Okay, installing a mast on a flat composition roof near the edge presents a few challenges that need to be addressed to ensure a watertight and secure installation. Here's a breakdown of considerations and recommendations for flashing and boot selection

Key Considerations:
Roof Material

You mentioned "composition - not metal." I'm assuming this is either a modified bitumen (mod-bit) roof, built-up roof (BUR), or possibly a single-ply membrane like TPO or EPDM. It's absolutely crucial to know the exact material, as the compatible sealants and flashing methods will differ.



Proximity to Roof Edge

This is a major factor. Closer to the edge means increased exposure to wind-driven rain, and the potential for water to "back up" under the flashing.

Mast Diameter

The size of the mast determines the size of the boot required.

Local Building Codes

Always check your local codes for specific requirements regarding roof penetrations and mast installations.

Warranty

Improper installation can void your roof warranty. Consider having a professional roofer handle this if you're not completely comfortable.

General Recommendations and Options


Universal Roof Flashing Boots (Adjustable)



Pros

Designed to accommodate a range of pipe/mast diameters. Some have flexible, paintable EPDM collars. Good for situations where the exact mast size isn't perfectly known beforehand.

Cons

Can be less durable than boots specifically designed for a certain pipe size. Ensure you choose a model rated for your roof type (modified bitumen, TPO, etc.). Can be difficult to seal perfectly if the mast diameter is at the extreme end of the boot's range.2.

Specialty Roof Flashing Boots (Specific Diameter)



Pros

Provides a more precise fit and tighter seal, and therefore, better long-term leak protection.

Cons

Requires you to know the exact diameter of the mast beforehand. Less flexible if you ever need to change the mast later.3.

"No-Calk" or "Retrofit" Flashing Boots



Pros

Can be installed without removing the mast if you are replacing an old boot. Split design allows for easy installation.

Cons

Not ideal for new installations, as they typically aren't as robust as a properly flashed full boot.4.

Flashing Material (for Custom Flashing)



Aluminum Flashing

A good option, especially when coated. It is durable and can be easily formed. Needs to be compatible with your roofing material to prevent galvanic corrosion. Generally must be used in conjunction with a boot (aluminum flashing alone isn't a complete solution).

Copper Flashing

Extremely durable and long-lasting, but more expensive. Also needs to be compatible with your roofing material.

Lead Flashing

Traditional, very malleable, and conforms well to irregular shapes. However, it is less common due to environmental concerns.

Installation Steps (General Outline - Consult Specific Product Instructions)


Preparation



Locate and Mark

Precisely mark the location where the mast will penetrate the roof. Consider the location of rafters or joists below for secure mounting.

Clean the Area

Thoroughly clean the roof surface around the marked area. Remove any dirt, debris, loose granules, or old sealant. Use a compatible cleaner recommended by the roof manufacturer.

Dry Fit

Place the boot over the mast and onto the roof without sealant to check the fit. Make sure it sits flush and that you have enough space to work.2.

Cutting the Hole



Pilot Hole

Drill a small pilot hole through the roof at the marked center.

Cut the Opening

Using a hole saw of the appropriate diameter for the mast, carefully cut the opening through the roof. Be sure to cut straight and avoid damaging the surrounding roofing material.3.

Sealing and Flashing (This is where it gets specific to your roof type and flashing material)



Modified Bitumen (Mod-Bit)

Apply a thick bead of modified bitumen-compatible sealant (e.g., a polyurethane or MS polymer sealant) around the cut opening on the roof. Slide the boot over the mast and press it firmly onto the sealant, ensuring full contact. Use a torch to heat the mod-bit flashing, bonding it to the existing roof.

This requires skill and caution to avoid fire hazards or damaging the roof. Consider hiring a professional. Apply additional sealant around the top edge of the flashing where it meets the mast.

Built-Up Roof (BUR)

Apply a layer of asphalt roofing cement around the cut opening. Place the flashing boot over the mast and press it firmly into the asphalt cement. Apply additional layers of roofing felt and asphalt cement, extending out from the boot in shingle fashion. This creates a layered, waterproof seal.

Single-Ply Membranes (TPO, EPDM)

Clean the membrane thoroughly with a compatible cleaner. Use a TPO or EPDM-compatible primer on the area where the flashing will be applied. Apply a TPO or EPDM-compatible sealant around the cut opening. Slide the boot over the mast and press it firmly into the sealant. Use a hot air welder (for TPO) or a compatible adhesive (for EPDM) to weld or adhere the flashing to the membrane.

This requires specialized equipment and skill. Apply seam sealant along the edges of the flashing for added protection.4.

Securing the Mast



Attach to Structure

Ensure the mast is securely attached to the roof structure (rafters or joists) below. Use appropriate lag bolts or screws. If necessary, add blocking between the rafters to provide a solid mounting point.

This is critical for wind resistance.5.

Final Sealant



Top Edge

Apply a final bead of sealant around the top edge of the boot where it meets the mast to prevent water from entering.

Exposed Edges

Seal any exposed edges of the flashing material.

Specific Recommendations for Near-Edge Installations



Increased Flashing Size

Use a larger flashing boot than you think you need. This provides more surface area for sealing and helps to divert water away from the penetration.

Collar/Weather Shield

A separate collar or weather shield installed above the flashing boot can provide extra protection from wind-driven rain. These can be purchased or fabricated from metal or durable plastic.

Water Diverter

Consider installing a small diverter (a small metal or plastic strip) above the mast on the roof slope to help redirect water away from the area.

Professional Installation

Seriously consider having a professional roofer install the flashing. Their experience and expertise can ensure a watertight and long-lasting installation, especially near the roof edge. This is particularly important for single-ply membrane roofs.

Important Notes



Compatibility

Always use sealants, primers, and flashing materials that are specifically designed to be compatible with your roofing material. Incompatibility can lead to premature failure, leaks, and damage to the roof.

Manufacturer's Instructions

Always follow the manufacturer's instructions for the flashing boot and sealant.

Inspection

Regularly inspect the flashing and sealant for any signs of damage or deterioration. Reapply sealant as needed.

Wind Loading

The mast and its mounting hardware must be able to withstand the wind loads in your area. Consult with a structural engineer if you are unsure.

In summary, to give you the best recommendation, I need to know


Exact roofing material

(e.g., Modified Bitumen, TPO, EPDM, Built-Up Roof)2.

Mast diameter.3.

Distance from roof edge to the center of the mast penetration.Once I have that information, I can provide more specific advice on boot selection, flashing techniques, and sealant recommendations. Good luck with your installation! Remember, safety first! Flag for review