Flat or low slope roof conversion to sloped roof design:

Roof Re-Cover procedures, roofing codes, fire hazard & moisture warnings. This article describes & illustrates the construction of a sloped gable roof over the flat-roofed home shown above. The original tar and gravel low slope roof lasted many years but ultimately a combination of roof age, wear, ponding, and leaks led the owners to construct a gable roof over this building.

Photographs of the gable roof conversion structure are illustrated below. We use flat roof conversion photographs from several homes to discuss fire and building code concerns when a new roof is constructed atop an existing structure.

Flat Roof Conversion: Retrofitting a Pitched Roof over a Flat or Low Slope Roofed Structure

Construction of a sloped roof over an existing flat roof is a common procedure and one used by many builders for a variety of reasons including these principal objectives:

• Fix a chronically leaky flat roof at or past end of useful life
• Avoid the extra cost of a complete tear-off of the existing flat roof
• Avoid exposing the occupied building space to water damage should rain occur during the building project
• Add insulation to the building envelope
• Correct a difficult-to-vent moisture trap in a flat roof (FLAT ROOF MOISTURE & CONDENSATION)

Our photo (left) makes clear that for this roof-over construction project the original flat interior ceiling and its original structure were retained. The new pitched roof was constructed to bear on the top plates of the original building walls.

Note that there is an important distinction between adding a pitched roof, typically 4 in 12 or more, over an existing flat or low-slope roof - leaving the original roof structure and covering in place - and converting a flat roofed structure to a cathedral ceiling. On a flat or low sloped roof building that includes parapet walls, the builder may construct the new roof to bear on the parapets.

A cathedral ceiling conversion will require complete removal of the original structure, while the simpler addition of a pitched roof over an existing flat roof is typically handled by bearing on the existing building structure, its walls, and possibly by including sleepers and an "overframing" approach.

At below left you can see the intersecting gable roofs that were constructed atop the flat roofed structure shown at page top. Converting to a pitched roof helped eliminate chronic roof leaks in the original structure.

At above right and in additional photographs below construction details include temporary bracing below a valley rafter (below left),

and the construction of a knee wall to address original differences between two building section roof heights (second photo below).

The photos above and at below left show that in these different homes from the one shown at page top the builders left the original tar and gravel built-up roof (BUR) in place, along with original flat roof vents (below left). Should a building fire occur the fire department may not be happy to discover that there are multiple layers of roof structure upon which older roofing was left in place in the building. Building codes refer to this approach as a "Roof Re-Cover".

As you will see, there are some easy solutions that avoid having to perform a horrible old-roof tear-off in confined space.[1][2]

Roof recover: the process of installing an additional roof covering over a prepared existing roof covering without removing the existing roof covering.

1510.3 Recovering versus replacement. New roof coverings shall not be installed without first removing all existing layers of roof coverings where any of the following conditions occur:

1. Where the existing roof or roof covering is water soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing.

2. Where the existing roof covering is wood shake, slate, d clay, cement or asbestos-cement tile.

3. Where the existing roof has two or more applications of any type of roof covering.

Exceptions:

1. Complete and separate roofing systems, such as d standing-seam metal roof systems, that are designed to transmit the roof loads directly to the building's structural system and that do not rely on existing roofs and roof coverings for support, shall not require the removal of existing roof coverings.

2. Metal panel, metal shingle and concrete and clay tile - roof coverings shall be permitted to be installed over existing wood shake roofs when applied in accordance with Section 1510.4.

3. The application of a new protective coating over an existing spray polyurethane foam roofing system shall be permitted without tear-off of existing roof coverings.

1510.4 Roof recovering. Where the application of a new roof covering over wood shingle or shake roofs creates a combustible concealed space, the entire existing surface shall be covered with gypsum board, mineral fiber, glass fiber or other approved materials securely fastened in place. - [1] CA Building Code Chapter 15

Below the black stains on the roof sheathing of the "new" gable roof constructed over the original flat roof show that this roof was not adequately vented.

Clearing up the Definition of Roof Re-Cover or Roof Recovering

Building codes and facilities management experts use the term roof re-cover generally to refer to covering an existing roof with additional layers of roofing materials without removing the original material. As expert sources point out,

Re-covering can postpone more expensive roof replacement projects, but if a re-cover is performed without due consideration, the new membrane can exacerbate existing problems within the roof. On the other hand, a new roof is a better way to guarantee protection, but it comes at a cost.

Whether a re-cover or replacement project is best depends on many factors. Insurance considerations, building codes or other regulations might prevent re-covering a roof. For example, the International Building Code prevents more than two systems on a roof deck. A need to replace insulation or portions of the deck are among the other factors that can force replacement. - http://www.iccsafe.org/ and facilities net [4]

Watch out: when retrofitting a sloped roof over an existing low slope or flat-roofed building be sure to check with your local building department.

Don't just plow ahead building a non-conforming structure. Building permits will almost certainly be required and you may require the services of a design professional, an architect or structural engineer whose drawings will confirm the adequacy of the new structure.

Also see FIRE RATINGS for ROOF SURFACES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2019-04-26 - by (mod) -

Carol

Construction and roofing costs vary considerably by country and city within country.

Generally in the U.S. you'd figure $5 to $10. per square foot to build a new simple gable roof above an existing flat roof.

If your 1800 square foot house is meant to say 1800 feet on one level - i.e the footprint of the house, and if the existing house is a simple rectangle so that the roof structure itself is a single, simple gable roof, figure $9,000 to $18,000. depending on where you live.

That cost may increase for the costs of

- disposal of debris - fire codes require removal of old roofing materials though the original wood structure can normally remain
- roof ventilation
- provision of an attic-access from inside the structure
- insulation improvements

On 2019-04-25 by Carole

What is the approximate cost to covert a flat roof to a pitched roof with the minimum pitch for about a 1800 Sq. Foot house?

On 2016-02-09 - by (mod) -

Trevor,

We cite some concerns in the article above. It's very tempting to just build a new roof over the old one, maybe slopping some insulation atop the old flat roof under the cover of the new sloped one, and many do just that. If you check with your local fire department you may hear first-hand their concerns including difficulty fighting a house fire. Beware also of roofing over and enclosing wet materials that cause mold trouble.

On 2016-02-04 by Trevor Hislop

We have purchased a 1952 built house that originally had a flat roof but had a pitched roof of corrugated iron with no building paper built above the flat roof 3 years later due to leaks. Should we insulate directly on top of the flat roof or not?

On 2015-11-02 - by (mod) -

DK

YOu'll find some suggestions by searching InspectApedia.com for "attic insulation" and "hot roof" designs.
Or hop over to

https://inspectapedia.com/Energy/Insulation_Location_Attic.php

On 2015-10-31 by DK

I recently bought a house that has this re-roofing done. The house is cold and I want to insulate the attic, but where do you insulate? It looks like the previous owner had blown in insulation on top of the old roof but then some sections of the original roof had some fiberglass batts on top of the dry wall ceilings but zero insulation on top of the tongue and groove ceilings so I can't tell what is going on. I want to do it properly, how are builders doing this when this type of construction is done?
Thank you!

On 2014-03-06 - by (mod) -

Lea Ann I'll be back in Minnesota next week and will check further on that ceiling as I have access to the building; I believe it's a plasterboard material or a cement board material.

Generally if ceiling tiles are in good shape and space allows, the absolute minimum hazard is achieved by coating or covering-over the old material. If it must be removed then you can either treat it as PACM - presumed asbestos containing material, or have a sample tested - which may make sense if significant costs are involved.

On 2014-03-06 by Lea Ann

My question actually involves the photo above with the beams (2nd photo). What are the ceiling tiles made of. I have come across those and I'm wondering how to replace and what they contain. Roof is circa 1963 and I was told the large white tiles are actually acting as the roof sheathing.

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Continue reading at MEMBRANE & SINGLE PLY ROOFS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• BUILT UP ROOFS
• CONCEALED SPACE FIRE CODES
• DEFECTS LIST - ROOF LOW SLOPE
• EPDM, RUBBER, PVC ROOFING
• EPDM ROOF COATING REPAIRS
• EPDM ROOF LEAK REPAIRS
• EPDM ROOF SEAM REPAIR TAPE
• FLAT ROOF DRAINAGE SYSTEMS
• FLAT ROOF MOISTURE & CONDENSATION
• LOW SLOPE ROOFING
• LOW SLOPE ROOF CONVERSION
• MEMBRANE & SINGLE PLY ROOFS
• MODIFIED BITUMEN ROOFING
• MODIFIED BITUMEN ROOF DEFECTS & ROOF LIFE
• ROLL ROOFING, ASPHALT & SBS

Suggested citation for this web page

LOW SLOPE ROOF CONVERSION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING ROOFING

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