMNEY PASSAGES

Or see ROOF VENT if NO SOFFIT

Suggested citation for this web page

CONCEALED SPACE FIRE CODES at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to ARTICLE INDEX to BUILDING VENTILATION

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